# PS Circuit with Adjustable Constant Current Mode



## vicbin (Dec 7, 2003)

Found this simple circuit with PCB mask while browsing www.epanorama.net :


0-30 Vdc Stabilized Power Supply With Current Control 0.002-3 A 


Since a digital adjustable CV & CC digital bench power supply is out of my budget, this circuit component price including the step down transformer is only cost about one 1 LS here.

Starting at 0.002 to 3 A with 30 V is more than enough for LED experimentor like us, and I'm planning to equip it with two cheap China made digital multimeter for voltage and current monitoring.


Now questions before I start etching the PCB :

- Any comments or suggestion on the circuit design ? or should I say is it really worth building it ?

- Is it possible to replace the current sense resistor 0.47 Ohm to 0.1 Ohm for easier current monitoring without sacrificing anything ? or is there any other component value need to be adjusted ?


For electronic hobbyist like me, I only understand the module like the reference part and the power driver part but not the overall circuit. Please, to any EE advise is needed. 

Mr.Al are you there ? /ubbthreads/images/graemlins/grin.gif

Regards,
Vic


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## MrAl (Dec 7, 2003)

Hi there Vic,

I took a quick look at the circuit and here are some
notes i'd like to share with you.


Nice looking circuit, although it sure uses a lot of parts


There are simpler solutions but you dont get max current
adjustment with them unless you add more parts.

If you want to reduce R7 to .22 ohms you have to change
one of the resistors. I'll get back with the value a
little later. Then you can get away with a 5w unit.
Keeping R7 at 0.47 ohms however, i'd use a 10 watt
unit rather then a 5w one. The power in a 0.47 ohm at
3 amps is almost 5 watts so that's too close.

One big point here: The heatsink for the big transistor
is going to have to be a REAL nice size...4 inches by
4 inches perhaps, with lots of fins. The power given
off as heat will be quite high: at 3 amps output at
4 volts output (assuming 24vdc input) will be 20 times
3, or SIXTY watts !!!
This means it could be worthwhile to provide some
thermal feedback via a thermistor on the heatsink,
and/or use a special transformer with at least two
taps: one at 'normal' input and one at half 'normal'.
The half normal setting could be used when you need
low voltage and high output current, with the normal
setting used when you need high output voltage.
Transformers of this kind are readily available. All
you have to do is pick one that has a center tap and
switch to the center tap when low output voltage
and high current is needed.


Im not too sure why they chose to 'or' the voltage
regulation and current regulation signals at
the current reference point...im not sure if this is
a temperature stable idea, if that matters. A slightly
better way is to 'or' them AFTER the two difference
amplifiers. This means the references for the two are
never dependent on a temperature varying device of any kind.
How much temperature variation there is would have to
be found by testing, but there's a chance it's low enough
to not worry about for apps that dont have critical
temperature demands.

You can also ask if it really needs output zero offset
adjustment, but i guess if you go through the trouble
of assembling all the other parts the few more wont
hurt.

After all is said and done, if i was going to rate
the circuit i'd have to give it a "B".
With the construction article the rating improves because
it's very well written and very clear, plus even the
circuit board layout is provided!
It's hard to beat that

I'll try and take a more in depth look a little later
today, perhaps a few simulations to see how this thing
behaves.

Take care,
Al


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## vicbin (Dec 7, 2003)

Hello Mr.Al,

As always, thanks for the comment.


[ QUOTE ]
*MrAl said:*
There are simpler solutions but you dont get max current adjustment with them unless you add more parts.


[/ QUOTE ]
Mind share the idea or the circuit it self ?


[ QUOTE ]
*MrAl said:*
If you want to reduce R7 to .22 ohms you have to change one of the resistors. I'll get back with the value a little later. Then you can get away with a 5w unit. Keeping R7 at 0.47 ohms however, i'd use a 10 watt unit rather then a 5w one. The power in a 0.47 ohm at 3 amps is almost 5 watts so that's too close.


[/ QUOTE ]
Not 0.22 Ohm, it is 0.1 Ohm I'm looking to since it is easier to use with DMM on it for current measurement.
Yes, I noticed that issue too and mine are 10 Watts /ubbthreads/images/graemlins/grin.gif


[ QUOTE ]
*MrAl said:*
One big point here: The heatsink for the big transistor is going to have to be a REAL nice size...4 inches by 4 inches perhaps, with lots of fins. The power given off as heat will be quite high: at 3 amps output at 4 volts output assuming 24vdc input) will be 20 times 3, or SIXTY watts !!! 
This means it could be worthwhile to provide some thermal feedback via a thermistor on the heatsink, and/or use a special transformer with at least two taps: one at 'normal' input and one at half 'normal'. The half normal setting could be used when you need low voltage and high output current, with the normal setting used when you need high output voltage. Transformers of this kind are readily available. All you have to do is pick one that has a center tap and switch to the center tap when low output voltage and high current is needed.


[/ QUOTE ]
Yes, don't worry, my secondary winding got 6 taps which are 6,9,12,15,18 and 24 Volt and here I have 1 pole 6 positions rotary switch (5 Amp rating) is ready to reduce the dissipation at the poor 2N3055. Also I'm planning to use a quite DC fan (Panaflo) for active cooling the TO3 heatsink.


[ QUOTE ]
*MrAl said:*
Im not too sure why they chose to 'or' the voltage regulation and current regulation signals at the current reference point...im not sure if this is a temperature stable idea, if that matters. A slightly better way is to 'or' them AFTER the two difference amplifiers. This means the references for the two are never dependent on a temperature varying device of any kind.

How much temperature variation there is would have to be found by testing, but there's a chance it's low enough to not worry about for apps that dont have critical temperature demands.


[/ QUOTE ]
Explain a bit further please.


[ QUOTE ]
*MrAl said:*
You can also ask if it really needs output zero offset adjustment, but i guess if you go through the trouble of assembling all the other parts the few more wont hurt.


[/ QUOTE ]
Yes, absolutely, show us please. /ubbthreads/images/graemlins/cool.gif


[ QUOTE ]
*MrAl said:*
I'll try and take a more in depth look a little later today, perhaps a few simulations to see how this thing behaves.


[/ QUOTE ]
Thank you ! /ubbthreads/images/graemlins/smile.gif


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## MrAl (Dec 7, 2003)

Hello again,

After spending a few more minutes with this circuit i 
have to change my 'rating' to an A-

and the only reason for the 'minus' is because the one
improvement to the circuit would be to use a temperature
stable reference diode instead of a zener for the 
reference voltage (D8).

Other then that, this circuit looks well thought out.
Also, the 'or' part of the circuit i was talking about
uses a diode but that diode is on the output of an integrator, so
the temperature dependence is not there, so i have to
retract what i said about the 'or' part of the circuit.
The only temperature dependence left is the zener, which
is probably good enough for a test bench supply.

If R7=0.22 ohms then R18=100k ohms.
R18 roughly DOUBLES when R7 HALVES.
This means if you want to try 0.1 ohms, then 
R7=0.1 and R18=200k. I would try it first with
0.22 ohms then work it down to 0.1 ohms.
After the change, adjust the pot for max output current
and test with a load to see if the current goes much
above 3 amps before the voltage begins to decrease.
R7 sets the current limit but R18 sets the max adjustment
for the current limit (if you change only R7 to 0.22 you
effectively double the max output to almost 6 amps).

I see you thought out the heat sink issue,
that's good 
It's a shame that these nice linear regulators 
have such a nasty power dissipation when output
voltage is low. It's very good that you already
thought about this and have decided to use taps
on the transformer secondary, complete with a
multi-pole switch. That's neat!
If you want to add thermal sense feedback probably
all you have to do is epoxy a themistor to the
heatsink and wire this perhaps in parallel with the
pot P2 and lower R18 to half it's value. It 
might require a series resistor too though
of around 5k to 10k.
The thermistor i have in mind is the Radio Shack
10k @ 25 deg C model (only one they have i think).
It would be worth looking at this more if you 
think you want to do this also.

All i meant about the zero offset adjustment is that there
is one already in the circuit (RV1) that requires the
extra parts Rv1 and R10. Perhaps these can be
eliminated without side effects, but if you are building
it i guess it wont hurt to add these also.
If you decide to try it without, you can probably 
also get away with using the LM358 type op amps
which dont have any offset adjustment.
Since the circuit is adjustable already anyway,
i cant see even 4mv of input offset affecting anything
other than the zero voltage output adjustment.
Who cares if you cant get an output adjustment of
0.008 volts? If you need to get outputs down this low
then you might need the offset adjustment.

Lastly, the filter cap value seems adequate if you keep
the dc buss up about 7 volts above the output voltage.
Any lower then that at full output current and some
ripple will begin to show up on the output.

Simpler circuits without current adjustments:
The LM317 for example, one ic does the voltage regulation,
then if you need current limit you can use another device
to set the current limit.

If you build this circuit i hope you keep us updated
on your progress and results etc. ?


Take care,
Al


----------



## vicbin (Dec 30, 2003)

Thank you for the input MrAl,

Sorry, been a while, just back from vacation /ubbthreads/images/graemlins/grin.gif

[ QUOTE ]
*MrAl said:*
After spending a few more minutes with this circuit i have to change my 'rating' to an A- /ubbthreads/images/graemlins/smile.gif , and the only reason for the 'minus' is because the one improvement to the circuit would be to use a temperature stable reference diode instead of a zener for the reference voltage (D8).


[/ QUOTE ]
Now plan to make it an A+ circuit. /ubbthreads/images/graemlins/smile.gif

How about replacing the 5.6V Zener with LM113 1.2 Volt temperature compensated low voltage reference diode ?

Beside the adjustment at the amplication factor at R11 and R12, are there any other component need to be adjusted ?

Any consideration/comment about the negative supply rail ?

[ QUOTE ]

*MrAl said:*Lastly, the filter cap value seems adequate if you keep the dc buss up about 7 volts above the output voltage.Any lower then that at full output current and some ripple will begin to show up on the output.

[/ QUOTE ]

What is the best value or formula ?

[ QUOTE ]
*MrAl said:*Simpler circuits without current adjustments:
The LM317 for example, one ic does the voltage regulation,then if you need current limit you can use another device
to set the current limit.


