# LM317 Simple Constant Current Circuit & Luxeon ??



## reviewum (May 4, 2004)

I was going to email my buddy Doug Owen about this, but thought I'd post my questions here so other noobs like me can benefit!

Here is the great and simple constant current circuit Doug gave me which produces a current of 26 ma with “any” voltage provided above 6v (also pictured is the Luxeon I got from Doug):






My questions:

1) How do I calculate the value of the resistor X to get a current of Y? 
2) Is there anything I have to worry about concerning the resistor wattage rating, or the LM317 rating so I don't "cook" them?
3) I was thinking about using this little Luxeon, but don’t know the specs for it. Say I want to run this at the spec values, what size / value of resistor would I use to get what current?

Thanks in advance!


----------



## Doug S (May 4, 2004)

1. If X=resistance in Ohms, Y=current in amps then 
X=1.25/Y

2.That version of the LM317 [TO-92 package] has an internal current limit of 150mA. It is also thermally protected limiting die temp to around 160C. With the 150mA current limit, you don't need to worry much about the resistor power rating. If you need more than 150mA you need to use the LM317 in the TO-220 package which current limits at around 2A. In this case, the wattage rating of the resistor should be at least 1.6/R.

3. The Luxeon is rated at 350mA but see #2 above. If you want to get the maximum out of that LM317 pictured [about 150mA] replace the resistor with a short.


----------



## reviewum (May 4, 2004)

Doug S.... another great Doug! 

Thanks for that great info! So, if I short out the LM317 I've got then it will run the Luxeon I've gor at 150mA?

Do I need to worry about a "max" voltage I'm supplying? I'd like to use this in my car, but when the engine's running I get up to 14 volts.

Thanks in advance.... what a great little circuit! With this circuit and the one Doug Owen did for the Milky Candle I can have LOTS of fun!


----------



## Doug S (May 4, 2004)

[ QUOTE ]
*reviewum said:*
Doug S.... another great Doug! 

Thanks for that great info! So, if I short out the LM317 I've got then it will run the Luxeon I've gor at 150mA?

Do I need to worry about a "max" voltage I'm supplying? I'd like to use this in my car, but when the engine's running I get up to 14 volts.

Thanks in advance.... what a great little circuit! With this circuit and the one Doug Owen did for the Milky Candle I can have LOTS of fun! 

[/ QUOTE ]
Yes, if you want the full 150mA, you do need to worry about the voltage. That TO-92 package can only dissapate about half a watt. If your supply is 14V and your Luxeon is say 3V, then 14-3=11V would be dropping across the LM317. 11Vx0.15A=1.65W. The IC would go into thermal limit and reduce the current to probably less than 50mA. If you need to dissapate that much power, you need to use the TO-220 package and the appropriate resistor to set the current.


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Honestly I would NOT do this mod....Well not if your going to be using 9-volts...The LM317 is Horribly Inefficient, You will need to heatsink the LM317 and the LED (seperately) if your planing on running that luxeon at 350ma.

Now for constant power source like your truck it should be ok, but it would be better to run Multiple luxeons off 1 LM317 if your going to use a 14 V source. Here is why, say your running the luxeon at 350ma, and the luxeons Vf at 350ma is 3.5 volts. 14-3.5 = 10.5 volts that are wasted. 10.5x350 = 6.65 watts that hte LM317 is responsible for disipating. It is going to get HOT!

Now with an LM317T variety this is ok if you have good cooling. But Horribly inefficient, now add 2 more Luxeons to the equation and you will wee that the LM317T does not have to disipate nearly as much power! Weird huh!

Anyways to answer the question wether or not it will be able to handle 14V it really depends on the variety of the LM317 your purchase, the LM317T shoudl be able to do it, Can't rememebr the max V but I think it was like 30 at 1 amp or something (Look it up if you need). Oh and beleive me heat adds up fast, I was gettign readings over 200F when driving a couple Lux III's (at 1 amp! meaning it has even more power to disipate as heat)

Also now you can see why it would be horrible for driving things on batteries.

BTW a 4 ohm 1/2 watt resistor should cover it 1.25/4 = 3.125 ma


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

This is good stuff! Thanks guys!

Maybe I'll use 5mm LEDs in series / parallel using the LM317 for this application.


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

run them in series...

JI


----------



## unclearty (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Would you be willing to draw out this regulator circuit for the rest of us? Why series for multiple 5mm leds?


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

typicaly it is always better to run leds in series with this circuit because you are limiting the current through all of the LED's to particular value. Bad things can happen when you wire them in Parallel. Think of it this way current is liek a flow, and you want 350ma to flow through one LED and the same current to flow into the other. If you were to wire them in parallel you will probably fry your LED's. Trust me I have killed Luxeons this way before...I forget what exactly happens but perhaps the currents add in some manner and smoke your lights.


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Here is the one I'm working on to add two lights to my interior in my car:






I'm not a super schematic guy, so let me know if it is wrong.

I'll take some pics when it is done.

On my test bench I get a constant 40 mA to each LED from 10 to 15 volts supplied without any head to the resistor or the transistor.


----------



## Doug S (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Reverse the polarity shown for the battery.


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Comeon doug... does it really matter? /ubbthreads/images/graemlins/wink.gif

Fixed in the image above!


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

It should look more like this..

http://linear1.org/gm/archives/00000147.php

Note that the order of the Pins may be different on your LM317. I believe you want the neg wire hooked right into the LED.


----------



## IsaacHayes (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Check out plans for my car dome light that I will build when I get time. It'll be mounted on a 1/8" thick aluminum plate for heatsinking the luxeons and the LM317T, but the LM317T must be isolated from the plate, as the plate will be tied to ground from the luxeons..


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Yea, I believe the back of the LM317T is common with Vout..


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Okay, I give up... I'll let someone else post the CORRECT schematics!

/ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/banghead.gif


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

The link I gave you not only shows the correct schematics but it explains in detail the entire circuit using the LM317T for Current regulation. (I guess it can do voltage regulation also)


----------



## evan9162 (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Don't worry. Your schematic is correct. Pay attention to the pin designation that he's drawn for the LM317. 

Justin might be confused since your circuit isn't drawn in the traditional left--> right manner that most people are used to (power source on left, load on the right).


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

edit: N/M I see it now, looks good. I didn't realize the LED's were in the Diagram /ubbthreads/images/graemlins/banghead.gif


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Okay, I'm looking at the circuit running on my desk and it is working perfectly! I've got 40 mA running through 2 sets of LEDs (20 mA each right?)... when I crank up the voltage the current stays the same. 

Here is what the circuit looks like... is this better or worse?






Maybe I need to turn on the A/C this heat is cooking my brain!


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

are you sure that is the circuit on the desk? The battery sign looks reversed, and acording to this diagram the LED's are run before the Vin on the LM317 which makes no sense! This one is wrong, go back to your origional....

GO back to your origional Diagram, I would NOT run LED's in parallel using this circuit. I have Fried LEDs in this way before. I'm not sure what the affects of hookig them up in parallel would be, but I know it fried 2 of my blue Star_O's before where as I had no problems running them in series... I had it set for 350ma for the blue stars, and it fried them! so it appears that the current was Greater than it should have been, I would measure the current if I was you it should be simple to do. I would think there would be Less current going to each but hey too little current should not fry things and they were toasted!

STICK TO YOUR FIRST DIAGRAM /ubbthreads/images/graemlins/twakfl.gif

the circuit in your origional picture looks nothing like this new one.


----------



## Doug Owen (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Rob,

I see you've been having some fun today.....

First off, your first circuit is correct as I just saw it, the second needs labels on the 317 (AOI as drawn, left to right). Secondly the polarity of the LEDs is backwards as drawn (should 'point the other way'). Otherwise OK except it's confusing to some because the traditional symbol is IAO left to right. The schematic on the page cited by Justintoxicated is closer to traditional and therefore easier for folks to understand.

You can think of the LM317 and the resistor as a two terminal network (input to adj pins) that's a 'smart resistor' that will only pass current defined by 1.2 divided by R (gives the total current in amps). Put this resistor in series with the supply and LED(s) (in any order) across the supply. Watch the polarities, and if the supply is more than say 3 Volts above the total Vf it will regulate as designed. 

