# Led amperage basic question



## meyerovb (Feb 10, 2013)

I'm having a little trouble grasping the basic concepts. I want to drive a XM-L2 U2 at max brightness (but not overdrive it) using a standard ac-dc wall plug. Can I do this with a wall plug rated at 5V 1A? I know I need a driver in between, but am not sure if a buck/boost affects voltage or amperage.


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## videoman (Feb 10, 2013)

No. 
And here is why I would not do it.
AC DC adapters voltage ratings are usually underrated by a large margin, up to twice their rated value. That 5V DC one you have, if you measure it's output may read like 7V or more. The XM-L2 wants to see like a 3 volt voltage across it and it is the voltage that will determine the current flowing through it. As it is most likely a non-regulated type, as soon as you connect it to the led, the 5v( or more ) will ramp up that current way past the 3A for a short time but just enough to probably fry it. I may be wrong but let's see what other CPF members think about. Good question though.
The buck/boost driver keeps the current in regulation by adjusting the voltage across the led (s). It affects both voltage and amperage ( current )


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## Illum (Feb 10, 2013)

Theres two forms of AC/DC power supplies: the Wallwort "Linear" unregulated version or the SMPS "tabletop" regulated version. Wallworts are designed to be loaded roughly 50-70% at any given time [or else the heat resulted from the stepdown would melt the casing], by unregulated it means if no load is applied, the output voltage will read much higher than the rated output. 5V wallworts are pretty common nowadays and most are regulated [make sure it specifies 100-240V input]. Always make sure the power supply you are using is regulated, or else you will fry your driver the next time you plug in or turn on. 

Driving an XML-2 at Max brightness would be driven by constant current at 3A, forward voltage would start at ~3.4V cold and slowly decrease as the emitter gets hot. This shift in forward voltage as a function to temperature is why its important to drive LEDs using a constant current driver and not a constant voltage driver. If current is not regulated, the LED will pull more current as it heats up, accelerating it to pull even more current and eventually die of thermal runaway. 

With unregulated supplies especially, if its rated for 1A and you pull 3A, the transformer will saturate and funny things will happen, a number of which may result in an electrical fire. A wall plug thats rated at 5V 1A yields 5W total, to drive an XML at spec requires 3.4x3 = 10.2W, assuming 80% on your driver, you're looking at least 10.2/.8=~13W input. Unless you figure out a way to create energy from nothing I don't know how to help you. 

Last time I tried driving an XML on 5V I had to use a 5V 3A regulated supply and a TI PTN78060W as the 3A driver. It worked okay... the output was a tad shy of 900 lumens under forced cooling and heatsink temp at 70F, really nothing to call home about. I never found why XML was so appealing. :thinking:


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## meyerovb (Feb 10, 2013)

Thanks for the info Illum. To better my understanding, what exactly does the boost driver do, and why wouldn't it help in my situation?


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## Yoda4561 (Feb 11, 2013)

With any voltage source above around 3-3.2v you want a buck driver, not a boost driver. You use buck drivers when your voltage source is higher than the LED needs, you use a boost driver when it's lower. Leds work sort of like water valves, with the applied voltage being the valve handle that controls how much current can flow through it. Too much voltage and the LED will burn itself up trying to flow more current than it's capable of. While the wall wart may only be rated at 3 amps that just tells you how much it can deliver reliably, they will dump more short term at the expense of shortened lifetime. The XML may live long enough for the wall wart to become overloaded and suffer voltage drop (at 5v an XML will look like a dead short to the voltage source), but it's not doing either the LED or wall wart any favors.


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## meyerovb (Feb 11, 2013)

So if I were to get a switched regulated 3.3v power supply rated at 3A, will I still need a driver since even a switched regulated power supply might spike and not be true constant current?


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## SemiMan (Feb 11, 2013)

meyerovb said:


> So if I were to get a switched regulated 3.3v power supply rated at 3A, will I still need a driver since even a switched regulated power supply might spike and not be true constant current?