[/ QUOTE ]

Yes, I'm aware of it, infact built one using two LM317 in series, one for current control and voltage control at the end.

But it has limitations such as lack of CC indicator and negative supply rail collapse protection mechanism.
For led circuits experiment, sometime 1.25 Volt is too high ! /ubbthreads/images/graemlins/smile.gif

By the way, other purpose for building these circuit or should I say this thread is for fun and improving my "basic" knowledge, thanks to you MrAl. 

Just curious, if you watch closely at the PCB layout, the Zener diode D8 is placed close to R1.
What is the purpose of R1 ? Doesn't that generate some heat (energy wasted) ? 
or is this some old trick to keep the Zener temp stable ? just my wild guess tho.

Any comment on this circuit perfomance on the load transient response and ripple rejection ? Honestly, this is far beyond my skill level. /ubbthreads/images/graemlins/icon23.gif

I will update and inform the progress when I build it.

Best regards,
Vic


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## MrAl (Dec 30, 2003)

Hello again Vic,

The LM113 is a good choice, but probably better are the cheaper ones.
On the National site they have much cheaper shunt ref diodes and some are
'adjustable' so you can keep the other parts the same if you 
adjust to 5.6v .

What did you have in mind about the negative supply rail?
The only thing that might matter is the maximum plus
supply because of the supply range of the ic chips, which
is partly determined by the level of the negative supply
voltage level.

With the filter cap, the more cap the better  
Starting with the value they give, when you double the cap
value you half the ripple. This not only means less ripple
reaches the output, but also the voltage doesnt have to
be as high to avoid the ripple 'dips'. A ripple dip that
goes too low will appear in the output as a VERY large 
ripple component which is highly undesireable. Keeping
the input voltage ac dips higher then the output voltage
plus about 4 volts prevents this (sometimes called 'Overhead').
The peak is 1.41*Vac and the ripple is about i*.006/C
and the diodes drop about 3v,
so for 15Vac and 3300uf this gives us a dip to about 12.5v,
but with 6600uf the dip is to about 15.4v.
This means with 3300uf we could get up to about 8.5v out,
but with 6600uf we could get up to about 11.4v
(for 15vac input).

It looks like the purpose of R1 is to make sure the caps run
down when the supply is shut off. I doubt it's to help 
the regulation of zener because a variation in input
supply voltage would cause a temperature variation in
the zener which in turn would vary the output with temperature.
In fact, it may be a bad placement on the PCB after all.
It's interesting that you thought of what you did because
a higher absolute temperature might make the delta T cause
a lower delta V in the zener, but if it's not a stable 
reference temperature it will cause more delta V overall.
Not only that, it would have to be enclosed in a mini oven
to make sure air currents didnt change the thermal coupling
between the resistor and diode. Might actually make a nice
air speed indicator though 

This should give you some idea of what the load transient
response and ripple rejection is like:

Output voltage no load to 2A load transient response time: dip 5%, 20us
Output voltage short response time: 200us
Output voltage short to open: 100us, no overshoot
Output voltage 2A load to no load: +/- 10%, 500us
Ripple output 0.04% of input (the published claim is 0.01%)

BTW, a quick way to improve skill level with this stuff is to
look into downloading a circuit simulator, where you can quickly
experiment with various circuits, in case you're interested.


Take care,
Al


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## billw (Jan 4, 2004)

*Re: PS Circuit with Adjustable Constant Current Mo*

i don't suppose anyone happens to know of a schematic for
a CCCV power supply based on the 317 that uses 317's for
ALL the active components? i can find examples in the
app notes that use a second 317 for the negative supply
reference to get down to 0V 0mA, and parallel 317s for
more current, and pre-regulators and switching regulators...
But nothing puts them all together. A 0-30V, 0-2A, CCCV
with switching pre-regulator using only 317s, a switching
transistor, and passive components would be cool...

BillW


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## vicbin (Jan 5, 2004)

Hello MrAl,

Many thanks for mentoring this humble newbie.

[ QUOTE ]
*MrAl said:*
Output voltage no load to 2A load transient response time: dip 5%, 20us
Output voltage short response time: 200us
Output voltage short to open: 100us, no overshoot
Output voltage 2A load to no load: +/- 10%, 500us
Ripple output 0.04% of input (the published claim is 0.01%)

BTW, a quick way to improve skill level with this stuff is to
look into downloading a circuit simulator, where you can quickly
experiment with various circuits, in case you're interested.


[/ QUOTE ]

Please, I'm interested, especially on how you came out with those number. please suggest one.

About the dissipation problem, found this simple switching preregulator and it uses ordinary discrete component rather than custom switcher IC :

Switching Preregulator (PDF file) 


As always, your comment please ! /ubbthreads/images/graemlins/smile.gif


Hi BillW,

The above URL is a switching preregulator and it use standard LM317 as the final regulator with basic configuration, no CC though.


Regards,
Vic


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## MrAl (Jan 6, 2004)

Hello again,


BillW:
The two 317's in parallel sound interesting. I'll have to
look into those types of circuits and see what comes up.
Two in parallel would give 2A which isnt bad, as long
as it doesnt take too many extra components.


Vic:
To get say the no load to 2A load spec:

A switch is connected to the output which is set to turn on
after a small delay. When the switch turns on, a 5 ohm
resistor is connected to the output causing 2A to flow
with the output set to 10 volts.
The power supply thus turns on and comes up to normal
operating current with no load, then after the delay
the switch turns on and connects the load. This action
causes the output to dip slightly and remain that way
for a small time period before the voltage comes back up
to 10 volts. This dip in voltage and the time period
is measured and noted.
The other specs are found in about the same way except
the switch opens in some cases (instead of turning on)
and the load is changed to 0.1 ohms to test the shorted
conditions.

To test the ripple spec, a small 120Hz ac ripple voltage
source is placed in series with the dc input source and
set for some ac voltage Vi. A 10v battery is placed in series
with the output to subtract the dc output voltage level 
(such as 10 volts) so that only the ac component is
measured at the negative terminal of this battery.
The ac output voltage Vo is measured and compared as a
ratio to the input ac voltage. The ratio is then:
Vo/Vi
and is noted.
BTW, i made the mistake of dividing the peak to peak output
by the peak input instead of using the same measurement
for both input and output Vac, so a better approximation to
the output ripple voltage percent is 0.015, which is
pretty close to the published value.


One more thing:
It was noted that the response to a short (or some other
overload) is partly dependent on the value of R7. 
Reducing that value to 0.22 means the response to an
overload transient is increased approximately two times.
If C8=100pf and C4=300pf then this condition is
corrected to some extent, but unless you have
a scope to check this out in the real life circuit
im not sure if it's a good idea to reduce R7
(and change C8 and C4) to lower then 0.22 ohms.
If you are worried about being able to measure
current as simply as you can when using a 0.1 ohm
resistor then you can connect a pot (or two resistors
in series) across R7 (100 ohms) and connect the meter
to the arm of this pot. It can then be calibrated
by measuring the current with another meter and adjusting
the pot until both readings are the same.
The pot will function as an approximate divide
by 2.2 network which will provide 0.1 volt output
for each amp of current (same as using a 0.1 ohm
resistor) and is easily readable on a standard
low cost digital meter.

I like the idea of using a switching regulator on the input
but didnt think you would want to go to that more complex
circuit. If you dont mind then it's a good idea for sure.


Take care for now,
Al


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## vicbin (Jan 24, 2004)

Hi Mr.Al,

Just want to update you this circuit works and did test with 12V , 3 A load for 6 hours without problem.

The stability is a great since I replace the voltage reference part (the opamp and zener) with TL431 adjustable reference as you suggested, and its cheap too. (about 40 cents ?)

The CC mechanism works ok and stable for hours with different room temperature on high load about 2 Amps and also in low load couple mili amp test. I didn't notice any shift in my DMM in ma resolution.

Too bad don't have scope to see the ripple and transient response. /ubbthreads/images/graemlins/frown.gif

Now what I'm thinking now is how to hook it up with a simple switching pregulator such as LM2576-ADJ or LM2678-ADJ (National simple switcher families) to reduce the dissipations.

Actually this suggestion came from a friend that asking how to use the simple switcher IC as preregulator for common LM317 while maintain it's adjustment flexibilities ? e.g. maintain a constant 3 to 4 volt drop across the post linear regulator ? Mind help us on this ?

Regards,
Vic


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## MrAl (Jan 24, 2004)

Hi again Vic,

Wow it sounds like it's working pretty well.

You probably dont have to worry too much about the 
ripple, it must be pretty low if the numbers came
out that good.
As for the transient response, yes that would be good
to see on a scope. I'd like to see it too and compare
to the simulations. Chances are it's worse then the
simulations, but at least the recovery doesnt overshoot
so that is probably good enough.

I'll take a look at the LM parts you suggested hopefully
a little later today, or perhaps tomorrow morning.
Yes, maintaining perhaps a 4 volt drop or so across the
linear series pass elements is the idea.


Take care for now,
Al


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## MrAl (Jan 25, 2004)

Hi here Vic,


It looks like a general method might work for most linear regulators
driven with a switching regulator.

The main goal is to keep the output of the switching regulator about
4 volts above the output of the linear regulator. What looks like
a simple way to achieve this is as follows.

The switching regulator output circuit is set up to reach up to 37 volts
(typical of these kind), while the input circuit is set up to 
regulate at 4 volts (ADJ type regulator ic). The selection of
the feedback resistors makes the input look like a 4 volt regulator.
This puts a voltage divider (two resistors) at the input of the
ic chip fb pin as shown on the data sheets. Now instead of running
the upper feedback resistor to the output terminal of the switcher
(as shown on all the data sheets) the feedback resistor will run
to the output of a difference amplifier with a gain of 1. The
two inputs from this difference amplifier come from the two outputs:
one from the switcher and one from the linear power supply.
The non-inverting input connects to the output of the switcher
while the inverting input connects to the output of the linear.

Required parts:

Switcher:
ic switcher and associated parts that go with the ADJ type circuits.