Power dissipation is always volts times amps. You can measure Vf (or make a conservative guess), you know that the voltage across the resistor is 1.2 Volts at 'normal current' (less below that) and that the voltage across the 317 is the supply voltage minus the Vf total minus 1.2 (or you can measure that as well if you want). And you can measure or know from your selection of the resistor what he current is. Multiplying that current (in amps) by the voltage in question will give you the dissipation of the part in question. As the other Doug (rightly) points out, the TO92 case part is good for maybe half a watt, the TO220 for say 1.5 (without extra heatsinking).

FWIW, I wouldn't advise depending on the internal current limit on the TO92. The 150 mA number is the guaranteed *minimum* value, I've seen it far higher. If you want 150 mA, use the correct value resistor (8 ohms).

Doug Owen


----------



## Doug Owen (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Oh, yeah, while the LS I gave you is a 350 mA device, it shouldn't be driven past say 100 mA without a heatsink.

Even then it's likely to be kinda hot.

Doug Owen


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Doug Owen said:*
Rob,

I see you've been having some fun today.....

First off, your first circuit is correct as I just saw it, the second needs labels on the 317 (AOI as drawn, left to right). Secondly the polarity of the LEDs is backwards as drawn (should 'point the other way'). Otherwise OK except it's confusing to some because the traditional symbol is IAO left to right. The schematic on the page cited by Justintoxicated is closer to traditional and therefore easier for folks to understand.

You can think of the LM317 and the resistor as a two terminal network (input to adj pins) that's a 'smart resistor' that will only pass current defined by 1.2 divided by R (gives the total current in amps). Put this resistor in series with the supply and LED(s) (in any order) across the supply. Watch the polarities, and if the supply is more than say 3 Volts above the total Vf it will regulate as designed. 

Power dissipation is always volts times amps. You can measure Vf (or make a conservative guess), you know that the voltage across the resistor is 1.2 Volts at 'normal current' (less below that) and that the voltage across the 317 is the supply voltage minus the Vf total minus 1.2 (or you can measure that as well if you want). And you can measure or know from your selection of the resistor what he current is. Multiplying that current (in amps) by the voltage in question will give you the dissipation of the part in question. As the other Doug (rightly) points out, the TO92 case part is good for maybe half a watt, the TO220 for say 1.5 (without extra heatsinking).

FWIW, I wouldn't advise depending on the internal current limit on the TO92. The 150 mA number is the guaranteed *minimum* value, I've seen it far higher. If you want 150 mA, use the correct value resistor (8 ohms).

Doug Owen 

[/ QUOTE ]

what exactly causes LED's to blow when you run them in parallel with this circuit Doug?

The only thing I don't like about the second diagram besides the power being backwards is that there is no way the LEd's are seeing the Vadj, because even if he reverses the power it just does not make sense. The first diagram looks much better.


----------



## Doug Owen (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Justintoxicated said:*

what exactly causes LED's to blow when you run them in parallel with this circuit Doug?

The only thing I don't like about the second diagram besides the power being backwards is that there is no way the LEd's are seeing the Vadj, because even if he reverses the power it just does not make sense. The first diagram looks much better. 

[/ QUOTE ]

Nothing. That is if you correct the polarity in Rob's circuit it should (and does) work fine. For sure putting a second LED in parallel can only *lower* the current in the first in a true current regulator.

While the first diagram might make more sense to you, the second will work fine (with the reversal of the LEDs), assuming the pin the the middle is the output. If, instead you wire it as the adj pin then yes, the regulator will run wide open (since it never 'sees' the necessary 1.2 Volts between out and adj) which will kill the LEDs unless the supply poops out.

Doug Owen


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Sorry for the confusion... I just drew the diagram as I'm looking at the circuit and the transistor with the flat side facing me.

Regarding LEDs in parallel, I know that this isn't "reccommended" without each LED having its own resistor, but I figured at this low current (20 mA each) it wasn't an issue.


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Doug Owen, then why is it than when I ran them (2 blue luxeons) in parallel I toasted my blue Luxeons? When I ran them in series after that (another set)they never had a single problem. I'm pretty sure it is better to run them in series!!?!?! 

I still don;t see how the second diagram is right if he simply switches the signs, the power source cannot be between Vout and Vadj...I think we are thinking the same thing though since the LM317 is not labeled, I jsut figured he would have to change to location of the LED's so that they are drawing power from Vadj...anyways stick with the first diagram and it will work!

I'd still like to know however, why running my Blues in Parallel fried them, and then replacing only the LED's with another set in series they ran fine... Any ideas man?


----------



## gadget_lover (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Parallel leds:

Two devices in parallel with the same resistance will split the current evenly. Devices that are in parallel with different resistances will split the current in proportion to the resistance of each.

If you use a current regulator, it will vary the voltage till the correct current is reached. If one of the LEDs has a different VF (remember they use ranges of almost 1/4 volt) it will pulll more current than the others. This will cause it to heat up, which causes the resistance to drop more. This is known as thermal runaway. The hotter it gets the lower it's resistance and the more current which makes it get hotter and so on. They sometimes lower their resistance to that of a short.

If you have two LEDs in parallel, the regulator will need to put out twice the current that either can handle, I.E. 700ma for a pair of 1 watt LEDs. That means that the LED in runaway can use almost all of that. Many 1 watts will run for a while using 2 times thier rated current.

If you use 4 LEDs in parallel, you end up with 1.5 amps. If one of the LEDs goes into runaway, it will almost certainly burn out. That leaves 3 LEDs to split the 1.5 amps... Oops, will they last at that level?

Runaway can be triggered by one of two identical LEDs that has less heatsinking or that's near another heat source.

That's my understanding of the problem.

Daniel


----------



## Doug Owen (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*reviewum said:*
Sorry for the confusion... I just drew the diagram as I'm looking at the circuit and the transistor with the flat side facing me.

Regarding LEDs in parallel, I know that this isn't "reccommended" without each LED having its own resistor, but I figured at this low current (20 mA each) it wasn't an issue. 

[/ QUOTE ]

Rob,

You're still confusing me, and I'm sure at least one other CPFer, it's not a transistor, it's an IC, right? It only uses a transistor package because it has two leads....

And I don't think there's anything really dangerous paralleling LEDs as long as you're using 'fairly identical' ones. Vf varies a lot with current, the small differences cause only a mA or two shift at the levels we're talking about. For sure a lot of commercial LED products do it 'all the time'.

And I'm usually pretty conservative.

Doug Owen


----------



## Justintoxicated (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

I do believe that the LED's had different Vf in my case so that would make sense. However this then nulifies what was said above that running in parallel will split the current betweent the lights when using a current limiting resistor. Does it not? 

Then again, why won't LED's with differn't Vf go into runaway when they are in series? Wouldnet one still need more voltage than the other? I have successfuly run them in series using this circuit with different Vf, see Avitar /ubbthreads/images/graemlins/smile.gif

Also if the LM317 Varries voltage, which I keep hearing it does, then why does the Lower Vf LED give me a lower Vf than the higher Vf LED? and why do I get exactly 350ma between them? I don't understand the depths of this relationship between voltage and current well enough to comprehend this. But it seems to me that they should both have the same Vf if the regulator was truely regulating current by voltage?


----------



## reviewum (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Doug.. you are correct... I guess it is an IC. Shows how much I know!

I was just trying to take what you had built and make it work in a different application... I guess I'm better working with my hands on this vs. schematics!

Maybe I should stop typing and just keep reading!


----------



## Doug Owen (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Justintoxicated said:*
Doug Owen, then why is it than when I ran them (2 blue luxeons) in parallel I toasted my blue Luxeons? When I ran them in series after that (another set)they never had a single problem. I'm pretty sure it is better to run them in series!!?!?!

I still don;t see how the second diagram is right if he simply switches the signs, the power source cannot be between Vout and Vadj...I think we are thinking the same thing though since the LM317 is not labeled, I jsut figured he would have to change to location of the LED's so that they are drawing power from Vadj...anyways stick with the first diagram and it will work!

I'd still like to know however, why running my Blues in Parallel fried them, and then replacing only the LED's with another set in series they ran fine... Any ideas man? 