It is best that you look at the data sheet for an XML and look at the curve of voltage versus current. That will give you a bit of an understanding of the the XML responds to current/voltage. You need to realize though that this curve shifts by approximately +/- 250mV purely based on process variation. I.e. One XML could hit a target current at 3V, and one could hit it at 3.5V. You will not know till you get the part in your hand. To add to that, those values will change over the first 50 hours or so, and they change w.r.t. temperature. Forward voltage goes down as the temp goes up. So while it may hit a particular current at 3.2V when first turned on, you may only need 3.1V or less when it is hot.

For this reason, you need a constant current driver specifically for an LED, not a simple constant voltage power supply. You could start with a 12V, 1.5A power supply, then buy an appropriate LED buck driver that will hit the current you need. The LED driver automatically adjusts its output voltage so that the target current is maintained.

Semiman


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## meyerovb (Feb 11, 2013)

SemiMan said:


> It is best that you look at the data sheet for an XML and look at the curve of voltage versus current. That will give you a bit of an understanding of the the XML responds to current/voltage. You need to realize though that this curve shifts by approximately +/- 250mV purely based on process variation. I.e. One XML could hit a target current at 3V, and one could hit it at 3.5V. You will not know till you get the part in your hand. To add to that, those values will change over the first 50 hours or so, and they change w.r.t. temperature. Forward voltage goes down as the temp goes up. So while it may hit a particular current at 3.2V when first turned on, you may only need 3.1V or less when it is hot.
> 
> For this reason, you need a constant current driver specifically for an LED, not a simple constant voltage power supply. You could start with a 12V, 1.5A power supply, then buy an appropriate LED buck driver that will hit the current you need. The LED driver automatically adjusts its output voltage so that the target current is maintained.
> 
> Semiman



Thanks, that's another question I had, what would be the difference between using, say, the 12V 1.5A psu and a 5V 4A psu (let's assume using a driver that can handle either, like a b3flex)? Could both power the led with to it's max brightness? Any advantages/disadvantages to either option?


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## argleargle (Feb 11, 2013)

First of all: I love this thread! 

A lot of home made direct-drive LED stuff only works with the battery it was designed with because perhaps the designer didn't realize that the properties of the battery itself were limiting the voltage and current through the circuit. Perhaps the designer was attempting to come up with a cheaper solution by using direct-drive of the led (with or without a limit resistor?)

Example: With no calculations or math, using simple  trial-and error, someone manages to come up with a driver-less direct-drive LED circuit that works when powered by "12 volts of specific button batteries." The tinker then hooks his circuit to a 12v car battery, thinking it "should work." Most likely, the circut blows. Why? The driver-less build relied upon resistance (reactance) of the LED and internal resistance of the battery stack to not explode. Different batteries have different capacities and each one has its own internal resistance. These things, of course, affect the circuit. Note: Button cells tend to supply extremely low peak current.

Related search terms: "internal battery resistance," "peak discharge amperage," "voltage sag," "current-limited regulator," "voltage-limited regulator." 

The solution is a driver circuit of some type, although you can usually get away with tinkering with specific batteries and a limit resistor. This is also called "driverless" or "direct drive," and some implementations try to even leave out the limiting resistor. *Warning: *using different (higher current capable) batteries may well  fry your home made experiment if you try this method.

Also: I completely agree with everyone else in this thread stating that each specific led might have different electrical properties than another one of the same type, even same maker.



meyerovb said:


> Thanks, that's another question I had, what would be the difference between using, say, the 12V 1.5A psu and a 5V 4A psu (let's assume using a driver that can handle either, like a b3flex)? Could both power the led with to it's max brightness? Any advantages/disadvantages to either option?



If you've made a driverless circuit, the 5v psu with its 4a max might very well blow something that the 12v 1.5A psu will not. If the circuit design relies upon the PSU to *NOT* provide the full current that the electric circuit is "asking for," then it could definitely blow with other power supplies.

Another way to say it: A circuit that depends on "voltage sag" to not catch fire is definitely vulnerable to different battery types.