Difference amplifier:
Op amp such as LM358 or similar, plus four 50k resistors 


Once the circuit is built up you can check across the output transistors of
the linear supply to see that they dont drop more then 4 volts. If they
do you will have to increase the upper feedback resistor of the switcher
in order to increase the difference voltage.

Stability will have to be checked also to make sure the two supplies
dont oscillate when connected.

I'll try to look at this a little more tomorrow sometime, and see if
i can get a pseudo circuit set up in the simulator program to check
stability.

So far this looks like it's going to be a really
nice power supply!

Take care for now,
Al


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## MrAl (Jan 26, 2004)

UPDATE 1/26/2004

Hello again,

It looks like a 100pf to 1000pf cap across the difference
amplifier helps the stability of the switcher. The cap
connects as follows...
one lead directly to the inverting terminal of the op amp
and the other lead directly to the output of the op amp.
This turns the difference amp into an integrating difference
amp.
The larger the value of this cap the better the stability,
but the slower the transient response for the condition of
switching no load to full load.

Also, it might be possible to turn the 4 volts down
to 3 volts and still maintain regulation at full load.
This is easy to achieve by simply lowering the value
of the upper feedback resistor of the switcher. This
is something that could easily be done after the circuit
is built up.


Take care,
Al


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## RussH (Jan 27, 2004)

Hey Al, thanks for the info, just to let you know Vic is not the only one interested in this....... -RussH


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## MrAl (Jan 28, 2004)

Hello there RussH,

Oh that's great. Maybe you can post some info here too
if you decide to build this thing up. I'm almost 
tempted to build one myself and might do that.


UPDATE #2

In addition to the previous update (one or two posts ago)
i have another update. This new result was verified with
a circuit simulator program as usual.

Update #2 is basically the idea of replacing the difference 
amplifier with a single PNP transistor and a few other
passive parts (zener plus two resistors and small cap).
The zener voltage value sets the difference that will
appear across the linear power supply's output stage
transistors. 

Basic connections...
The zener connects to the emitter, 10k to base, 1k to 
collector. Free end of zener to switcher output,
free end of 10k to linear ps output, free end of 1k to
ground. Collector runs directly to the switcher ic
'fb' pin with no voltage divider needed. 100pf cap
across base to ground. Extra 1N4003 diode connects
across base emitter junction in reverse to transistors
internal diode.

The simple idea here is when the difference across the
output stage of the linear supply reaches approx the
zener voltage level it conducts, causing the transistor
emitter collector to conduct more heavily causing a voltage
to appear across the 1k resistor. Since this voltage also
appears at the fb terminal of the switcher ic when it
reaches the internal voltage reference value (1.2 volts)
it causes the switcher ic to begin to cut back its output
voltage so that the difference stays at about the same
level as the zener voltage value.

Since it's a little harder to change the difference value
(because the zener would have to be changed) it may be
easier to substitute a series string of 1N4003 diodes
in place of the zener. Start with 4 diodes in series.
If more voltage is required simply add another diode in 
series, if less then remove one. The voltage difference
will change by about 1/2 volt or so with each diode.


Take care,
Al


----------



## vicbin (Jan 29, 2004)

Hi again MrAl,

I'm back, thank you for the circuit realisation and the idea /ubbthreads/images/graemlins/smile.gif

Honestly, I'm getting very excited about this switching preregulator setup and quite surprised if you've almost tempted to build it since I thought this kinda circuit is for beginner like me, not a sensei level like you. /ubbthreads/images/graemlins/ooo.gif

Actually tried using two LM317, one as pregulator and one as the final linear using 1/2 LM358 as the difference amplifier without success at the breadboard level.

Also created a simulation test with Linear Switcher CAD and using the LT1083 (LM317 better clone) to simulate the switcher and the post linear regulator. Yes, I also use the same device as the imaginary switcher (aka LM2576) since use the same feedback voltage mechanism 1.25 V (1.23 to be exact), is this setup ok just for testing ?

The problem is I can not make it work with both models as you suggested, hope you have time to take a look at this simulation to correct it if I made a mistake. /ubbthreads/images/graemlins/bowdown.gif

PS: Sorry, my ASCII art for schematics is a bit rusty for this kind of circuit.

UUEncoded FILE: "Linear Preregulator Test w OpAmp.uue"

[ QUOTE ]

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MK`AIB`4?/K;[L8U#]02P$"%``4````"`!"<3TP-QB[S)P"``!&"0``)```
M```````!`"``MH$`````3&EN96%R(%!R97)E9W5L871O<B!497-T('<@3W!!
<;7`N87-C4$L%[email protected]`````!``$`[email protected]```-X"````````
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end


[/ QUOTE ] 

*NOTE FOR OTHER READER : This is an .UUE encoded file, to use it copy and paste the above quoted text to Windows Notepad program and save it with .UUE extension. To open it use the popular WinZIP to extract it into final .ASC SwitcherCAD file. 
BTW, you can download and "use" the nice Switcher CAD from www.linear.com site for free. Yes, its a freeware.*


Another second model using the discrete PNP transistor version , really sorry since I'm a bit confused where to put the IN4003 diodes at ?

UUEncoded FILE : "Linear Preregulator Test w Discrete.uue"
[ QUOTE ]


_=_ 
_=_ Part 001 of 001 of file Linear Preregulator Test w Discrete.zip
_=_ 

begin 666 Linear Preregulator Test w Discrete.zip
M4$L#!!0````(`%1Q/3#_S.D"B`(``-D'```G````3&EN96%R(%!R97)E9W5L
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end



[/ QUOTE ]

Hi RussH,

Great for you to drop in this thread, it is encouraging. After several posts, was thinking no one ever would interested with this kind of thread. The idea from the 1st time I started this thread is to share with other users that I know doesn't even have a decent PS to be a LEDaholic. Thanks. /ubbthreads/images/graemlins/grin.gif 

Regards,
Vic


----------



## MrAl (Jan 29, 2004)

Hello there Vic,

Could you post the schematics somewhere?

If you need to make a picture of them you can use
the 'Print Scrn' key.

You can email them to me if you like that idea better.
I'll be happy to take a look at them and see
what's going on.

One of the problems with the uue file is that XP doesnt
know what to do with it. Any ideas?

Take care,
Al


----------



## vicbin (Jan 29, 2004)

Hello MrAl,

4 files sent thru email, 2 are the CAD files and 2 are the capture JPG files.

Please inform me if you have trouble with it.

Regarding the UUE file, do you have WinZip installed ? If not , the XP default can not deal with it. Though many other program will also can decode an .UUE file but I think WinZIP is easier and most popular.

Regards,
Vic


----------



## MrAl (Jan 29, 2004)

Hello again Vic,

I downloaded your schematics and 
i see you got the details of the circuits perfectly.
You must have lots of experiance with these kinds of
circuits right?

I wasnt exactly clear on one point earlier...

When i said 'ADJ type regulator' i should have said
"Adjustable type regulator with a feedback (fb) input"
as the type most of the switcher ic's have.

The type LM117 and similar with an ADJ input dont work
quite the same, so the concept of using the difference
as an error signal has to be tested a little differently
if using another LM117 as the pre-regulator. It turns 
out this is no problem at all.

For one thing, the LM117 has a floating reference so the
easy way to acheive this is to disconnect the lower (R2)
resistor from ground and simply connect it directly to
the output of the 2nd linear regulator. This works, but
of course this doesnt really test the concept of using the
difference circuit to control the output so a few changes
have to be made in order to see how this really works
with an actual switching pre-regulator (using the first
linear to simulate it for now).

The changes to the circuit with the transistor and zener are
as follows:

The lower LM117 resistor (R2 on data sheets) changes to 5k.
The upper LM117 resistor (R1 on data sheets) changes to 240 ohms.
Take an NPN transistor and connect a 50 ohm resistor to
its collector. Ground the emitter. Disconnect the 1k
resistor from ground (going to the existing PNP transistor collector)
and instead connect it to the base of the NPN transistor.
This 1k now connects PNP collector to NPN base.
Thus when the PNP conducts the collector drives the NPN 
base through the 1k. Next connect the loose end of the 50 ohm resistor
to the ADJ pin on the pre LM117. Change the 100pf cap to 1000pf.
Add a second cap from ADJ pin of pre LM117 to ground of value 0.001uf.
Note there are no changes made to the second LM117 part
of the circuit.

The above additions are mainly for testing the concept using
the transistor and zener type difference amp. If it becomes
desireable to test the op amp style difference amp concept then
it would be easier to use a different type of regulator to
simulate the switcher rather then the LM117 type.
To create this new pre-regulator all that would be required
is to drive an NPN transistor base with the output of an LM358 through
a 1k resistor. The non-inverting terminal gets a battery of 1.2 volts
connected to it. The load connects
emitter to ground, which would be the second linear regulator.
The inverting input of the op amp gets the voltage divider
and the top resistor goes to the output of the difference 
amp (op amp style). The difference amp connects as before.


Take care for now,
Al


----------



## vicbin (Jan 31, 2004)

Hi Mr.Al,

Point taken, there is no need to test using LM117 anymore since it wont reflect the real situation.

Since the test on power switching circuit can't be easily done on breadboard, I think I'll go testing it on real PCB.

Please verify the downloaded schematic you receive both version (op-amp and transistor) and if there is still any correction needed.

Btw, what simulation software do you use ? Just curious. /ubbthreads/images/graemlins/smile.gif

Again, thank for the LM117 explanation, really learn something here .

Regards,
Vic


----------



## MrAl (Jan 31, 2004)

Hello again Vic,

You had turned the notes into a circuit schematic very
well so there is little to add here 

You probably figured this out already too, but i'll
note this here for any other readers that might be
interested too...

For reference (op amp diff amp):
In your schematic with the op amp, R5 is the upper feedback resistor,
R6 is the lower feedback resistor (switcher). On the data sheets
these are labeled as R1 and R2 usually. U4 on your schematic is the
op amp itself.

For a 4 volt difference using op amp:

R5=2.7k (switcher upper feedback resistor)
R6=1.2k (switcher lower feedback resistor)
100pf cap might be necessary:
inverting input of U4 to output of U4 (directly to pins)


The transistor circuit needs no changes so far.