[/ QUOTE ]

The answer is because you were driving it from a voltage, not current source. At least that's the short of it.

The higher Vf of two in series against the available voltage is entirely different than when they're in parallel. This can lead to a 'other than square' I/V curve. It's very easy to see how this works with DD lights (kind of an extreme case), but even with resistored lights it can be a problem if Vr is less than Vf total for the series set up. So, it's not a current regulator (or it's not working), all bets are off.

Be careful of the 'relabel' on the diagram. The pin on the right in input, right? Out in the center, adj on the left (I know I did it the other way around last time. Remember the load (LEDs) can go 'above or below', in fact you can put one from plus battery to input, and the other from adj to minus battery (conventional current, real electrons go the other way.....).

Or so I'm betting (with your Luxeons, of course).

Doug Owen


----------



## gadget_lover (May 4, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Justintoxicated said:*
Then again, why won't LED's with differn't Vf go into runaway when they are in series? Wouldnet one still need more voltage than the other? I have successfuly run them in series using this circuit with different Vf, see Avitar /ubbthreads/images/graemlins/smile.gif

Also if the LM317 Varries voltage, which I keep hearing it does, then why does the Lower Vf LED give me a lower Vf than the higher Vf LED? and why do I get exactly 350ma between them? I don't understand the depths of this relationship between voltage and current well enough to comprehend this. But it seems to me that they should both have the same Vf if the regulator was truely regulating current by voltage? 

[/ QUOTE ]

You are confusing Vf's relationship to the current. I deduce from your last paragraph that you are thinking that the Vf varies depending on how much current is pushed into a circuit. In reality, current is a measure of flow and voltage is the measure of pressure. Given a set resistance, the current will change when the voltage is changed.

So your 317 has internal circuitry that lets more voltage through until it senses that the target current is flowing. 

That brings us to Vf. Think of Vf as the voltage at which the push is strong enough to push the rated current through the LED. That's simplified, but close enough. If your regulator is set for 350ma and the LED has a VF of 3.57v, your volt meter should show that voltage across the LED's terminals when 350ma is flowing. If you add a resistor in seres with the LED, the resistor will use part of that voltage but all that current will flow through both.

If you are driving two 1 watt LEDs in series at 350ma, you are probably pushing around 7 volts. The low Vf led will consume part of the power and the higher will consume the rest. Assume one with a Vf of 3.5v and one at 3.7 and a regulator set for steady current at 350ma. Total voltage will be 7.2 volts. LED a will consume 1.225 watts (.350a * 3.5 v) and LED B will consume 1.295 watts (350a * 3.7v). LED has a computed resistance of 10 ohms and LED be has a computed resistance of 10.5714 ohms.

If either of the LEDs is allowed to overheat, the resistance will change and it might trigger thermal runaway.

Did that help?

Daniel


----------



## Doug Owen (May 5, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

This is getting heavy, fast. Last night I typed this all up and managed to do the wrong cut and paste and lost the lot. So I took the cue and went to bed....

Let's try again.

While for some parts we can use pure resistance or other properties, most real parts are a mixture and therefore cannot be modeled well this way. LEDs are a good example of this. We need some more powerful tools for this than 'just Ohm's Law'. In this case we'll need Thevenin's Theorem 

If we look at the 'Vf against I curve' for say the Luxeon we see it's more or less a straight line (which is purely resistive in Circuit Analysis terms) but it's *offset*, that is it doesn't pass thought zero. In fact at low currents it gets all weird anyway, which is why it's usually not drawn down there. However the *slope* of the line defines a resistance (in this case this is for the most part 'bulk resistances' and lead resistances (including bonds) in the device). You'll note that a change of 250 mA causes a .25 Volt change in Vf? This is ..25 Volts divided by .25 Amp or roughly one ohm. This is often referred to as the 'dynamic resistance'. I varies with device construction. It is also the "Thevenin resistance", Rth. There is also a "Thevenin voltage" involved, Vth. If you extend the straight part of the curve down to the x axis (zero current) you'll see this is about 3 or so Volts. This is, for the most part the 'quantum well depth', the energy that goes into releasing the photon we see. It varies with color. Of course both vary a bit from device to device (hence binning).

So the proper Thevenin equivalent of the LED is a three Volt 'perfect battery' (Vth) in series with one ohm (Rth). This is what the real world 'sees' as it 'looks into' the two leads from the LED. These properties come from various parts of the device, but the sum effect can be modeled that way, a Vth in series with a Rth. FWIW, the same thing happens with batteries. You have Vth (the open circuit voltage, what you measure with no load) and the 'internal resistance', Rth, that causes the terminal voltage to drop as the battery gets loaded.

This model of the LED, a Vth of 3 and Rth of one, explains what we see. Very low currents show Voltage up to Vth with 'no light'. Once we exceed Vth, current flows, causing a drop across Rth that adds to Vth and makes Vf. Changes in current are reflected by this 'dynamic resistance', not Vth.

Note this is a current perspective (which is the way we really should deal with LEDs), but voltage analysis also works (as it needs to if the Thevenin Theorem is to be useful).

There, that didn't hurt all that much did it? For a first lesson in Circuit Analysis? Be sure you put it on your resume......

Doug Owen


----------



## Draco_Americanus (May 6, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

I think my brain just melted. I belive the same applies to transistors as well?
Has anyone tried making a regulator using lm555 timer IC's ?
Useing pulse width modulation would be a lot better and less wastefull. Or has some IC maker made a chip that can do that easy (I.E Maxum's buck boost converter but from a step down insted of a step up point of view)


----------



## Justintoxicated (May 6, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

I still don't understand why my 2 Blue LED's fried when hooking them in parallel to a current limiting LM317 set for slightly under 350ma...

Any takers? 

Hooked some new ones up in series and I have never had a problem...

I'm also told not to run different color LED's off the same LM317 Circuit...Why not?


----------



## Draco_Americanus (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Justintoxicated, your lm317 probly failed to limit the total current to 350ma. "if" the total current was regulated correctly at 350ma(total current) that would have not toasted the luxeon as it could safly disipate 350ma. one led would have probly been brighter due to it turning on before the other one due to it's slightly lower forward voltage. As was explaned in detail by other members no two leds will turn on at the exact same voltage.


----------



## shiftd (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

justin, different color leds is fine, as long as it is blue, green or white together. Red and orange behaves differently, which might make them unsuitable to be paraleled with other colored luxes. 
also, i think your lux died because you mount them on top of + oriented sink. That is assuming your lm317 really did maintain 350 mA. over heated also the possibility, though i bet you would have known it by yourself.


----------



## unclearty (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

I'm seeing smoke coming out of my ears....I know this may be asking alot, but would someone be willing to draw a schematic using the 317 reg for those of us...ahem..less knowledgeable about this item??? Perhaps using 6volts in and driving a few 5mm leds..?? Something that most of us could easily play with?? /ubbthreads/images/graemlins/help.gif

I think this is a very cool idea..creating a simple diagram that alot of us could easily work out on our workbenches...this is the fun stuff.


----------



## gadget_lover (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Are you thinking of a picture based diagram as opposed to the schematic at the top of this thread? 

Daniel


----------



## unclearty (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

What I'm asking is: a diagram...6 volt input ( a little more common) to drive ,say, 2 or 4 5mm leds at 30ma as opposed to a Luxeon that gets driven harder. I think alot of people probably have alot of 5mm 's laying around, and a 6 volt lamp is pretty common.


----------



## reviewum (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

unclearty, What you ask is EXACTLY what I was attempting to draw out (except using 11 + volts), so we're on the same page you and I!

I'm trying to run 4 5mm LEDs at 20mA each. I have two serial sets of two in parallel. Two LEDs in series that require 3.3 volts PLUS the circuit that uses 3 volts will require 9.6+ volts to run. Using only two LEDs in parallel, you'll need about 6.6 volts to run them with this circuit (if I'm wrong someone will correct me). This is the circuit I had running on my desk and it's been running just fine.

I'm DEFINATLY more of a visual / actual-picture-of-the-circuit kind of guy. My brain isn't to the point of interpreting schematics yet.