As always, someone jump in and correct me if I'm wrong please.


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## meyerovb (Feb 11, 2013)

My question was if I use a driver like the b3flex to power a xm-l at its spec max (3.4V3A), would there be any difference between powering that driver with a 12v1.5a psu and a 5v4a psu?


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## SemiMan (Feb 11, 2013)

meyerovb said:


> My question was if I use a driver like the b3flex to power a xm-l at its spec max (3.4V3A), would there be any difference between powering that driver with a 12v1.5a psu and a 5v4a psu?



The answer is a solid probably on the 5V, 4A supply. The required overhead of a B-Flex at 3A is 1.1V. The XML typical may be 3.4V, but it could be almost 4V, and if the 5V sags, you may not have enough oomph to get all the way there. You will probably be fine though. The 12V, 1.5A should be just fine.


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## ianfield (Feb 11, 2013)

meyerovb said:


> So if I were to get a switched regulated 3.3v power supply rated at 3A, will I still need a driver since even a switched regulated power supply might spike and not be true constant current?



A 3.3V switch mode PSU will be regulated within reasonable limits and might give you some light from the LED, which really wants about 3.4V when cold. Its probably going to be easier to find a wall wart with too much voltage than not enough, so a buck regulator is probably the way to go. Unfortunately - constant current output (essential to the life of your LED) is rather more tricky, but there are designs online if you're a patient searcher - a much easier solution is one of the many LED driver chips on the market.

If you stick with the 3.3V supply - you'll need a buck/boost converter (Sepic - Cuk etc) these are more complex and usually have 2 inductors + a hefty power transfer capacitor. Its only realy worth the extra expense for battery operation, where the fresh charge voltage starts off above 3.4 & end of life is somewhat lower than 3.4V


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## meyerovb (Feb 11, 2013)

Thanks for all the info. If I have a 4amp buck, can I still run the XM-L at 3A somehow? What would I need to add between the buck and the XM-L?


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## SemiMan (Feb 11, 2013)

Ignore Ianfields comment as it just confused the discussion (no offence). The Bflex is perfect for what you need. Use it in conjunction with a power supply say 6-12V with at least 15 watts to give yourself overhead. 

Semiman


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## meyerovb (Feb 11, 2013)

I get all that, thanks. I'm wondering if you can use a driver rated at higher amperage than the led without overdriving the led.


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## Steve K (Feb 12, 2013)

meyerovb said:


> I get all that, thanks. I'm wondering if you can use a driver rated at higher amperage than the led without overdriving the led.



Can you adjust the output of the driver to put out the desired 3 amps? If so, you should be in good shape. In fact, it's usually nice when the driver has a somewhat higher current capacity than you are using, as it tends to stress the parts less and provide a longer lifetime.


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## meyerovb (Feb 12, 2013)

But just to clarify, if I stick an xm-l behind a non-adjustable 4amp driver, I'd be 'overdriving' it? Or would the driver provide voltage protection while the led just pulled 3 amps?


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## argleargle (Feb 12, 2013)

meyerovb said:


> But just to clarify, if I stick an xm-l behind a non-adjustable 4amp driver, I'd be 'overdriving' it? Or would the driver provide voltage protection while the led just pulled 3 amps?



My understanding is that you'd be overdriving the crap out of it, the led doesn't "know" to only pull 3 amps. The driver "tells it what to do" and the power supply tells the driver "what it has to work with."

A "buck driver" limits the circuit down to a certain number. As the battery wears out, eventually the light "falls out of regulation." 4 amps to a single led would overdrive the leds I'm aware of, assuming your battery or power supply can actually provide the 4 amps. If the supply isn't up to the task, voltage and current will both sag beneath 4 amps and you'll have "undesired operation" either way you slice it.

Help any? Ask more questions if we are unclear. It's cheaper than an electrical fire.

If you used a 4 amp buck driver to drive a couple of leds in parallel, then you'd get away with it. Too much for single leds I'm aware of.