I use Switchcad but it's been a while. I use another
one called "Micro Cap" that seems a little easier
to draw parts in, but it's limited to a certain 
number of nodes and you can't go beyond that point.
For example, you can't analyze two LM117's at once
because together their internal circuits use up 
more nodes then it allows you to use. If i want
to do this i have to use a simpler model of the
LM117. I dont think Switchcad has this problem.


Take care for now,
Al


----------



## vicbin (Feb 1, 2004)

Hi MrAl,

Well, looks like I'll start making a simple layout for this switcher the see it's perfomance as a preregulator.

It will take a while, will update you the result.

Thank,
Vic


----------



## MrAl (Feb 3, 2004)

Hello again Vic,

Thanks, im looking forward to hearing more about it.

UPDATE

Im not sure how interested you are in this but
a quick look at the dc characteristics of the 
switcher pre-regulator with linear output
provided the following equation for the output
voltage:

Vo=(E1-4-1/A)/2+sqrt(E1^2+8*E1+16-2*E1/A+8/A+1/A^2)/2

where
Vo is the output voltage
E1 is the reference voltage
A is the gain of an op amp open loop
The difference voltage is set at 4 volts constant

A is taken as a large number, typically 1000 to 100000.

For A=1000 and choosing a 10 volt output, E1=10 also.
Vo therefore comes out to:
9.9992857 volts.

Letting A approach infinity, Vo becomes:

Vo=E1/2-2+sqrt(E1^2/4+2*E1+4)

Amazingly, this equation is basically an identity if E1>0 (always 
the case for a positive regulator), so skipping the details we can write:
Vo=E1

This result verifies the concept at least.

The assumptions are:
1. No delays or integrations in the feedback paths
2. Perfect regulation response from the feedback inputs
etc., etc.



The only thing left is the ac response, which looks good
when the two feedback paths are identical and perhaps
good when the linear feedback path has a slower integration
relative to the switcher feedback path. This says that
perhaps the 100pf cap shouldnt be included, or at least
test it first without the cap then add if necessary.
In fact, the transistor circuit may be the more desirable
way to go with the difference amplifier because it will
be so fast.
I'll try to take a more detailed look at these things 
hopefully within maybe the next few days or so.


Take care,
Al


----------



## vicbin (Feb 3, 2004)

Hi Mr.Al,

This is great, if it is faster to accomodate the changes at the linear final, then the transistor version is the way to go. 

Btw, it is simpler to me. /ubbthreads/images/graemlins/smile.gif

Thanks,
Vic


----------



## vicbin (Sep 11, 2004)

Hi Mr.Al,

Really sorry for not updating you and I hope you still remember what it is all about. I was "offline" at the field work for few months and the PS building was continued by my friend after I supply him the etched the PCB + all components and it is as a gift for him to play around ! /ubbthreads/images/graemlins/smile.gif

Now it is my obligation to report that *IT IS WORKING AS PLAN * /ubbthreads/images/graemlins/clap.gif /ubbthreads/images/graemlins/happy14.gif

Measured voltage drop across the power transistor is consistent with the range 4 to 5 volts on various output voltage starting from 0.5 - 24 volts and also with various load from 10mA to 3A. /ubbthreads/images/graemlins/cool.gif

Although the plan to reduce dissipation at linear part is proven, we encountered some problems which was not exist if we bypassed the switching preregulator.

The problems are :

- At low load on few mA to 100 mA (CC mode on), the voltage is not stable (measured at DMM) and we suspected the ripple was too high (sorry, no scope here). One of my friend's suggestion was to fix the problem by putting a constant current drain circuit approx. at 200mA (the popular 2 transistors circuit) between the switching output and before linear part, and the problem was gone ! Is this normal ? 
This solution created another problem, cause when we crank up the voltage, it will dissipated way to much heat ! /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/mecry.gif

- Locally it is difficult to find a low ESR cap, so for COut at switching part, we use 5 x 1000uF/100V in parallel, is this enough ?

- The voltage reference part, we still use zener instead of dedicated reference diode because of some reason at PCB layout, and it was proven not stable. We used hair dryer to heat the components and the voltage was drifting ! Time to modify the PCB artwork to use real low ppm and accurate reference diode. Any better idea ? /ubbthreads/images/graemlins/mpr.gif

Regards,
Vic


----------



## MrAl (Sep 12, 2004)

Hi Vic,

Could it be possible you've got TOO MUCH cap on the output
of the switcher? Did you try a lower value such as
100uf or even 1000uf?
Also, did you try to load the switcher alone, as that
might help if some min load is required on the 
switcher?

If the voltage reference diode is unstable it must be 
something simple bothering it, such as bias current
too low or too high, either of which could cause the
output to vary quite a bit and not behave like a 'perfect'
zener would.

Let me know if any of this helps...

Take care,
Al


----------



## vicbin (Sep 23, 2004)

Hi Al,

5000uF is too much ? In the LM2576 datasheet, it suggested for C-Out is 2000uF and I thought the bigger the better so I put on 5 X 1000uF, since I can not get low ESR cap. Is that "way" too much ? Again, I thought that supposed to improve the ripple ?

By the way, in order to run properly, is this kind of simple buck switching IC need a minimal load ?

Yes, we did try the switcher capability (infact we separate the switcher and linear by different module/pcb) and it can survived with a full 4 Amps load at Vin=35V and Vout=15V for hours and with minimal heatsink.

About the reference, is it possible to replace the whole reference parts (zener+opamp) with this popular/generic KA/TL-431 Adjustable Ref. (PDF datasheet) ?

I'm fully aware that it needs a low ESR cap for switcher, how low the ESR for an "ordinary or el cheapo" electrolyte cap ? 

Thanks & regards,
Vic


----------



## vicbin (Sep 25, 2004)

Hi again Al,

The modified circuit already sent, please check your email.

Vic


----------



## MrAl (Sep 25, 2004)

Hello again,

Vic:
Sometimes a switcher will run more stable with less cap on the
output. Yes, more caps will reduce ripple, but stability is
first. A cap with too low an esr could cause instability also.
The easiest way is to try different caps and see what happens.
I'd try as low as 100uf and as high as 5,000uf as you have already.

Some switchers need a minimal load, yes. Im not sure if this
one does or not...i'll look at your new schematic as soon as
i can (should be a little later today).

You tried the switcher ALONE with no load and tested the
output voltage?

The TL-431 should be able to be used because it's a ref diode too.
Shouldnt be any problem if you use it according to data sheet.

Electrolytic cap should be ok according to the data sheet.
The esr is higher but that just means a little more ripple
usually. Also, parallel caps result in lower esr too.
Higher voltage electros are supposedly better with lower esr.

I'll check my email in a few minutes...thanks for sending it.
I'll post it and comment a little later today.


Take care,
Al


----------



## MrAl (Sep 25, 2004)

Hi again,

Here's the circuit so far:






Vic:
Looks like i need the part values again (sorry).
I'll update the schematic as changes occur.

I've noted one interesting thing new:
If you put a resistor or two diodes in series with
the linear main output transistors collector that
transistor only has to handle maybe 12 watts at full
4 amp output rather than 20 watts. The power lost doesnt
change, but the transistor heat sink can be almost half
the original size.
Also, along the same lines, it may be possible to change
the drive circuit slightly to conserve power in the last
output stage (same transistor). If we can reduce the 
voltage drop across the transistor to 3 volts that makes
a big difference at 4 amps out!
Also, regarding stability at no load...
I've notice that the two regulators might fight each other
a little at no load, which might explain the need for
a small min load. I think the linear pass section has to
be kept in the linear mode with light loads so there may
always be the need for some small load.

The cc load being used now might be improved if the
LED's are replaced by a resistor maybe 30 ohms, and the
22k resistor changed to 5k. The second transistor has to
be able to handle some 3 watts however. This change will
allow the cc load to work down to lower voltage and so
stability down to 1 volt should be achieved. The circuit
may not work down below 0.6 volt or so, well, it will
work just wont go down lower than that perhaps, at least for
now 

Take care for now,
Al


----------



## vicbin (Sep 25, 2004)

Hi Al,

Thanks for the comments and the edited pic's and also Pablo for hosting it. I'll make this as fast as possible, your request is my command ! /ubbthreads/images/graemlins/grin.gif

[ QUOTE ]

I've noted one interesting thing new:
If you put a resistor or two diodes in series with the linear main output transistors collector that transistor only has to handle maybe 12 watts at full 4 amp output rather than 20 watts. The power lost doesnt change, but the transistor heat sink can be almost half the original size.
Also, along the same lines, it may be possible to change the drive circuit slightly to conserve power in the last output stage (same transistor). 


[/ QUOTE ]
Great news ! /ubbthreads/images/graemlins/smile.gif No more cooling fans means ..... *less moving parts* means ..... longer life and LOW MAINTENANCE ! (except for drying cap of course) and I really love this idea !! Anyway, I always hate to clean up the dirty clogged cooling fans. /ubbthreads/images/graemlins/mpr.gif

Do you mean connecting 2 diodes is 2 high Amps diodes from the Collector to Emitter of the Q4 transistor ? And how high is high ? Mind explain on how it work ?

[ QUOTE ]

If we can reduce the voltage drop across the transistor to 3 volts that makes a big difference at 4 amps out!


[/ QUOTE ]
Yes, I'm aware of this issue, again, too bad I don't have scope to monitor how this will this affect the load transient response ? Especially with this kind of discrete built design on load jumps in several Amps ? That's why I'm planning to replace the zener in the reference part (D8) with an adjustable zener (xx-431) that can start as low as 2.5 volts. And in my place this TL-431 cost no more than LM317 ! /ubbthreads/images/graemlins/smile.gif

Is it possible to replace the ref. part (U1 opamp,R4,R5,R6 and D8) with *ONLY* TL-431 with bias resistor and pot ?

[ QUOTE ]

Also, regarding stability at no load...
I've notice that the two regulators might fight each other a little at no load, which might explain the need for a small min load. I think the linear pass section has to be kept in the linear mode with light loads so there may always be the need for some small load.