I think this circuit, along with a few other would be great to put on a DIY page with a ton of good pictures... Obviously I'm not to the point to be doing the drawings, etc. but if people want to put some DIY articles together I can put them on a website for the world to use!


----------



## unclearty (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

I love the idea..a forum dedicated to simple projects for simple minds....no..wait..not what I meant...anyway, a place for people to show what they are playing with and others, more knowledgeable, to tear it apart or perfect it. I love trying these little electronic projects. Do you agree that a 6 volt in would be a little more common for the average tinkerer? /ubbthreads/images/graemlins/popcorn.gif


----------



## php_44 (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

The schematic shown in the tenth entry to this thread will work fine for any voltage over 6V and under about 12v for 30mA output with an LM317L. Use a 60 ohm resistor for 20mA, 40 ohm resistor for 30mA, and a 30 ohm resistor for 40mA. [In general the resistor should be 1.2/(desired current) , example 1.2/0.04 = 30 ohms for 40mA=0.04A.] Here is the picture again:







Now - The leads on an LM317L in the small TO92 plastic package are identified as follows. If you lay the device on a table with the leads pointing toward you and the flat face pointing up, then the left lead is adjust (A), the middle lead is output (O), and the rightmost lead is input (I). Ideally you'd want one regulator circuit per LED with a 6V input. With a 12V input you could easily connect two LEDs in series for this circuit.

Also, I probably wouldn't bother with an LM317L for a 6V battery for a white, blue, cyan, UV, or true green LED (the type that need >3V). If you take the LED's voltage of ~3.3V and add the regulator's 1.7V minimum dropout voltage you're up to needing 5V just keep the LED lit. Soooo, when your four 1.5V cells drop to 1.25 volts (they'd still have a lot of life left) your light will be getting ready to quit. For a 6V light - I'd use a 56 to 68 ohm resistor in series with each LED to give ~40-48mA per LED with a fresh battery with a gradual tapering off as the battery wore off. Each to his/her own, though!!!


----------



## reviewum (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

As mentioned earlier, this is terribly a innefitient circuit... the way to go (in my opinion) for a battery application (like a flashight) is the beautiful circuit Doug Owen created for the MilkyCandle. It is SUPER efficient and has a "dropout" of almost nothing, so you can run an LED that requires 3.3 volts off of something from 3.4 volts up to something like 12 volts! Great little circuit!

If you shoot him a message I'm sure he'll send you the schematics... he'd probably even send you the parts to try to put it together.... Doug is a great guy like that!


----------



## unclearty (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

His site is here:
http://worldtorch.com/web/modules/tinycontent/index.php?id=5


----------



## Draco_Americanus (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*reviewum said:*
I'm trying to run 4 5mm LEDs at 20mA each. I have two serial sets of two in parallel. Two LEDs in series that require 3.3 volts PLUS the circuit that uses 3 volts will require 9.6+ volts to run. Using only two LEDs in parallel, you'll need about 6.6 volts to run them with this circuit (if I'm wrong someone will correct me). This is the circuit I had running on my desk and it's been running just fine.


[/ QUOTE ]
just a quick coment. if you have two 3.3 volt LEDs parallel with each other the voltage is still 3.3 volts. It's the current that needs to dubble to gain full brightness, as stated before too if each LED does not have it's own current limiter you risk blowing up the LED.
/ubbthreads/images/graemlins/dedhorse.gif


----------



## Justintoxicated (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

If anyone actualy read the link I posted, the LM317T is capable of running 8 Luxeons in series!

The thing that no1 has been able to answer clearly is why I cannot run 2 of the same color LED with different Vf or even with the same Vf in parallel, but this works fine in series.

From my personal experience, it seems the Circuti OVERDRIVES the leds when run in parallel, but everyone on here says no? Then why did it kill my LEDs? Anyone?


----------



## evan9162 (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

It will work fine in parallel, with LEDs of approximately the same Vf. The LED with the lower Vf will have more current flowing through it. 

For LEDs with substantially different Vfs (I.E. white + red), the LED with the lower Vf will light up, while the higher Vf may only be very dim.

For example, lets say you have a white Luxeon and red Luxeon. The white Luxeon's Vf at 350mA is 3.3V, the red's at 350mA, 2.8V. Put these in parallel with an LM317 setup in constant current mode at 350mA.

Now, how the LM317 works in current mode is that it adjusts the output voltage until the current matches what you set it at. It accomplishes this by measuring the voltage across the current set resistor, then adjusting the output voltage until the voltage across that resistor is 1.25V.

So, you've configured the 317 to output 350mA. Now, the red luxeon will allow 350mA to flow through it at 2.8V. So, the output voltage (after the current set resistor) will never exceed 2.8V, because that's all the higher it needs to be for 350mA to flow. It will, in fact, be a little lower. At 2.8V, about 10-20mA will be flowing through the white LED (dimly glowing). So, in reality, about 340mA will flow through the red LED, and about 10mA through the white LED.

Assuming you've wired everything correctly, and nothing bad happens (shorts, bad components, overvoltage, etc), once you've wired the LM317 up as a current source to output 350mA, there is NOTHING you can do to the load to make more than 350mA flow from that regulator - nothing. It doesn't matter what you attach to the output - bulbs, a resistor, LEDs, a cat, a short circuit - no more than 350mA will flow from the regulator (less may flow if the voltage drop of the load is too high).

Now, lets assume that you use two identical LEDs with identical Vfs. Then when you wire those in parallel, each will get exactly 1/2 the current flowing through them. Remove one, and the remaining will have the full current flowing through it.

For LEDs in series, there is only one current path, so they all have the same current flowing through them. What this "looks" like to the regulator is a load with twice (or N-times, given the number of LEDs) the voltage drop. Remove one, two, all of the LEDs, the same amount of current will flow from the regulator.

Now, as to why your LEDs were fried - we can't tell without knowing everything about your circuit configuration (and having a look at it). I do know this - if you wired something wrong, you could have shunted Vin just through the resistor to your LEDs. With a 12V source, this would fry your LEDs in no time. Another possible failure mode is if the Adj pin got shorted to the output pin. If this happens, the regulator runs wide-open. Now, an LM317 is supposed to have a 1.5A internal current limiter - however, it can supply 3.4A pulses momentarily. This easily exceeds the bond wire fusing current of a 1W luxeon, and, if the Vf differences are large enough (and the current distribution unbalanced enough), it could cause one of the luxeons in parallel to blow...then immediately after, the other (since all of the current is flowing through the remaining luxeon). If you had a short, where Adj got shorted to the output pin, it could have pumped that much current through your Luxeons, overheating or frying them in the process.


----------



## Draco_Americanus (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Well I am sorry then, I have read every post in this thread it it explanes very well to me why you can not use LEDs in parallel UNLESS you use a current limiter for each led. 
leds of diffrent colors have a slightly diffrent internal make up and because of that each one uses slightly diffrent voltage. even a red and green led have a diffrent forward voltage even though they are 20 ma. If you have these in parallel and wish to run both of them at 20 ma and use a single current limit device for the total 40ma. (for full brightness each led will need 20ma each since current devides in parellel and voltage stays the same one will need 40ma)LEDs are unlike lightbulbs once you meet the min turn on voltage they will fully conduct as mutch current as they can unless it's limited, think of it as a wire when it's turned on. In parellel the led with the lowest turn on voltage will conduct first and turn on before the other one will. It will take the full 40ma and quickly burn out. The next led will then turn on and burn out as it's current has been exceeded. This problem is solved if EACH led has it's own current limiter. you can prove this in an expement. hook two leds in parellel and only use 1 current limiter as in the origial circuit BUT set the max current at 20ma. you will notice that one led will be brighter then the other one even if they are the same bin number. Since the total current was preset at 20ma nothing will burn up. I am sorry if it still sounds confusing as leds are a totaly diffrent beastie then normal lightbulbs. I still find them to be amazing devices and a bit confusing too.


[ QUOTE ]
*Justintoxicated said:*
If anyone actualy read the link I posted, the LM317T is capable of running 8 Luxeons in series!

The thing that no1 has been able to answer clearly is why I cannot run 2 of the same color LED with different Vf or even with the same Vf in parallel, but this works fine in series.