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## meyerovb (Feb 12, 2013)

Perfect, thanks. What about if I use 3 1 amp bucks in parallel to drive a 3amp led, then dynamically switch the circuit to exclude 2 of them and drive a different 1amp led? Would that work? I understand each buck might have a +-5% margin on it's output, so it would increase the margin the more bucks I add, but just wanted to know if the theory is sound. Thanks.


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## ianfield (Feb 12, 2013)

SemiMan said:


> Ignore Ianfields comment as it just confused the discussion (no offence). The Bflex is perfect for what you need. Use it in conjunction with a power supply say 6-12V with at least 15 watts to give yourself overhead.
> 
> Semiman



My apologies for confusing you.

Its important to make the point that you can't just use any old wall-wart to drive a LED. It must have higher voltage than the LED requires - and some provision *MUST* be made to regulate the current.

If your wall wart hes too low voltage, you can boost it with a flyback converter - if your wall wart has too high boltage you can drop it with a buck converter.

There are plenty of chips on the market that do specifically constant current for driving a LED - the manufacturers supply application notes with example circuits that make it almost as easy as a pre-packaged kit.


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## SemiMan (Feb 12, 2013)

meyerovb said:


> Perfect, thanks. What about if I use 3 1 amp bucks in parallel to drive a 3amp led, then dynamically switch the circuit to exclude 2 of them and drive a different 1amp led? Would that work? I understand each buck might have a +-5% margin on it's output, so it would increase the margin the more bucks I add, but just wanted to know if the theory is sound. Thanks.



You described a 5V, 4A buck regulator. You would not use that to drive an LED. That is a constant voltage output product and you need constant current for the LED. One of several things may happen if you did this. The unit would limit the output to 4amps (unlikely), the unit would shut down for any variety of reasons (possible), the unit would drive considerably more than 4 amps trying to get up to 5V and then either blow the LED, fail, or engage a thermal shut-down from over heating.

Most LED drivers, i.e. buck-pucks can be paralleled for more current. No guarantees. Best to ask the supplier.

Semiman


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## meyerovb (Feb 12, 2013)

I'm trying to understand how paralleling buck regulators would work. If I parallel 3 1amp bucks (that support being paralleled), would that give me a 3amp current to my led?


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## ianfield (Feb 12, 2013)

meyerovb said:


> I'm trying to understand how paralleling buck regulators would work. If I parallel 3 1amp bucks (that support being paralleled), would that give me a 3amp current to my led?



Assuming each of the bucks had a higher voltage than required for the LED - you would be using a current limiting resistor in series with each buck output, as well as protecting the LED from drawing too much current, the resistor for each buck would help share the current draw for each regulator.

Its important to take into account; the smaller the voltage headroom you have, the smaller the resistor to get the required current, so regulation accuracy is more important. As the LED Vf is pretty much fixed, any voltage deviation is largely expressed across the resistor - small changes in voltage across a low resistance can mean significant changes in current.


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## ianfield (Feb 12, 2013)

PS - parallel operation of buck modules designed for driving an LED directly should probably be OK, as they should be rigged for constant current not constant voltage - that is an imprtant point to make sure of, and no balancing resistors are needed.

If the regulators are constant voltage; due to manufacturing tolerances, one may have slightly higher voltage than the others - it will then try to to supply more than its share of the load.


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## meyerovb (Feb 12, 2013)

Let's say I'm only referring to 3 constant current 3.7V1A bucks being driven by a single 12V5A ac-dc switched regulated wall plug psu. In that case, can I just wire all 3 bucks to the psu, then wire the single xm-l to all 3 bucks? Will that drive the led at 3A without overdriving? Will the circuit need more components (like a resistor) in between somewhere?


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## ianfield (Feb 12, 2013)

meyerovb said:


> Let's say I'm only referring to 3 constant current 3.7V1A bucks being driven by a single 12V5A ac-dc switched regulated wall plug psu. In that case, can I just wire all 3 bucks to the psu, then wire the single xm-l to all 3 bucks? Will that drive the led at 3A without overdriving? Will the circuit need more components (like a resistor) in between somewhere?