[/ QUOTE ]

Ok, nothing to complain and I love it, infact that dummy load serves as a great & glorious pilot light ! I believe most of CPFer or LEDalholic like me did their LED testing or tweaking in low ambience light to see the LED/flashlight result right ? ...hint...hint...hint... /ubbthreads/images/graemlins/grin.gif

[ QUOTE ]

The cc load being used now might be improved if the LED's are replaced by a resistor maybe 30 ohms, and the 22k resistor changed to 5k. The second transistor has to be able to handle some 3 watts however. This change will allow the cc load to work down to lower voltage and so stability down to 1 volt should be achieved. The circuit may not work down below 0.6 volt or so, well, it will work just wont go down lower than that perhaps, at least for now 


[/ QUOTE ]

Al, I think you misread the value, it is R22, not 22K. But once I send you the complete circuit *with value* on every component will clear this out.

If you're suggesting to remove the CC LED, then how about the CC indicator ? I think it is important.

Since I'm planning to start the second one with alot of modifications, honestly, it makes me think whether this backend linear circuit can be better in term of transient response, reliable CC mechanism and ripple rejection from the switcher rather than using ordinary linear regulator LM317 or LM350 such as Pablo *but* with some modifications/add-on for a stable CC mechanism down to low mA resolution ? 

My objective is to create a DIY adjustable (volt & current) PS with CC that is suitable for CPFer, I think most of the time CPFer are toying with low voltage circuit, a reliable CC mechanism to avoid blowing expensive parts is important e.g.:simulating cell/battery driven circuits. And one of the most important thing is the lesson learned & gained while building it. What do you think Al ? 

Thanks & regards,
Vic


----------



## MrAl (Sep 26, 2004)

Hi again Vic,

Note exactly... break the collector connection only, then insert two
diodes or one 0.5 ohm resistor. I'll draw it up. Yeah, unfortunately
the diodes would have to be rated 4 amps or better. The resistor maybe
15 watts -- i'll double check the rating.
The main idea behind this is that the smaller drive transistor always
has some voltage across it meaning the total across the large transistor
is higher than need be. Inserting a voltage drop in the collector
of the large transistor means the smaller transistor gets its voltage
while the large transistor gets a decrease in voltage drop. This means
some of the power moves from the transistor to the diodes, which means
it's distributed better. If the diodes can take it, they keep the 
transistor cooler and so a smaller heatsink. The difference was almost
half that's why i mentioned this. The heatsink will still have to take
care of maybe 12 watts, but that's better than around 20 watts.
Care has to be taken that the smaller transistor doesnt have to supply
too much more base drive, which would overheat it. This means too much
voltage drop and the little transistor might overheat...that's something
to watch out for. I simulated with a 0.5 ohm resistor and it looked fine
however. I'll next try the two (or one) diodes.
Im currently using a heatsink with my LM317 ps (like Pablos original)
and i've tested it up to 8 watts and it does get very warm but the
chip doesnt shut down. The heatsink is 3x3x1.5 inches, which isnt that
big really.
As long as the transistor is always in linear mode the transient response
should still be ok, or at least almost as good as before.

What about that 22uH and 220uf filter though? Where did that come from?
That introduces a resonant point that could be a problem with stability.
Try not using that (short out inductor and disconnect one lead of cap
to see the difference).
That filter could cause some problems with transient response too.
Did you determine you needed that somehow?

About the TL-431...
I'll look into replacing the ref section with that one device and post
a schematic.

Oh ok, if i made a mistake with the R22 then i'll wait until you tell me
the correct values of all the parts.

If you need an indicator that should be very simple to add...no problem.

If you're interested in replacing the linear section with an LM350 that's
cool too. Im planning on using a linear LM317 (lower current version)
with the switcher we end up with here. With the LM317 and the switcher
i'll cut down on the heat at the LM317 to maybe as little as 3 watts
at 1 amp out. Very small heat sink!

I think this will make a very good DIY project for CPF'er's as you 
noted. I think even simpler would be to use the LM350 which is almost
a power supply in itself. How do you feel about the LM350?

Another possibility to try to reduce power in large transistor...
If you experiment with diodes in series you might be able to 
replace the 4.7v zener with maybe four or five 1N4001 diodes to
reduce the voltage across the output stage. To find out,
connect five or even six diodes in series and then bias with
1.2 ma and check the voltage across all the diodes. If it's around
4 volts you can try it as a substitution for the zener. You can
then try shorting out one diode and try it again (for lower voltage).
The critical point where the diodes voltage drop matters most is
when the circuit is outputing the full load 4 amps. If the diodes
voltage is too low, the circuit wont be able to put out as much
current as before. If the voltage is too high, the output stage
puts out too much heat.

Take care,
Al


----------



## PEU (Sep 26, 2004)

I'm listening, but the technical details are above my knowledge /ubbthreads/images/graemlins/smile.gif I understand the logic of the comments, but not the why's /ubbthreads/images/graemlins/smile.gif

Anyway, just waiting for the gurues to agree on the final design!


Pablo


----------



## vicbin (Sep 26, 2004)

Hi again,

Al,

Email sent, the whole circuit includes the switching preregulator in Microcap format. Phhewww.. /ubbthreads/images/graemlins/mpr.gif it tooks me hours to draw all of them and since this is my second day on Microcap, please bear with me if there are errors & mistakes ! /ubbthreads/images/graemlins/crazy.gif

Pablo,

Hey I am no guru, just another hobbyist, infact I know nothing rather than reading circuit schematic and translating it to PCB & solder them, no more than that ! /ubbthreads/images/graemlins/blush.gif
The real guru is our sensei Mr.Al and through this thread *alone*, I believe I've learned lot of kungfu from him. /ubbthreads/images/graemlins/grin.gif

Vic


----------



## vicbin (Sep 26, 2004)

Hi Al,

[ QUOTE ]

What about that 22uH and 220uf filter though? Where did hat come from ? That introduces a resonant point that could be a problem with stability. Try not using that (short out inductor and disconnect one lead of cap to see the difference). That filter could cause some problems with transient response too. Did you determine you needed that somehow? 


[/ QUOTE ]

Yeah, that is my st*pidity, /ubbthreads/images/graemlins/banghead.gif I followed the instruction like a silly sheep in the LM2576 datasheet on reducing ripple down to ten times lower.

Again, I didn't have any idea it will affect the overall circuit like you've explained ! Great learning here, many-many thanks. /ubbthreads/images/graemlins/smile.gif I'll inform my friend to do what you've suggested and wait for the result !


[ QUOTE ]

If you're interested in replacing the linear section with an LM350 that's cool too. 

How do you feel about the LM350? 

[/ QUOTE ]

How do I feel ? Of course, infact I got two LM338 with TO3 casing in my drawer (LM338 the eldest brother of LM317 or LM350 families) and it can serves 5 Amps load ! /ubbthreads/images/graemlins/grin.gif

But, please if you don't mind, create a design with a *stable & reliable CC loop* down to mA resolution, with the target audience for CPFer which most of the time toying with LEDs or low voltage circuits. /ubbthreads/images/graemlins/bowdown.gif

I hope, I'm not asking too much on this and again I don't mind adding external components to make it happened. 


Thanks & regards,
Vic


----------



## MrAl (Sep 26, 2004)

Hi again Vic and Pablo,

As we work this circuit out we might find more and more 
improvements and make it simpler too perhaps.

Vic, if you're interested in a simpler circuit i'll look
into the possibility of simply driving the LM338 with
the switcher. That should make a pretty simple circuit
i think. It wont be too hard to add precision current
limiting either, and that's a very good idea! I'd like
that on mine too.
Since you already have some of these very good parts around
maybe it would be a good idea to go with that and skip
the 'many parts' linear circuit we used last time?
Pablo has an LM338 already too, and i have LM317 for
lower current version, so we're all almost there already /ubbthreads/images/graemlins/smile.gif

I'll check my email in a few minutes, thanks for sending
that.

Oh i didnt mean you were stupid or anything...in fact,
it's a good idea to try that filter i think, but you have
to check with and without to make sure it's not going
to mess up the regulation loop...that's all. I'd be
trying it too and scope it out to see if it really
improves the circuit or not.

BTW, where did you get your inductors? I could look
them up and see what info there is available on them
too perhaps. For my own purposes, i'd probably order
from Digikey. Since so many people can order from them
it may be a good idea anyway, this way anyone can build 
one up.

Let me know if the simpler circuit appeals to you too 
(LM338 instead of the many-parts linear)...


Take care,
Al


----------



## PEU (Sep 26, 2004)

I vote for KISS , but please understand that the last S is not a personal attack /ubbthreads/images/graemlins/smile.gif /ubbthreads/images/graemlins/smile.gif /ubbthreads/images/graemlins/smile.gif

I mean I would like to use the LM317/350/338 solution

Pablo


----------



## vicbin (Sep 27, 2004)

Hi Al,

Thanks for everything. /ubbthreads/images/graemlins/bowdown.gif

I think you've been too polite and patience with me, with your support I say we stop & *drop* this long & exhausted thread and go with the simple 3 legged linear regulator.

With your kungfu, I believe you can design "a very CPRer friendly" bench power supply.


[ QUOTE ]

Pablo has an LM338 already too, and i have LM317 for lower current version, so we're all almost there already /ubbthreads/images/graemlins/smile.gif


[/ QUOTE ]

Almost ? No, I think we (or at least it's me) is ready once you've confirmed that you want to start thinking to design it ! /ubbthreads/images/graemlins/grin.gif


BTW, where did you get your inductors?

Inductors ? Yes, the word "inductor" has been haunting me in the past during switching circuits and finally I got one local shop that has almost all type of core, any wildest inductor that you can imagine starting from 1 mm dia. toroid to tyre size one (no joking). /ubbthreads/images/graemlins/grin.gif 

I'll send you the pics of my inductors and my other preparation. 

[ QUOTE ]

Let me know if the simpler circuit appeals to you too (LM338 instead of the many-parts linear)... 

[/ QUOTE ]

You know, I've been waiting this from you for a very long time.


Pablo,

Can't disagree more with your "KISS" statement, still I feel like to /ubbthreads/images/graemlins/whoopin.gif you. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/wink.gif

Regards,
Vic


----------



## MrAl (Sep 27, 2004)

Hi Vic,

Wow cores that big? Where is this place?
Can you find out the type of material they are
made out of (the ones you have already).
Sounds very interesting!