From my personal experience, it seems the Circuti OVERDRIVES the leds when run in parallel, but everyone on here says no? Then why did it kill my LEDs? Anyone? 

[/ QUOTE ]


----------



## Justintoxicated (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*evan9162 said:*
It will work fine in parallel, with LEDs of approximately the same Vf. The LED with the lower Vf will have more current flowing through it. 

For LEDs with substantially different Vfs (I.E. white + red), the LED with the lower Vf will light up, while the higher Vf may only be very dim.

For example, lets say you have a white Luxeon and red Luxeon. The white Luxeon's Vf at 350mA is 3.3V, the red's at 350mA, 2.8V. Put these in parallel with an LM317 setup in constant current mode at 350mA.

Now, how the LM317 works in current mode is that it adjusts the output voltage until the current matches what you set it at. It accomplishes this by measuring the voltage across the current set resistor, then adjusting the output voltage until the voltage across that resistor is 1.25V.

So, you've configured the 317 to output 350mA. Now, the red luxeon will allow 350mA to flow through it at 2.8V. So, the output voltage (after the current set resistor) will never exceed 2.8V, because that's all the higher it needs to be for 350mA to flow. It will, in fact, be a little lower. At 2.8V, about 10-20mA will be flowing through the white LED (dimly glowing). So, in reality, about 340mA will flow through the red LED, and about 10mA through the white LED.

Assuming you've wired everything correctly, and nothing bad happens (shorts, bad components, overvoltage, etc), once you've wired the LM317 up as a current source to output 350mA, there is NOTHING you can do to the load to make more than 350mA flow from that regulator - nothing. It doesn't matter what you attach to the output - bulbs, a resistor, LEDs, a cat, a short circuit - no more than 350mA will flow from the regulator (less may flow if the voltage drop of the load is too high).

Now, lets assume that you use two identical LEDs with identical Vfs. Then when you wire those in parallel, each will get exactly 1/2 the current flowing through them. Remove one, and the remaining will have the full current flowing through it.

For LEDs in series, there is only one current path, so they all have the same current flowing through them. What this "looks" like to the regulator is a load with twice (or N-times, given the number of LEDs) the voltage drop. Remove one, two, all of the LEDs, the same amount of current will flow from the regulator.

Now, as to why your LEDs were fried - we can't tell without knowing everything about your circuit configuration (and having a look at it). I do know this - if you wired something wrong, you could have shunted Vin just through the resistor to your LEDs. With a 12V source, this would fry your LEDs in no time. Another possible failure mode is if the Adj pin got shorted to the output pin. If this happens, the regulator runs wide-open. Now, an LM317 is supposed to have a 1.5A internal current limiter - however, it can supply 3.4A pulses momentarily. This easily exceeds the bond wire fusing current of a 1W luxeon, and, if the Vf differences are large enough (and the current distribution unbalanced enough), it could cause one of the luxeons in parallel to blow...then immediately after, the other (since all of the current is flowing through the remaining luxeon). If you had a short, where Adj got shorted to the output pin, it could have pumped that much current through your Luxeons, overheating or frying them in the process. 

[/ QUOTE ]


Very strange then...Because The circuit is still in use today, All I did was replace the LED's Blue Star_O's and wire the new ones in series and no more problems! THe circuit was mroe complicated than just the LM317, but the rest just converted Variable AC to DC cap rectifier etc...

Thanks for the good explanation though. That actually Makes sense! On the other hand, now it appears it would be more dangerous to run them in series? 

THe odd thing is, when I ran multiple Luxeons in series, both luxeons had a differnt Vf and boh measured the current I had the LM317T set for...Since I know for sure (repeated the measurements multiple times with a Fluke 77), it appears that the LM317 does not regulate the voltage? (how could I get different votlages? I even swapped the order of the LED's and they both still pulled a different voltages (the one with the lower was still lower and the one with higher vf was still higher despite the order) and both still measure the same current????????


SO again, I am confused

Drako, you can run multiple LED's you just have to use the LM317 with the LED's in series, this forces the set current to run through all of them. I don't understand both these strange phenomanons with my experimentation but I did blow them up in parallel and I did measure different voltages in series! lol I would NOT run different color LED's in series Unless they have the same or almsot the same VF (if you don't mind overdriving one slightly) OF course from my exerimentation, the LEd's ran at their desired Vf's anyways, Perhpas this deals with the LED's internal resistance as was mentioned and not with the voltage the LM317 is providing. 

If all this information is correct then it would be better to run the Low Vf LED After the High Vf one (talking 1 bin difference) this way the resistance of the high Vf LED might just take a little strain off the Low Vf one? /ubbthreads/images/graemlins/banghead.gif /ubbthreads/images/graemlins/banghead.gif /ubbthreads/images/graemlins/dedhorse.gif /ubbthreads/images/graemlins/knight.gif


----------



## evan9162 (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Justin,

You're confusing yourself again. It makes no difference what order you put the LEDs in when they're wired in series. Each LED will drop a certian amount of voltage at its rated current - this is known as the Vf. 

Lets put the red and blue LEDs in series with the LM317 set for 350mA. 350mA flows through both LEDs, since there is only one current path. The blue will drop 3.3V, the red, 2.8V. If you measured the total voltage across both LEDs, you would get 6.1V.

Now, if you had a 5W LS, you could put that in series with the red. The LM317 is still set for 350mA. A 5W LS may have a Vf of 6.8V at 700mA, but only say 6.5V at 350mA. So, with the 5W in series with the 1W red, both will have 350mA flowing through them. The 5W will drop 6.5V, the red, 2.8V. If you measured the voltage across both LEDs, it would show 9.3V. 

The order of the LEDs makes no difference. Look at it from the regulator's standpoint. With the red/blue in series, the regulator "sees" a load with a voltage drop of 6.1V. Re-arrange the order of the LEDs, it will still be 6.1V. With the red and 5W, the regulator sees a load with a voltage drop of 9.3V. As long as the current is set to a safe level for all components that are in series, you can put whatever you want in series with whatever else you want.

So, configure the LM317 to output 20mA. You can now put a diode, 5mm LED, resistor, light bulb, red luxeon, and 5W luxeon all in series with each other, and only 20mA of current will flow. You can remove any one of those components, and the circuit will behave exactly the same as when it was in place. Each component will have the same voltage across it as before, and the same amount of current will be flowing as before.

If anything, a series circuit is safer. You are assured that every component has the same current flowing through it. 

If a component fails open-circuit (non-conducting), then no more current flows, saving other devices from destruction - in the parallel configuration, if a device fails open circuit, then the current that was flowing through it must flow through the remaining devices in addition to the current already flowing through those devices.

If the device fails short-circuit, then the remaining components still have the same current flowing through them as before. In the parallel configuration, the device that fails short-circuit will have ALL of the current from the regulator flowing through it. The other devices will stop working, and other bad things could happen as a result - the shorted device could heat up tremendously then start smoking, catch on fire, etc (this is assuming that several devices are in parallel, so the shorted device now has 3-4 times normal current flowing through it).

With a series configuration, you can put a single fuse inline with the devices, so if any overcurrent situation happens, the fuse blows. You can have a low-value fuse that will blow quickly and protect all devices in series.

Finally, with them all in series (I kinda hinted at this), you can remove a device from the series without needing to reconfigure your circuit to provide the proper current for the remaining devices. 

[ QUOTE ]

THe odd thing is, when I ran multiple Luxeons in series, both luxeons had a differnt Vf and boh measured the current I had the LM317T set for...Since I know for sure (repeated the measurements multiple times with a Fluke 77), it appears that the LM317 does not regulate the voltage? (how could I get different votlages? I even swapped the order of the LED's and they both still pulled a different voltages (the one with the lower was still lower and the one with higher vf was still higher despite the order) and both still measure the same current????????