As long as your buck modules truly are constant current output, there shouldn't be any problem connecting the outputs in parallel to make up the full rated forward current of your LED.

Adding resistors in that case would simply waste the power dissipated in those resistors.


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## SemiMan (Feb 12, 2013)

meyoerovb,

Do not refer to it as a 3.7V, 1A buck regulator. Refer to it as a buck regulator perhaps with a regulation range from 2.5 - 8V .... i.e. it could drive 1-2 LEDs of varying specs. It will adjust the output voltage to hit the set point of 1amp. Don't worry about the exact output voltage, you just need to ensure the product covers the range of the LED(s).

Semiman


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## ianfield (Feb 13, 2013)

SemiMan said:


> meyoerovb,
> 
> Do not refer to it as a 3.7V, 1A buck regulator. Refer to it as a buck regulator perhaps with a regulation range from 2.5 - 8V .... i.e. it could drive 1-2 LEDs of varying specs. It will adjust the output voltage to hit the set point of 1amp. Don't worry about the exact output voltage, you just need to ensure the product covers the range of the LED(s).
> 
> Semiman



You've just described a constant current regulator.

I've just thought of a possible complication with ganging several constant current LED drivers, the buck circuit is just the same as any other, but the regulation circuit senses the current drawn by the load rather than the buck output voltage - this is usually done by a current-sense resistor (internal to the regulator) inserted in the LED cathode return.

I'm not exactly certain what effects manufacturing tolerances will have on 3 ganged bucks (a single 3A one would be far better!) but it might be as well to start by ganging just 2 and check nothing overheats before adding the third.


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## SemiMan (Feb 13, 2013)

ianfield said:


> You've just described a constant current regulator.



Yes that was my point! .... the op was not quite understanding.

Some regulators will allow ganging, some will not. The issue will not be how they sense current, but the feedback loops and whether they co-operate with each other. Most will some will not. Adding a bit of extra resistance is usually all that is needed to make them work.


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## argleargle (Feb 13, 2013)

ianfield said:


> You've just described a constant current regulator.



A true constant current regulator would be what we call a "buck/boost driver." We will all "get on the same page" eventually. Nobody panic! 



ianfield said:


> I'm not exactly certain what effects manufacturing tolerances will have on 3 ganged bucks (a single 3A one would be far better!) but it might be as well to start by ganging just 2 and check nothing overheats before adding the third.



The new crop of some leds supposedly can run overdriven at 2.8A (each.) 2x1A bucks should be fine if we are talking of driving a single recent-high-tech led. This is a different story if the hypothetical experimentor is attempting to run 2 or more LED in parallel. I'm basically agreeing that 2x1a buck to feed one led COULD work, depending on a few things.

If we're talking about running a parallel-wired triple star of some recent leds, 3x1a bucks WITHOUT PULSE MODULATION should work. A lot of these stars are only "advertised" running at 100-500 mA, though.

This can easily get confusing and complicated, particularly if we are all talking about different components. The simplicity of one proper driver to one proper led is kind of nice. Some drivers support multiple output options, which is also nice.

A driver that uses PWM (pulse width modulation) would probably act really strangely when used in a gang to drive one LED.

Don't think of a LED as a "resistor" with a constant resistance. It's actually a *reactive* component, with its electrical properties changing based on its drive at a particular instant and even temperature and such. 

Short version: When you feed several simultaneous waveforms (pulse width modulation) to a single LED, the math gets REALLY WEIRD. It's because an LED is "dynamically reactive" and not "uniformly resistive." You can't really say that an LED is "3.7V." That wouldn't be accurate.

Right? Let's have some questions. If I've said anything wrong, standard disclaimer: jump in and let's hear it.


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## Steve K (Feb 13, 2013)

argleargle said:


> A true constant current regulator would be what we call a "buck/boost driver." We will all "get on the same page" eventually. Nobody panic!