Ok, so i guess you'd like to go with the three terminal
back end regulator too, so i guess it's time for me to
start banging my head against the ol' makiwara again /ubbthreads/images/graemlins/smile.gif

Hope to have something together by later today or
tomorrow...I guess it should be in a new thread then...

Take care,
Al


----------



## vicbin (Sep 27, 2004)

Hi Al,

Ok..ok.., I was a bit exaggerated, but that giant toroid OD is about 50 cm (20"), ID and thickness about 15 cm (6") and what interesting is it has a small gap about 5 mm wide. Was curious and asked about that monster and the shop keeper said that "stuff" was used most of time by giant government contractors in power plant building !!! /ubbthreads/images/graemlins/ooo.gif I was totally chicken out in silent and stopped asking more about it ! /ubbthreads/images/graemlins/blush.gif

The one I used for the 1st preregulator is covered with yellow epoxy with the size OD = 25mm, ID = 17mm and thickness = 10mm. It worked well on the test for standalone switcher part only with 4 Amp loads and the switcher IC was barely warm. 

Regarding the material, apparently that company is a wholesaler or distributor and it looks like the shop keeper is only good at processing inventory list or processing invoices (tons of it) and they lost interest once they found out that I was planning to buy in pieces ! /ubbthreads/images/graemlins/yellowlaugh.gif

But I did get a very brief explanation from the shop administration clerk and it was suitable for several Amps switcher with something like *J Type* & high flux but no info about it's mu. Don't know what J type means. I believe the complete data must be somewhere in those piles of inventory papers. /ubbthreads/images/graemlins/grin.gif

In this shop I also found a lot of high mu material such as the one we discussed here -> Inductor Core Questions , but didn't use it since it got saturated at 1-2 Amps. (switcher IC was hot)

Will send you pics the bare and the completed one with it's value on my LCR meter.

Please don't bang too hard at your ol' makiwara /ubbthreads/images/graemlins/ohgeez.gif, and yes I think it is the time you start a very special new thread on it. Just take your time and no need to do it in hurry. Don't worry, I got many op-amps including precision one and reference ICs for disposal in this project ! /ubbthreads/images/graemlins/grin.gif

Thanks,
Vic


----------



## MrAl (Sep 27, 2004)

Hi again Vic,

Oh i see ok. That's pretty big still.
So you cant get any more cores then?

Well, J type is about u=3000 i think.
The only thing i havent figured out yet is how to
make a custom gap size in the core without breaking it
(like i told Viren a while back...come to think of it,
where did Viren go i wonder??? )

Yes, i'd like to see the pics and thanks.

I'll take my time with it yes, but i've already started
on it now 

Take care,
Al


----------



## PEU (Sep 28, 2004)

Hi Al & Vic

can a LM2678 be used instead of the LM2576 ? 

The 2678 is easier to find here, slightly more expensive, but findable


Pablo


----------



## vicbin (Sep 28, 2004)

Hi Pablo,
I also have that LM2678 too, and it has more powerfull mosfet switch with max load. 5 Amps compare to LM2576 3 Amps max load with bipolar switch and better efficiency too.

But I'm planning to try with LM2576 first since it has lower switching frequency 52Khz (LM2678 switch freq is 260 Khz), hence more relax on the PCB layout and the requirement for low ESR (high freq) caps. Am I correct Al ?

I got this info from this document -> AN-1149: Application Note 1149 Layout Guidelines for Switching Power Supplies (PDF) 

But again, I think you should try it first or maybe we should hear what Al say about this ?

Hi Al,

Since you said my toroid core u=3000, I'll try to wind another one to measure it's inductance to see if this value fit in. 

Don't worry, I still can get any core I want at that shop, but I doubt I can get any real data or spec about them. They're dirt cheap anyway, so I bought 5 each type that I "feel" suitable. /ubbthreads/images/graemlins/grin.gif

I know you've mentioned it on other thread about making a gap on the toroid core, this make me think to find my friend which has a jewellery/gem workshop, may they have a suitable cutting tool ? In the past, I tried to cut/saw it using jigsaw and no success since ferrite core is so brittle & they broke in pieces. /ubbthreads/images/graemlins/frown.gif

And also I'm wondering if there is any difference for LM317/350/338 ripple rejection perfomance if we use different switcher frequency ?

Regards,
Vic


----------



## MrAl (Sep 29, 2004)

Hello again,

Pablo:
Yes, your chip should work fine as long as you keep the
circuit layout traces short. You'll also find you can
use a lower value inductor and smaller output cap so
you have a better part there! I guess if you follow
the data sheet you shouldnt have any problems. I'll include
extra info for this chip in addition to the LM2576 if
anything special turns up.

Vic:
That's a better chip you can use if you wish. It will 
give you about 4 times faster response too.
I'm planning to use the 2576 first too just to see how things
roll along. Yeah, the layout is more relaxed. The output
ripple is ESR*di.
The u=3000 is the initial perm. probably, if it's really J type
material.
Oh, if you can get lots of cores then i guess it's cheaper for
you to make your own inductor, if you can.
Also, since you can get so many cores for dirt cheap you can
always send me one (i'll pay for it no prob) and i'll test it
with my switcher once i get it built. I can then let you know
if it needs a gap or not. Without a gap, the inductance swings
wildly, but as on the Zetex circuit may still work adequately.

Gee i never thought about a gem shop cutting, but then, can
they cut very accurate slices out of toroidal shaped objects?
The gap would have to be very accurately cut, unless maybe you
have enough room to wind more turns in case the gap ends up
too wide.

I guess you're wondering about the high frequency rejection from
the switcher getting through the LM317/350/338 right? The data
sheets all spec ripple at 120Hz, so i guess it will have to be
measured once the circuit is built. Without a scope, you might
get some interesting results using the "Ripple Detect Network"
from the Zetex 300 project. This might give you some idea
what kind of output ripple you are getting (even at high frequency!)
That network was made to detect high frequency ripple (300kHz)
so it would probably be ideal for double checking the output
of the linear part of the ps.

Im still looking for some parts for this but it shouldnt be long
now.

Take care,
Al


----------



## PEU (Sep 29, 2004)

I found a LM2576HVT-ADJ provider, $8.50 a pop... *******s /ubbthreads/images/graemlins/smile.gif anyway if its simpler with this one, I'll use this one.

Thinking, seconds later, what if I order free samples from national... mmmm great idea... let's try... /ubbthreads/images/graemlins/smile.gif


Pablo


----------



## vicbin (Sep 29, 2004)

Hi Al,

When it come to inductors, I always wind it myself since I can't afford Digikey ($75 just for s/h + custom tax) and most of pre-wound inductors here locally are low quality one e.g: loose winding, cracked core, high resistance, chipped isolator at the wire and etc. /ubbthreads/images/graemlins/mad.gif 

Even though I've never seen it, I believe those cutting tool at gem shop can and *must* has a precise cutting ability, those "kind of stone" at one piece alone prolly equal to thousand pieces of LS. hmm...since when LS starting as a currency ? he..he.. /ubbthreads/images/graemlins/grin.gif

Talking about cut, do you mean only single cut (still one piece) ? or two cuts (splitting it into two pieces) ? and the gap, is it possible to fill it with non conducting epoxy without affecting the perfomance ?

I found an interesting data after staring for hours at the LM317 PDF datasheet, at page 7, Typical Performance Characteristics page and at the Ripple Rejection chart (upper right). Does it mean (with CAdj curve) at 100/120 Hz input ripple the rejection is 80 dB while on 50Khz region it will be about 30 dB ? Again, I'm not qualified to understand it completely, just some assumptions and don't know exactly what those graph and dB (decibell) means ?

Say 50mv at 52Khz ripple signal into the LM317, what is the the difference at 80dB and at 30dB ripple rejection result ?

Just send me your snail mail address through email, I'll take care of everything and hope US custom service will not see those ferrite cores as an atomic bomb parts ! You don't need to pay for it, it is my tuition fee ! /ubbthreads/images/graemlins/blush.gif But if you insisted, I only accept one 500Mhz digital scope, hmmm.. not bad for a few pieces of ferrite core..eh ? ....... j/k /ubbthreads/images/graemlins/grin.gif


Hi Pablo,

Yeah, those *******s ! Even at Digikey (known for a very-very high margin) it cost only $6.15 ! /ubbthreads/images/graemlins/icon23.gif 

But look at the bright side, those type with HV suffix can withstand 63 Volt input ! Better than the one without HV (max input 45 V). Too bad I can't find it here, with this HV model you can use *smaller transformer* ! /ubbthreads/images/graemlins/grin.gif

Fyi, here a LM2576-ADJ cost $1.5, and as rough comparison LM338 (TO3 casing) is about $ 2 while LM317 is about $ 20 cents. Beware, I don't know about your country, here we got counterfeit components and they cost 50%-70% less than the original one. These price above are for the original.

Regards,
Vic


----------



## MrAl (Sep 30, 2004)

Hello again,


Pablo:
Can you order from Jameco? Less than $2 i think! (non HV)
Were you able to get the samples?


Vic:
Yes, pretty soon we'll all be converting our currency to equivalent
number of LS's hee hee...good idea /ubbthreads/images/graemlins/smile.gif
Next time i go out for beer the guy will say
"Ok that will be twelve LS's and 5 Nichia's please..." /ubbthreads/images/graemlins/smile.gif
(It's ok though cause i'll probably have that on me anyway 

Actually, the gap on a toroid can be either a single cut in one
section of the core or through the whole thing splitting it in
half. In the latter case, the gap can be adjusted if you can
find a way to hold the final construction together.
I dont see why you cant fill the gap with non conducting epoxy
since the u of the core will be many times the u of air or epoxy.

Oh yes, i must have missed the ripple rejection curves on the
data sheet somehow 
I found with Cadj=0, at 60Hz about 65dB and at 50kHz about 40dB.

20db divide by 10
40db divide by 100
60db divide by 1000
80db divide by 10000

Using the above 80dB and 40dB, for 50mv input ripple the output
would be 0.050/10000 and 0.050/100 respectively.

500Mhz digital scope? Wish i had ONE of those hee hee.

Smaller transformer? How?