[/ QUOTE ]

The phenomenon you observed is exactly how LEDs behave. What you also observed is the principle of equivalent circuits. According to the regulator, the two LEDs in series look to it just like a load with a voltage drop that equals the sum of the Vfs of the two LEDs. The 317 only is concerned with regulating the current flowing out of it. The only thing that really matters is the overall load. Think of the load (your two LEDs in series) as being a black box with two wires coming out of it - + and -. When the load is hooked up to the LM317 in current mode, the only thing that really matters to the LM317 is making sure that X current flows through that load (where X is what you've set it to). It has no idea (nor really any concern) about what's inside that box, how many components there are in the load, or what order they are in. Just as long as X current flows through that box, the LM317 is regulating properly.

Electrical loads drop a certian amount of voltage at a given current (or pass a certian amount of current for a given voltage, depending on how you look at it). Purely resistive loads follow Ohms law - V=IR . Semiconducting circuits and motors are slightly different. A so called perfect diode will drop the same voltage regardless of current (except for 0, at which point there is no voltage drop, and no current, so it really doesn't matter).

So, let's use a perfect diode and a 10 ohm resistor as the load. At 350mA, the diode will drop 0.7V. At 350mA, the resistor will drop 3.5V (V=IR - V=10*0.35=3.5). Since they're in series, the total voltage drop is 4.2V. Regardless of which order you put them in, the voltage drop will always be 4.2V - the diode will always drop 0.7V (you can measure 0.7V across it), and the resistor will always drop 3.5V. The regulator will always see a load that drop 4.2V at 350mA. Now, drop your two LEDs in series in there instead. If one has a Vf of 3.3V, and the other, 3.6V, then the regulator will see a load that drops 6.9V. Regardless of the order, you will always measure 3.3V across LED #1, and 3.6V across LED #2.


----------



## Draco_Americanus (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

yeh, it's generally better to run them in series when you have the needed forward voltage. 
When I was also discribing the led experiment I would use the same color led and if possible the same bin numbers, 1 will still be brighter. I chose the little 20ma ones just incase there would be a "poof" /ubbthreads/images/graemlins/mad.gif if the current was set wrong. those are much cheaper then luxeons. 
The order in for your luxeons does not matter, you will still have the same voltages across the leds. 
yeh it's the leds internal resistance that dictates the voltage. technically leds are considered to be a quantum device. I don't fully understand the inner workings of them my self but with basic knowledge one can do great and bright things with them /ubbthreads/images/graemlins/cool.gif /ubbthreads/images/graemlins/evilgrin07.gif


----------



## Justintoxicated (May 7, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Ok thats actualy what I was thinking, just wanted clarification. I was told that it will regulate the same voltafe for both LED's before in which one LED would then be overdriven, but this made no sesne to me!

The last time this subject came up everyone told me it is impossible to have a circuit that will allow each LED to run at it's designated Vf and that I would need a different Regulator for each different color..I thought they were wrong and now that you have clarified I understand perfectly. It works exactly How I thought! True Current regulation, not voltage regulation...The voltage really does not matter, as long as you have more volts than the LED's need.

I'm glad I finaly got this clarified this is EXACTLY how I thought it worked a while ago, untill others told me I was incorrect.

Anyways I have also brought up the inline fuse idea, and it was dismissed as Pointless! Everyne tells me that an overdose of current would still blow the Luxeons before a Quick Blow Fuse...So I don't know if a fuse will work and I refuse to test the concept (I have killed enough of these guys)..I believe I am running at least one fuse for the 350ma just because I had already purchased it before the idea was dismissed as pointles So time MAY tell, but hopefully it will not!


----------



## HarryN (May 8, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Hi - I made a very - very simple ciruit with an Intersil 756 timer. It is just a basic astable mode and I drove a small Radio Shack RED led directly off of the output. (I have no electronics metrology, so it was just guessing)

R/S also has a small power supply paperback that shows how to make some basic circuits using 555 style timers. I used the Intersil ones because they are cmos (low power) wide input range, and claim good stability. They are pretty cheap as well.

The next step for me is to use it to drive a mosfet, and work my way up to a more or less "real" switching circuit. 

Learning to crawl, then


----------



## Doug Owen (May 8, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Justintoxicated said:*
If anyone actualy read the link I posted, the LM317T is capable of running 8 Luxeons in series!



[/ QUOTE ]

This has been a way cool discussion, lots of good experience and insight floating around.

As just another (hopefully useful) data point, the above is not a real limit. If you're careful to never exceed the maximum voltage rating of the part (37 Volts) there's no real limit to how many a single part can drive.

For sure the most common 'higher voltage' use for us will be nominal 12 Volt systems, meaning generally 3 (white) LEDs in series.

Doug Owen


----------



## Doug Owen (May 8, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Draco_Americanus said:*
Well I am sorry then, I have read every post in this thread it it explanes very well to me why you can not use LEDs in parallel UNLESS you use a current limiter for each led. 


[/ QUOTE ]

While you need to be careful with different colors (chemestries) and with grossly mismatched devices, and those driven near the edge, I don't think it's always true you need current distribution resistors.

There's about 15 ohms Rth in 5 mm parts, plenty to handle the task if the LEDs are close to start with. My testing shows that you don't even have to select sets by screening for Vf. LEDs from the same batch work OK in parallel within reason. 

For sure a lot of makers do this all the time, right?

Doug Owen


----------



## Doug Owen (May 8, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Draco_Americanus said:*
yeh, it's generally better to run them in series when you have the needed forward voltage. 

[/ QUOTE ]

Absolutely. First off, you get to 'use the same current twice' (or more). The 'extra voltage' is just wasted as heat somewhere, most likely in your regulator. Raising it's cost, shortening it's life, costing you more.

Yup, keep the *headroom* (voltage across the regulator, 'wasted energy') low, and the current draw low. Series does both these, parallel does neither.

Doug Owen


----------



## Doug Owen (May 8, 2004)

*Yup, stick with current limit*

[ QUOTE ]
*Justintoxicated said:*
Ok thats actualy what I was thinking, just wanted clarification. I was told that it will regulate the same voltafe for both LED's before in which one LED would then be overdriven, but this made no sesne to me!

[/ QUOTE ]

Yup. Shouda stood yer ground.

Doncha just hate that? You're right and some fool changes your mind for you. Sometimes it can take for ever for them to convince you you were right in the first place.....

IMO Current regulators are "the only way to go". Those who recommend (or use) voltage regulators either don't really know what they're doing, are covering up for some other design problems or considerations, or just like skating on thin ice. 

My advice continues to be 'don't go there'.

Doug Owen


----------



## Doug Owen (May 8, 2004)

*Playing with LEDs and stuff....*

[ QUOTE ]
*HarryN said:*
Hi - I made a very - very simple ciruit with an Intersil 756 timer. It is just a basic astable mode and I drove a small Radio Shack RED led directly off of the output. (I have no electronics metrology, so it was just guessing)

R/S also has a small power supply paperback that shows how to make some basic circuits using 555 style timers. I used the Intersil ones because they are cmos (low power) wide input range, and claim good stability. They are pretty cheap as well.

The next step for me is to use it to drive a mosfet, and work my way up to a more or less "real" switching circuit. 

Learning to crawl, then 

[/ QUOTE ]

Harry,

Cool. You might want to get a few 'real' 555s to play with as well. Pin for pin compatible parts, you can pull one and replace it with the other (generally). The 555 can source or sink 200 mA or so, you can drive Luxeons fairly well directly with them (resisor recommended of course), they don't use much current really, and they're cheaper.

More, since you, Rob and I are all local, perhaps we should get together sometime and play with electricity?

Fer instance, you're half way to your 'trail marker' idea. Use the 755, a small fet (like 2N7000) a resistor and *red* (or orange or yellow) LED with the 123 cell......

Doug Owen


----------



## Justintoxicated (May 8, 2004)

*Re: Playing with LEDs and stuff....*

Doug, I know it is not a set limit you might even be able to drive a few more, but you are really pusing it at Luxeon 8 LED's with Vf's over 3.5.

As Much as I hate it, I am glad I have it all straightened out now. It works jsut like it should, and you should also be able to drive different color luxeons (long as they are all the same type I, III, or V). Great news for me /ubbthreads/images/graemlins/smile.gif


----------



## evan9162 (May 8, 2004)

*Re: Playing with LEDs and stuff....*

Justin,

Since the 317 is a floating regulator, it has no direct connection to ground. the 37 volt limit is just the maximum voltage the 317 can drop, but has no relation to the input voltage that the 317 is regulating.