"buck/boost" refers to a type of switching power supply that can handle input voltages that can be both above and below the voltage at the output. With a buck regulator, the output voltage must be less than the input voltage.
Whether a converter regulates voltage or current is just a matter of the feedback method.



argleargle said:


> Don't think of a LED as a "resistor" with a constant resistance. It's actually a *reactive* component, with its electrical properties changing based on its drive at a particular instant and even temperature and such.



a reactive component is either a capacitor or an inductor. A purely reactive component only occurs on the pages of a text book, as any real device has both real and reactive elements. For example, a review of a datasheet for a capacitor will show that it has lead inductance, lead resistance, and shunt resistance. Likewise, diodes like an LED are primarily resistive with some lead inductance, some shunt capacitance, etc. The relationship between voltage and current does change as the LED current changes, but that is not considered "reactive".... at least not in the terms of the electronics world.
To appreciate the difficulty of describing all of the weird behavior going on inside a diode, take a look at how SPICE models one. It's surprisingly complex, and is why it's fairly hard to get a good model for whatever LED you happen to be working with.



argleargle said:


> If I've said anything wrong, standard disclaimer: jump in and let's hear it.



I wouldn't say you were wrong, but a little clarification was in order. 
When working with an audience as diverse as the typical CPF group, it's hard to explain complicated concepts without knowing the level of education of people you are talking to. In regards to the basic problem of running switching power supplies in parallel, I think the only answer is to ask the people who know.. i.e. the manufacturer of the device that you are considering using. Anything else is just speculation.


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## argleargle (Feb 13, 2013)

Great post, Steve! I used the words "reactive component" and compared to a regular resistor to get the basic idea across. I wasn't about to start talking about "imaginary numbers" and such with resolving waveforms in a reactive circuit.

I just hope we didn't blow OP's mind with all of this at once!


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## ianfield (Feb 13, 2013)

argleargle said:


> A true constant current regulator would be what we call a "buck/boost driver." We will all "get on the same page" eventually. Nobody panic!
> 
> You're absolutely wrong about that! A buck/boost regulator can transition between the two topolugies to cope with input below as well as above the required output voltage - such as battery equipment, like 5V TTL running off 5x NiMh, or a 3W LED in a properly designed 3D flashlight.
> 
> That's not to say a buck/boost converter can't be configured for constant current output - but I've yet to see one in a practical application.


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## argleargle (Feb 13, 2013)

argleargle said:


> A true constant current regulator would be what we call a "buck/boost driver."
> 
> 
> 
> ...


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## Steve K (Feb 13, 2013)

I'm open to hearing about altenative meanings for buck/boost, but I'm using what is standard for the electronics industry. For example, this is what Linear Technology uses as a header for their selection of buck/boost switching regulators:

"Linear Technology offers true buck-boost synchronous DC/DC converters. With their unique internal four MOSFET switch combination, these switching regulators can seamlessly transition from step-down mode, through 100% dropout operation and then to step-up mode to allow a fixed output voltage even when the input voltage fluctuates above or below the output."


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## Illum (Feb 13, 2013)

ianfield said:


> I'm not exactly certain what effects manufacturing tolerances will have on 3 ganged bucks (a single 3A one would be far better!) but it might be as well to start by ganging just 2 and check nothing overheats before adding the third.



Unless you are dealing with linear drivers, this setup will not work. Reason is this, Switching bucks employ a sense resistor between the feedback pin and ground. Paralleling three 1A bucks would mean all three bucks have their sense resistors decreased to about 1/3 of their original value [as in the LED cathode now has three separate routes to ground]. Due to the internal variations between drivers, they will not "mutually yield" 3A in 1A partitions. Of the three bucks, the one with the lowest impedance will produce the most current. At some point the current threshold will "burn through" the internal mosfet of the chip designed to provide 1A max. The controller used to drive the Mosfet may fry trying to compensate for it. Depending on your duty cycle you might end up with a cascading failure if you tried to jolt an LED using three separate bucks.