Take care,
Al


----------



## PEU (Sep 30, 2004)

[ QUOTE ]
*MrAl said:*
Hello again,

Pablo:
Can you order from Jameco? Less than $2 i think! (non HV)
Were you able to get the samples?


[/ QUOTE ]

Nope /ubbthreads/images/graemlins/frown.gif they declined to send

Anyway, I can obtain the parts here, shipping the parts in USA is only feasible if I have a friend coming, what MAY happen mid october, so if the design is finished in a week or so I may be able to order in USA and save a few bucks /ubbthreads/images/graemlins/smile.gif 


Pablo


----------



## vicbin (Sep 30, 2004)

Al,

Do you mean for this upcoming your PS circuit we should avoid "if possible" high freq switcher ?

It is quite a surprise for me to realize the big difference how this linear regulator behaves on difference frequency, especially those huge difference in dB multiplier ! Thanks a lot, I think I got a very good lessons here.

I'm experiencing a deja vu again, in the past the word "inductor" was a scary word and nightmare, but now the new one will be "*low ESR cap*". /ubbthreads/images/graemlins/frown.gif 

As you said the cap esr plays a big role in reducing the ripple. How serious is this ? Will parallel caps helps a lot ? or just a bit ? I don't have any idea what is the ESR for an ordinary electrolyte cap .

By the way, I have yet to receive your snail mail addy.

Pablo,

If you can not easily find a low ESR cap locally, I suggest you should add the low esr cap into your friend's shopping list. /ubbthreads/images/graemlins/grin.gif

Any way, pm me your snail mail address, you don't need to pay $8.5 for those *******s, its a rippoff. I'll send you one instead. It might take some time since it will travel half of this earth. /ubbthreads/images/graemlins/crazy.gif

Hey, with that price, two of them will equal to one good binned white LS. /ubbthreads/images/graemlins/grin.gif

Vic


----------



## PEU (Sep 30, 2004)

[ QUOTE ]
Pablo,

If you can not easily find a low ESR cap locally, I suggest you should add the low esr cap into your friend's shopping list. 

Any way, pm me your snail mail address, you don't need to pay $8.5 for those *******s, its a rippoff. I'll send you one instead. It might take some time since it will travel half of this earth. 

[/ QUOTE ]

I appreciate your offer Vic, lets wait for the final schematic and I will see if I can ship to some friend in USA, and then receive the goods when they arrive here.


Pablo


----------



## MrAl (Sep 30, 2004)

Hello again,

Pablo:
Oh ok, but remember the chip you have is actually a better chip.
The chip im going to use is a little cheaper so that's why
im using that one. Also, i need a few things from the same
place and they happen to have one so that's why im not getting
the more expensive chip (yet). I might get that one too later
sometime once i've done the first tests with the cheaper part.
If i can help you with parts somehow let me know too.

vicbin:
Sorry, i didnt mean to imply that the lower freq chip is better,
actually the higher chip is better, but i cant get one yet, but
i will at some point. For now, i'll use the cheaper one because
i can get that one easier right now and if it blows it's ok.

BTW, do you have your Schottky connected properly?
The schematic you sent shows it not connected right
i think...Check that?

The ESR of the cap is important because of the ripple voltage,
but it's not 'that' important because we might add a simple 
ripple filter like you tried, with different values perhaps.
I've found that when you can easily add a secondary ripple
filter it's much more affordable then trying to buy low esr 
caps just so you can have low ripple. I dont think we have to
worry too much about this, and when you add caps you also
lower esr because the esr's all act in parallel. Also, you
get higher rms rating by using caps in parallel too, and that
is important with high ripple current (just in case) so
it's better to use more caps in parallel than one with low esr
as long as you can get equivalent esr. Im not even sure yet
what im going to start with on mine -- probably cheap electro's
just to see how things go. If the ripple is too much, i'll
just add a filter with low L and C. Even small series L and
low C gets rid of lots of high freq ripple without interfering
too much with the control loop.
What if the ESR is 1 ohm? That's high, but let's see...
1 ohm with 1 amp inductor ripple gives us 1 volt ripple.
Will that be a problem at 52kHz? Let's see, after LM3xx linear
it takes it down to 0.010 volts ripple. If that's not good
enough maybe a small inline LC filter.
Do you have any kind of requirement for this spec?

Probably the simplest way to determine if your esr is good enough
or not is to measure the output ripple voltage, then decide, then
add filter maybe. We'll have to see. Once i get the caps, i'll
know more about mine...until then it's a guess because they dont
always show the esr for the cheap caps.

There's also the possibility of using two (cheap) inductors and 
two caps as a double pole filter, rather than the one inductor
and single cap. I havent looked into this at all yet so it's just
a thought for now.

Sorry about the addy, i meant to send it yesterday. I hope you're
not paying too much for these cores though. What i can do is this:
If you send the EXACT SAME core that you intend to use and tell
me the number of turns and what size wire i can construct that
exact same thing and test it with mine...that way i'll be able to
tell you exactly how well it works with the LM2576 chip with 
respect to ripple and whatever else.
If i think a different number of turns will work better i'll
try that and let you know what happens.
If you are using the same parts you will get the same results.
The cores must be the same, and maybe you should have purchased
them both at the same time too just to make sure.

I'll try to get the schematic done within the next few days.
I already have the basic layout however, so maybe i'll start
drawing it up and get the prelim posted so everyone knows
what parts we are using. I had lots to do yesterday so
didnt get much time then.


Take care for now,
Al


----------



## vicbin (Oct 1, 2004)

Hi Al,

[ QUOTE ]

actually the higher chip is better, but i cant get one yet, but i will at some point. For now, i'll use the cheaper one because i can get that one easier right now and if it blows it's ok.

[/ QUOTE ]

Exactly as my plan, try it with the cheapo LM2576 first since PCB constraint is not significant and also allows a lot of tweaking & modification such as adding more caps, replacing inductor or even testing it with LC filters, and once the overall circuit matured, I'm going replace it with 5 Amps switcher. I'm thinking to separate both linear and switcher part at different PCB so they will be modular. /ubbthreads/images/graemlins/grin.gif

[ QUOTE ]

The schematic you sent shows it not connected right i think...Check that?


[/ QUOTE ]

Ooops, sorry, that was done in hurry for you to see my modification. Yes, I'm fully understand how it works and the schottky diode should be placed before the inductor. 

About the ripple, I don't have any specific target, as long it is suitable for most CPRer such as powering low voltage circuits or small, delicate & expensive LEDs. /ubbthreads/images/graemlins/smile.gif

What is your expectations or what "realistic" ripple from this kind of circuit ? Of course the lower the better. I wont complain if you can make it work down to microvolts ? ...he...he...just j/k /ubbthreads/images/graemlins/grin.gif

[ QUOTE ]

There's also the possibility of using two (cheap) inductors and two caps as a double pole filter, rather than the one inductor and single cap. I havent looked into this at all yet so it's justa thought for now.

[/ QUOTE ]

Ok, this new to me /ubbthreads/images/graemlins/thinking.gif, is this supposed to improve the ripple ? 

Just take your time, no rush please ! /ubbthreads/images/graemlins/smile.gif

Regards,
Vic


----------



## MrAl (Oct 1, 2004)

Hi Vic,

I thought it was just the schematic but thought i would
say something just in case, that's all  Im glad
it's correct already.

My expectations? I dont have any really, i just figured
whatever we get will still be very useful to me, and
i think that if the ripple is easy to get much lower
(with some filter or trick) then we'll add that and
get very low ripple (at switcher frequency).

The only problem i ran into now is that when i go to
simulate the circuit, i dont have the actual model
for the LM2576 so it looks like i have to wait until
i get a package to try on the breadboard. I can
try the different filters except to get general results
because i dont have a good drawing of what exactly is
inside the ic itself, and the data sheet shows a poor 
drawing. I could guess, but then the dynamics could
still be way off because they dont even give a hint
about this. Funny, National usually has the best
internal diagrams.
All i can do now i guess is order parts and test with
various components once i get the stuff in the mail,
unless you have any ideas?

Microvolts of ripple? Hmmm, i dont know yet. As i 
was saying, i dont have the model to simulate a test
to extremely accurate specs like this...unless someone
can find one...

Well, the double inductor/cap would lower ripple but
without the model of the LM part i cant be sure without
testing it if it's going to be stable or not. It will
have to be tried on the breadboard im afraid.

Any ideas let me know.

Take care for now,
Al


----------



## vicbin (Oct 3, 2004)

Hi Al & Pablo,

Was bored this Sunday noon, and while watching with my kids the Toy Story II movie in the TV....this came out of my head.... /ubbthreads/images/graemlins/grin.gif

Again, put LS at some pics for size comparison.

LM338 & da switcher ...psst...Pablo...something was saying hello to you ! /ubbthreads/images/graemlins/grin.gif









The big family is ready to rumble 








Da CORE....oh.. I mean the inductor...its 263 uH 








....some talks at CORE STATION.... 








The CANDIDATES !!! For the Volt & Amp display !!!








....some chit-chat ........ 








.......meanwhile............somewhere downthere...





......naahhh, just kidding /ubbthreads/images/graemlins/grin.gif

See, I'm so ready !! /ubbthreads/images/graemlins/smile.gif

Vic


----------



## MrAl (Oct 3, 2004)

Hi again,

Hee hee hee hee hee, ha ha ha, LOL, hee hee hee,
The case of the talking IC chips!
And i thought only my ic's could talk...i was wrong!

Vic, you really made my day ! /ubbthreads/images/graemlins/smile.gif
When Pablo sees that he's gonna LHAO hee hee.

...i'd like to do some of those pic's too /ubbthreads/images/graemlins/smile.gif

You used a digital camera i guess? what kind?
Pic's came out very nice!
I have a scanner i could do some with i guess 


Thanks for the laugh, and take care,
Al


----------



## Zelandeth (Oct 3, 2004)

Heh, that is soooo the workbench in my room. Though the usual conversation is the equipment muttering about getting left in the wrong places...or taunting my DMM for being so old...

Those pics are now officially saved, very, very funny. 

Good luck with the project!


----------



## PEU (Oct 3, 2004)

Vic,

what can I say, YOU MADE ME LAUGHT, and worst, want to do the same in the office on monday... 