You could drive 20 luxeons in series with a 317. Assuming an average Vf of 3.3V at 350mA, the load would drop 66V. Anything between 70VDC and 90VDC input would work fine. The 317 would only be dropping between 3 and 23 volts.

Likewise, if the 317 is set up as a voltage regulator, just as long as the 317 doesn't drop more than 37 volts, you are doing fine. So you could regulate 100VDC given 110VDC input, or any similar situation.


----------



## Justintoxicated (May 8, 2004)

*Re: Playing with LEDs and stuff....*

Interesting, but I swear the spec sheet said there was definately a limitation n Vin, AND a limitation on the POWER it could drop...

Guess I'll have to take a look again when I get some more time.


----------



## Doug Owen (May 8, 2004)

*Re: Playing with LEDs and stuff....*

Evan,

That would be what I was trying to say, thanks for saying it in a different (and hopefully) clearer manner.

It's very true, the bird on the power wire is in no danger.

Doug Owen


----------



## Draco_Americanus (May 8, 2004)

*Re: Playing with LEDs and stuff....*

the spec sheet I belive has it set up as a normal pass type regulater with respect to ground, so it's not floating like it is with the current limit. it should beable to handle it as long as the wattage it disipates is with in it's spec and voltage across the part is with in the spec. /ubbthreads/images/graemlins/thumbsup.gif


----------



## Doug Owen (May 9, 2004)

*Re: Playing with LEDs and stuff....*

[ QUOTE ]
*Draco_Americanus said:*
the spec sheet I belive has it set up as a normal pass type regulater with respect to ground, so it's not floating like it is with the current limit. 

[/ QUOTE ]

If you examine the basic variable voltage regulator circuit for the 317 you'll see that while the output is indeed referenced to gound the device itself is not. Only to the positive supply input and the output. By definition, when it's running the adjust terminal is 1.2 Volts (or *less*) more negative than the output but otherwise very close to the input/output Voltages at all times. I've used the standard parts on 180 VDC supplies before, it works fine. The device doesn't know (or care) it's humming along 200 VDC above ground. Like the birds on the power lines.

Doug Owen


----------



## NewBie (May 9, 2004)

*Re: Playing with LEDs and stuff....*

[ QUOTE ]
*evan9162 said:*
Justin,

Since the 317 is a floating regulator, it has no direct connection to ground. the 37 volt limit is just the maximum voltage the 317 can drop, but has no relation to the input voltage that the 317 is regulating.

You could drive 20 luxeons in series with a 317. Assuming an average Vf of 3.3V at 350mA, the load would drop 66V. Anything between 70VDC and 90VDC input would work fine. The 317 would only be dropping between 3 and 23 volts.

Likewise, if the 317 is set up as a voltage regulator, just as long as the 317 doesn't drop more than 37 volts, you are doing fine. So you could regulate 100VDC given 110VDC input, or any similar situation. 

[/ QUOTE ]

90-66V = 24V

24V * 0.35A (350ma) = 8.4W

You're gonna be dissipating some serious heat there...plan ahead.


----------



## NewBie (May 9, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*Draco_Americanus said:*
Or has some IC maker made a chip that can do that easy (I.E Maxum's buck boost converter but from a step down insted of a step up point of view) 

[/ QUOTE ]

Yes, there are thousands of chips for doing switching regulator buck supplies, TI, Linear, Maxim, Semtech, Micrel, Fairchild, National Semiconductor, Intersil, and many, many others make them. It is quite easy to get in the 90-95% efficiency range, and 98% efficiency is possible.


----------



## Doug Owen (May 9, 2004)

*Re: Playing with LEDs and stuff....*

[ QUOTE ]
*NewBie said:*
[ QUOTE ]
*evan9162 said:*
24V * 0.35A (350ma) = 8.4W

You're gonna be dissipating some serious heat there...plan ahead. 

[/ QUOTE ]

Very important point. These rascals have self protection built in, over temperature shutdown. However, in higher voltage uses this means the differential voltage across it is going up as well. It won't die from overtemp, but overvoltage will get it....

I agree, plan on keeping it cool.

Doug Owen


----------



## Doug Owen (May 9, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*NewBie said:*
It is quite easy to get in the 90-95% efficiency range, and 98% efficiency is possible. 

[/ QUOTE ]

I'm not sure I'd agree with that, at least as far a *practical* lights made today. Numbers like 70 and 80% seem more common. A switcher that is low power, very efficient over a wide range of inputs, is small and cheap is not an easy design to resolve.

And I suggest it's only easy for a very few individuals, if even for them. Most CPFers are likely to find it challenging at some level.

At least as I see it.

Doug Owen


----------



## NewBie (May 9, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

Humm, typical cpfer for me...

Dat2zip, Georges80, and myself, I'm definitely average.

I bet DougS could even do it.

(heheh, I know, just kidding)


----------



## gadget_lover (May 13, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

I picked up a few LM317T to use with a wall wart to make a task lamp for use in my garage. Efficiency is not an issue there.

But I got to thinking... how efficient is the LM317 compared to a simple resistor? It's acting as an automatically variable resistor, so wouldn't it be about the same?

Daniel


----------



## Justintoxicated (May 13, 2004)

*Re: LM317 Simple Constant Current Circuit & Luxeon*

[ QUOTE ]
*gadget_lover said:*
I picked up a few LM317T to use with a wall wart to make a task lamp for use in my garage. Efficiency is not an issue there.

But I got to thinking... how efficient is the LM317 compared to a simple resistor? It's acting as an automatically variable resistor, so wouldn't it be about the same?

Daniel 

[/ QUOTE ]

Hmm very true!. The only thing I have used them for so far is to regulate current flow from a variable output voltage source...In other words, for an application a simple resistor would suck for...I wanted the light to turn on at low voltages 4+ but not be anymore overpowered or underpower when the votlage varied or reached a peak of 14V (Power was dependent on speed and engine RPM in this case) I didn't just want to have bright lights at full speed because I'm not always going that fast! it would also make sense for an application in which your turning on or off multiple lights...

Just to power 1 light from a constant voltage source I think a resistor would work also. The only advantage to the LM317 I can see is that it will keep the current constant even after the LED has heated up (since there is fluctuation of Vf with temperature), where a resistor is just a set value, but probably good enough)...

I also plan to use one to make an LED tester soon...


----------



## HarryN (May 21, 2004)

*Re: Playing with LEDs and stuff....*

Hi Doug Owen

Sorry for the delayed response. I had not checked this thread for a while. I would really enjoy the chance to get some opinions about some of my beginner project ideas. A get together would be great, especially if you are feeling patient that day.

I tried using a bipolar based NTE 555 timer from (radio shack or frys - can't remember), and the circuit did not work. Frankly, that is pretty normal for my circuits, so I was not that surprised, just disappointed that once again, I cannot seem to follow a simple diagram, or the parts are bad, or - who knows.

My longer term goal is to be able to run this light over a very wide Vin range, and a very wide Temp range, so I purchased the Intersil 756 part in a mil spec (-55 C to hot), and it will run from 2 V (3 V when really its really cold) up to 18 V with no additional regulation. I got it from Newark for around $ 5, so really not a big price issue. The same part rated for (-40 C) is about $ 1.

With this kind of Vin range, in theory, I can put 5 x 123 Li cells in series, and drain them really dry, maybe below what some would consider reasonable or desireable.

I switched to the ICM756 and rebuilt the circuit - and it worked this time, with a target frequency of 100 Khz. (based on the RC components, no idea what it actually is)

The LED driven was just a slightly larger than normal red LED from RS. I used 3 x D cell alkaline for power.

I was able to achieve two light levels by just playing with the Vcc pin - full brightness with Vcc hooked to Vin, and a lower brightness level by connecting Vcc to ground. I had hoped for more levels with different Vcc voltages, but it did not seem to work.

I am saving the second timer in the 756 to make the LED act as a flasher.

I tend to not be all that kind to electronics, which might explain my historical challenges with them, but it seems like I should be able to take the Vout pin directly to an IR MOSFET, and directly drive the a 1 - 3 watt Luxeon (without the resistor or inductor).