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## argleargle (Feb 13, 2013)

Thanks for mentioning cascade failure, Ilum. Excellent and on-topic points. People could do worse than to re-read what you've said, IMHO.

Steve, thanks much for your contributions to the thread (and this isn't even my thread!) I hadn't noticed your stuff before this thread, but now I know to keep an eye out. We have had some good discussion sparked here!

I just hope that we haven't scared off OP (Meyerovb.) Don't be scared! Most of us don't bite (much.) 

Since the thread was started with a basic fundamental questions, perhaps we can get back on track with something like "tips and suggestions" while we wait for more questions? I'll start.

*Tip*: Set your multimeter to amps (ammeter,) and put it in series (not parallel) with your circuit when you're playing around on your test bench. The results might surprise you. Electronics people throw around terms like "1 amp" and so on, but the real measured value can be quite different. If there is a problem with the circuit, watching the current through it as you test and play with it might tip you off so you can shutdown before there's a failure. (Hopefully.) It's easy to forget that what's happening in the circuit might be different than what you think.


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## Steve K (Feb 13, 2013)

argleargle said:


> *Tip*: Set your multimeter to amps (ammeter,) and put it in series (not parallel) with your circuit when you're playing around on your test bench. The results might surprise you. Electronics people throw around terms like "1 amp" and so on, but the real measured value can be quite different.



Good advice, but you'll only make that sort of mistake once (or you'll never tell anyone about the second time).

I'd offer the tip that there's no substitute for an oscilloscope when troubleshooting your circuit. You'll be amazed at all of the weird stuff going on that you never suspected was there.


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## ianfield (Feb 14, 2013)

Illum said:


> Unless you are dealing with linear drivers, this setup will not work. Reason is this, Switching bucks employ a sense resistor between the feedback pin and ground. Paralleling three 1A bucks would mean all three bucks have their sense resistors decreased to about 1/3 of their original value [as in the LED cathode now has three separate routes to ground]. Due to the internal variations between drivers, they will not "mutually yield" 3A in 1A partitions. Of the three bucks, the one with the lowest impedance will produce the most current. At some point the current threshold will "burn through" the internal mosfet of the chip designed to provide 1A max. The controller used to drive the Mosfet may fry trying to compensate for it. Depending on your duty cycle you might end up with a cascading failure if you tried to jolt an LED using three separate bucks.



Most implementations I've seen have a rectifier/reservoir cap arrangement, so the voltage developed across the current sense resistor for the chip's sense input, should be (mostly) DC - but there could be large ripple which could cause problems, so beware.

I did point out in an earlier post; that finding a single 3A buck would be a better idea than ganging 3x 1A bucks.

My best guess is - try 2 & see what happens, if it works then see if you can get away with 3.


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## ianfield (Feb 14, 2013)

argleargle said:


> Thanks for mentioning cascade failure, Ilum. Excellent and on-topic points. People could do worse than to re-read what you've said, IMHO.
> 
> Steve, thanks much for your contributions to the thread (and this isn't even my thread!) I hadn't noticed your stuff before this thread, but now I know to keep an eye out. We have had some good discussion sparked here!
> 
> ...



Bear in mind that the volt drop across the multimeter current shunt can be significant - especially with a regulator disigned to drive a single LED.

There might be a good reason why the results are quite different to what you were expecting.

I remember trying to measure the current on a NiCd charger - the volt-drop on the DMM test leads alone caused the charger to display a fault warning.


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## Steve K (Feb 14, 2013)

in regards to measuring current, you do need to know the detailed characteristics of the meter to get the desired accuracy. The voltage drop across the meter will be different when set on the "10A" input versus the "300mA" input. If in doubt, use a second meter to measure the voltage across the meter measuring current.

In some cases, your better method might be to use a very low resistance current shunt, and measure the voltage across it. A shunt of this type might be as low as 0.01 ohm or less. A good current shunt will have low inductance too, and should always have kelvin connections to reduce measurement errors.