/ubbthreads/images/graemlins/crackup.gif /ubbthreads/images/graemlins/clap.gif /ubbthreads/images/graemlins/crackup.gif


Pablo


----------



## PEU (Oct 3, 2004)

In the onsemi LM2576 datasheet at least are pcb board examples that we may use.

LATER: does this help?

LATER2: Article with good pointers to simulating regulators.


Pablo


----------



## MrAl (Oct 4, 2004)

Hi Pablo,

Yes, some interesting reading.

I have an approximate model for the LM2576, but what i
was hoping to find was an exact model, where you could
run it in a simulation environment and get near real
life results.
On-Semi's block diagram is a little better than
Nationals, and reveals some interesting character when
the device goes into current limit that i didnt previously
know about.

So far it looks like the only way i'll know for sure is
to wire one up and make some measurements. This of
course means it's going to take more time before a
truely working diagram can be produced.

While im here, can everyone working on this get the part
number LF347 or a similar or faster op amp package?

The main part components are looking like this:
1. LM2576 (or faster)
2. LM317 (or higher rated or LDO version)
3. LF347 op amp (or faster: slew=5v/us or better)
4. 3 amp Schottky
5. 100uH to 200uH low R inductor, 4 amp(or lower for 1A out)
6. Output caps, Electrolytic
7. Reference diode, 1.25 or 2.5 volts.

Did i forget anything?

Take care,
Al


----------



## PEU (Oct 4, 2004)

I tried hard using a web agent (www.copernic.com) to find a model to no avail, the national Switchers made simple app does not help?

in that page there is also a PDF with a model of a regulator, but not exactly the lm2576



By the way, we can build a parts list in digikey, here are the access numbers (go to order status)

Web ID 5282732 Access ID 21478

so far its only $8.88 + shipping/handling

Feel free to add the remaining components!


Pablo


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## vicbin (Oct 4, 2004)

*PS Circuit with Adjustable CC Mode w/[PICS]*

Greetings all,

Pablo,

....YOU...I know YOU...the riot maker at the office ! /ubbthreads/images/graemlins/yellowlaugh.gif he..he.. please share your work once you've done will ya ? /ubbthreads/images/graemlins/smile.gif
Thanks for those nice links, bookmarked and content saved to my harddisk, those are heavy stuff, need some time to digest it ! /ubbthreads/images/graemlins/mpr.gif

Too bad I can't afford Digikey, the price is not bad though.


Hi Zelandeth,

Thanks for dropping by, my pleasure...geeee... my honour that you did save those pics ! /ubbthreads/images/graemlins/grin.gif and thanks for the blessing too ! /ubbthreads/images/graemlins/smile.gif

I believe a lot us must have those messy working bench with all tools & components spreading around like it was got bombed ! /ubbthreads/images/graemlins/grin.gif



Al,

I hope my comics will inspire you in designing the PS circuit ! /ubbthreads/images/graemlins/crazy.gif Those pics was taken using Panasonic DMC-FZ10 digicam. 

Hey..please, make some comics from your components, tools and especially those antiques one.....must be hillarious !! /ubbthreads/images/graemlins/popcorn.gif

Great.....it seems like the circuit is almost in final stage isn' it ? /ubbthreads/images/graemlins/smile.gif

About the components , here is my list :

*1. LM2576 (or faster)*

Ok, you've seen it and also other type capable of 5 Amps load for later stages. /ubbthreads/images/graemlins/grin.gif


*2. LM317 (or higher rated or LDO version)*

Two LM338s checked ! hmmmm....LDO huh ? I remember some our local shop carry it, will take a look later. I though LDO mostly for low voltage or relatively low compared to LM317/338 no ?

*3. LF347 op amp (or faster: slew=5v/us or better)*

Checked, though not LF347, locally I can easily find this opamp :

MC34071A: Single Supply 3V to 44V (PDF) 

Brief feature :

Bandwidth: 4.5 MHz
Slew Rate: 13 V/ms
Single Supply Operation: 3.0 V to 44 V ...(*this is very handy for high voltage supply voltage*)
Input Offset Voltage: 3.0 mV Maximum
Output Voltage Swing: -14.7 V to +14 V (with +/-15 V Supplies)

This one should be ok right ?

Does old style op-amp can be used such as TL-082 or the legendary 741 ? Is it only the slew rate we need to worry ? 

*4. 3 amp Schottky*

No problem, got 10 Amps 60 Volt TO-220 schottky.

By the way, one of my fav. shop offered me this Zero Recovery Rectifier (PDF), and he said this one is better than schottky cause its "Zero Reverse and Zero Forward Recovery", since I 'm not so sure and don't understand clearly about it, I skip it. Its about $4 a pop, what do you think ?

*5. 100uH to 200uH low R inductor, 4 amp(or lower for 1A out)*

Ok, you've seen it and it is wounded with 2 wires (1.5 mm dia.) or approx. equal to 15 AWG wire with DCR measured approx. 0.07 Ohm. 

What is the ideal value ? I follow the one in the datasheet and it is range from 220 to 330uH. Does it make any "big" difference ?

*6. Output caps, Electrolytic*

Now, no low ESR caps here, my best bet is parallel them using 1000uF/100V.

*7. Reference diode, 1.25 or 2.5 volts.*

Ok, no problem, got 1.23 V reference not 1.25 V(ICL-8069) and 2.5 V(TL-431). Say if I can get a very low PPM like 10 to 20 PPM reference, is it worth the money ?


Regards,
Vic


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## MrAl (Oct 4, 2004)

*Re: PS Circuit with Adjustable CC Mode w/[PICS]*

Greetings Pablo and Vic, and all others who might be
interested in building this power supply,

Pablo:
Thanks for setting that up on DK, very good idea.
Why two 350 regulators though?
NS's Switchcad doesnt want to design anything using the LM2576
for some reason...keeps recommending a higher priced unit.

Vic:
Yes, the comics were funny, thanks!
Yes, almost in final stage. The only thing i dont like is
im sorta going into it blind, because i dont have an exact
model for the LM2576. I have approx model i made myself, so
that will have to do for now. I can probably draw the whole
schematic if you like because i do have close to final circuit,
but it will be subject to change *slightly* because of the lack
of the exact model...but then i guess all breadboards are like this
anyway 

Many of the LDO's are low voltage, but some are a bit higher and
have only 1.5 amp output (might be suitable for my purposes or others).
What i might do in the future is find a good LDO for my final version
so the output doesnt use much power (heat) and very very small
heatsink. That would make a nice 1.5 amp power supply in a very
little case, with maybe the LM2576 or perhaps a more eff unit.

Yes, the MC34071A looks like it will work with the high BW and
fast slew. The slew is important because this ic will be responsible
for the current limit, which i'd like to be at least as fast as
1 microsecond for high faults. the Ios voltage is better than the
347 i think too, which can be as high as 10mv.
Old style op amp? It's possible, but im not taking any chances
because i want speed. Some of the old ones are nice, but slow.
It's possible even an LM358, but why bother when the 347 is so cheap
anyway? Should be nice.

Schottky?
I think that Zero Rec Rect looks very interesting, but probably an
overkill for this project. The standard $0.25 1N5822 should be ok for now for not too high outputs.
For your high output you'll need a higher voltage
diode.
Also, the switch freq isnt that high either.

Inductor?
The value affects the performance in various ways...
Low inductance means higher ripple current, which means higher
ripple voltage (and caps have to take it), but faster response.
The ideal value is probably the one that lets the caps work
without a problem, but since i'm going to be working with random
electrolytic caps, im a bit in the dark here.
The inductor should be big enough to stop high ripple yet small
enough to not cause too long of a delay or surge. Also, the
max current of the switch might limit a high peak current value.
For now, i'm assuming 1 amp of ripple isnt too much.

Output caps?
Im going to parallel a couple of cheap ones too. Not sure
yet what their ESR is, but i'll measure it once i get them.

Reference diode?
I dont think you'll need a very high temp stable diode, because
the voltage for this only needs to be approximate anyway, but
decently stable. The current limit set point will depend on
this one, that's all. If it increases by 0.01 percent, then the
output current goes up by 0.01 percent if it's in current limit
mode. If you want super-dooper current limit stability with
temperature, then waste the money  I think almost any one will
work just fine though, but the final decision is yours.

I have started drawing this thing up, but i have to consider
such things as higher voltage input (45v or so). That's about
it. This will probably mean throwing another small regulator
ic into it, like 7815. For mine, the ic can work right off the
main power supply because im only shooting for about 20v output.
There's also the possibility of allowing the use of two 
wall warts, one for main power and the other for the negative
reference and op amp supply.

I'll try to finish up today and get something posted by later
tonight or tomorrow morning.

Sorry this couldnt be a little faster, but a few things came up
and i've also been mulling over some options for this, what's
going to be, a pretty darn nice power supply.

I've priced some power supplies and i couldnt find anything like
this. Yeah, linears at $300 but i dont want to spend that kind of
money NOR have a huge box with many air slots taking up space
and generating heat. Im hoping for a small box (not incld transformer)
that maybe gets a little warm, that's all. Then, i can look
at ways to get up to 3 amps with very little heat.
I like small heatsinks with only warm temperatures /ubbthreads/images/graemlins/smile.gif
not exceedingly hot fins to burn fingers on or need fans.


Take care for now,
Al


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## PEU (Oct 4, 2004)

*Re: PS Circuit with Adjustable CC Mode w/[PICS]*

[ QUOTE ]
*MrAl said:*
Pablo:
Thanks for setting that up on DK, very good idea.
Why two 350 regulators though?
NS's Switchcad doesnt want to design anything using the LM2576
for some reason...keeps recommending a higher priced unit.


[/ QUOTE ]

OOOps, my fault, probably hit 2 instead of one, feel free to add the remaining components needed, I may end purchasing everything there.

Can't wait to start /ubbthreads/images/graemlins/smile.gif /ubbthreads/images/graemlins/popcorn.gif !!!

Pablo


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## MrAl (Oct 4, 2004)

*Re: PS Circuit with Adjustable CC Mode w/[PICS]*

Hi again Pablo,

Im putting the new thread up now...

Take care,
Al


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