A broader, more general lesson would be to know the quirks and characteristics of all of your measurement tools! For instance, even when using an oscilloscope, there are a lot of things you should know about how to use the ground lead on the probe, and when it should be removed and replaced by a very short piece of wire.


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## SemiMan (Feb 14, 2013)

ianfield said:


> argleargle said:
> 
> 
> > A true constant current regulator would be what we call a "buck/boost driver." We will all "get on the same page" eventually. Nobody panic!
> ...


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## Steve K (Feb 14, 2013)

SemiMan said:


> ianfield said:
> 
> 
> > Actually semiconductor companies or at least their marketers throw around this term pretty loosely.
> ...


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## ianfield (Feb 15, 2013)

Steve K said:


> SemiMan said:
> 
> 
> > the inverting buck/boost is a handy design that is only useful for a load that is not connected to anything other than the power supply. Not that useful on average, but very useful for LEDs. It doesn't get the love that it ought to. The modern buck/boost controllers that generate a positive output voltage are relatively new, and only possible due to the ability to add extra mosfets to the IC. I've seen a variety of them in production applications, and they are pretty slick! From an EMC perspective (my personal concern), they are a bit tricky because the emissions change when they change from buck to boost.
> ...


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## meyerovb (Feb 15, 2013)

Well, lots of great info. I do not need a boost. What are the differences/advantages to a switching buck vs a linear buck?


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## Steve K (Feb 15, 2013)

ianfield said:


> Steve K said:
> 
> 
> > Its worth mentioning that the term "inverter" is another one used loosely - often referring to *ANY* switching converter.
> ...


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## Steve K (Feb 15, 2013)

meyerovb said:


> Well, lots of great info. I do not need a boost. What are the differences/advantages to a switching buck vs a linear buck?



Primarily, the switcher is more efficient, but the linear regulator is less noisy and simpler.

And to help confuse the terminology a bit more, since a linear regulator can only reduce voltage, it's not called a buck. I've been wondering where the term "buck" comes from, and just assume it's related to "bucking" as in a bucksaw. The act of reducing the length of a log is not that much different from the idea of reducing voltage, I guess.


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## argleargle (Feb 15, 2013)

...perhaps it is "buck" as in "to buck a trend," or oppose the full realization of something?


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## ianfield (Feb 15, 2013)

meyerovb said:


> Well, lots of great info. I do not need a boost. What are the differences/advantages to a switching buck vs a linear buck?



To the best of my knowlege there's no such thing as a "linear buck", a linear regulator is simply a controlled variable resistance in series with the load - as current flows a voltage is devoloped across that resistance and produces heat (wasted energy).

With most types of switching regulator, the control element alternates betwee on & off - when on the volt drop is small so minimal dissipation, when off no (almost) current flows, so also low dissipation. One way or another, the pulsed current finds its way through a LC filter, which gibes you (hopefully) smooth DC for your load.


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## MikeAusC (Feb 15, 2013)

argleargle said:


> Great post, Steve! I used the words "reactive component" and compared to a regular resistor to get the basic idea across. I wasn't about to start talking about "imaginary numbers" and such with resolving waveforms in a reactive circuit.
> 
> I just hope we didn't blow OP's mind with all of this at once!



"Reactive Component" has a very clear definition in Electrical Engineering and refers only to Capacitors and Inductors. 

You're just confusing people with your personal definitions.

An LED is a non-linear component, as it's current is not proportional to the voltage across it, as occurs for a resistor.


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## MikeAusC (Feb 15, 2013)

argleargle said:


> A true constant current regulator would be what we call a "buck/boost driver." . . . .



A true constant current regulator can be implemented using - 
- Linear buck
- Switchmode buck
- Switchmode boost
- Switchmode buck/boost


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## MikeAusC (Feb 15, 2013)

Steve K said:


> SemiMan said:
> 
> 
> > . . . The modern buck/boost controllers that generate a positive output voltage are relatively new, and only possible due to the ability to add extra mosfets to the IC. . . . .
> ...


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