The main concern I hear on this seems to revolve around Ipeak, which I cannot measure, but there are a lot of losses in the circuit, so maybe this is clipped enough to work.

I know that in the extremely high efficiency buck circuits on the forum, there is a desire to get past the linear part of the control transistor, so the focus is on Rds, but if I would be satisfied with 70% efficiency (at this stage), then the losses in the MOSFET would seem to be certainly no worse than having a managed resistor setup.

I keep thinking that there should be a way to take the Vin through some kind of comparator, and have it drive the Vcc pin with a voltage that would alter the frequency or perhaps the duty cycle enough to have a linear (enough) power supply for the Luxeon.

Anyway, that is what I am toying around with for fun. When I need something to really work, I have some boards from Georges80 and leddynamics.


----------



## Doug Owen (May 21, 2004)

*Re: Playing with LEDs and stuff....*

[ QUOTE ]
*HarryN said:*
I would be satisfied with 70% efficiency (at this stage), then the losses in the MOSFET would seem to be certainly no worse than having a managed resistor setup.



[/ QUOTE ]

"Managed resistor". I like the term. 

Yes, that's what a linear regulator tries to do. Be kind of a smart resistor. And for sure some loss is always present, at at Luxeon levels, 30% or so can be a lot of heat to deal with. This means heatsinks. 

I'm not sure about your overall goal, but by all means we should get together some afternoon and play a bit with this stuff. Something to look forward to.

Doug Owen


----------



## LEDboy (May 28, 2004)

*Re: Playing with LEDs and stuff....*

Hello,
Does any body know of a better solution than a LM317 which has a lower drop out voltage, say 0.2V?
I have found a solution on the net which uses 2 FETs and a resistor to do the job, but no details of the FET parts.
http://www.elecdesign.com/Articles/ArticleID/4914/4914.html
Has anybody tried this?
Does anybody know of a switching solution, with low component count?

Regards
James


----------



## reviewum (May 28, 2004)

*Re: Playing with LEDs and stuff....*

Doug's worldtorch / milkycandle circuit does this (I'm not sure how low the dropout is, but it is TINY. He'll send you a kit that is super easy to build for only $4.
http://www.worldtorch.com


----------



## LEDboy (May 31, 2004)

*Re: Playing with LEDs and stuff....*

Thanks for the link Flashaholic,
I should of said in my earlier post that I want to drive Luxeon LEDs. I am already aware of the LM334 based circuits, but it is the FET based design found here
http://www.elecdesign.com/Articles/ArticleID/4914/4914.html
that can solve the higher current issue. 
This article describes in great detail the circuit, except for the FET part details. It talks about using the latest FETs and was posteed back in 2000, so I am hoping 'the latest FETs' are now widly available.


----------



## Doug Owen (May 31, 2004)

*Re: Playing with LEDs and stuff....*

[ QUOTE ]
*LEDboy said:*
Thanks for the link Flashaholic,
I should of said in my earlier post that I want to drive Luxeon LEDs. I am already aware of the LM334 based circuits, but it is the FET based design found here
http://www.elecdesign.com/Articles/ArticleID/4914/4914.html
that can solve the higher current issue. 
This article describes in great detail the circuit, except for the FET part details. It talks about using the latest FETs and was posteed back in 2000, so I am hoping 'the latest FETs' are now widly available. 

[/ QUOTE ]

While this is an improvement on the all bipolar version, it's far from 'the solution' to LDO. By running the PNP on the ragged edge of conduction, Vbe of .55 Volts is had, but the drop out is still .6 Volts total. This is because Vsense is still big (550 mV).

The LM334 uses 65 mV, a *big* improvement. This can lower total drop out to under .1 Volt total. More over, while the 334 isn't all that stable WRT temperature, it's sure to be way better than the bipolar. I suspect the bipolar stability is really very poor.

There's nothing there to indicate (at least to me) that this circuit 'solves the higher current issue'. Current is dependant on the FET chosen (like it would be for the bipolar in the typical 334 based system).

As far a parts go, you're looking for a p chanel MOSFET, something like IRF9540.

FWIW, I don't know who told you you can't drive Luxeons with 334 based regulators, but they lied.....

Doug Owen


----------



## LEDboy (May 31, 2004)

*Re: Playing with LEDs and stuff....*

I think I have just answered my own question here.
http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html
Will try this with a IRL530 and skip the dimming bit.


----------



## Doug Owen (May 31, 2004)

*Know what you\'re up to before playing......*

[ QUOTE ]
*LEDboy said:*
I think I have just answered my own question here.
http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html
Will try this with a IRL530 and skip the dimming bit. 

[/ QUOTE ]

I wouldn't suggest that, as that's a *N channel* part, meaning the 'gate logic' is backwards from what you want. The regulator will 'go the wrong way' and cost you a LED each time you try. Note how the little arrow goes the other way? That's important......

MOSFETs are not interchangable. There's both P and N channel, enhancement and depletion mode to choose from (four total 'types').

IRF95X0 series parts will work (they're P channel), but I'd still go with the 9540 for lowest on resistance, highest current capacity (not that you need all them amps) and most sensitive gate.

Doug Owen


----------



## LEDboy (May 31, 2004)

*Re: Know what you\'re up to before playing......*

Thanks for the replay Doug,
I have changed cicuits, the one from the tripod link uses a N channel.
I have just built this circuit but used a Bs295 as my IRL530 is dead, also i used a BC108 instead of 2N2222 and made the sense resistor 0.6. I can confirm I get 280mA.

The reason I favor this design over LM334 is that the LED is not floating. Please correct me if I am wrong but is it possible to have low drop out and a non floating load with a LM334?


----------



## Doug Owen (May 31, 2004)

*Re: Know what you\'re up to before playing......*

[ QUOTE ]
*LEDboy said:*
The reason I favor this design over LM334 is that the LED is not floating. Please correct me if I am wrong but is it possible to have low drop out and a non floating load with a LM334? 

[/ QUOTE ]

You are incorrect. Using a similar N channel pass element, the same configuration (anodes to positive supply) is possible. You'll no doubt get better stability from such a circuit as well as lower drop out (although .2 Volts is getting right down there for sure).

How does your current vary with changes in battery voltage and temperature?

Doug Owen


----------



## LEDboy (Jun 1, 2004)

*LM334 Circuit for Driving Luxeons*

I don't know how to use a N channel with the LM334, did you mean P channel?
I have revisted the LM334 circuit and modified it, by changing the base resistor from 390 to 300, the sense resistor to 0.22 and the transistor to 2N6109 and keeping the load floating. The load is a 1W warm white Luxeon and I have measured the Vf of 4V at 280mA. I am getting 240mA max. The problem seems to be the transistor. So I tried disconnecting the base resistor from the LM334 and connecting straight to 0V, result 247mA, then same thing with 33 resistor, result 257mA. From this am thinking that the Vec of the transistor cannot go low enough and either way the LM334 cannot sink enough to drive it anyway.
So now I need to find a suitable transistor, or try a P channel FET.

I have also tried moving the load for both common 0V or common Vcc operation, either way the Vf of the LED robs the LM334 of working voltage and I get 70mA.
Doug I don't what circuit you had in mind from your last post.

As to the circuit from the tripod link, I am getting .4V across the sense resistor, not .2 thats required.
Thinking about it prehaps thats the problem, I am using a solderless breadboard, and maybe it can't cope with 350mA.

James


----------



## LEDboy (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

Forgive me,
It is the series resistance of my ammeter was causing low current readings!
I can confirm both designs work fine.

James


----------



## Doug Owen (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

2N6109 wouldn't be my first choice, but it should work fairly well. You really want more gain (30 minimum isn't much when you only have 10 mA of drive) and low Vsat (meaning you need to overdrive the base, high gain again). Some of the Zetex PNPs (look for part numbers with '717' in them) work well if you keep an eye on power.

Otherwise, you should be able to use a small signal PNP to drive the FET. Use it as I am but return the collector to ground through a few thousand ohms (not to the top of the shunt resistor (R input on the 334)). FET gate goes to collector. LED anode to plus, cathode to Drain, Source to the shunt (and R input), shunt to ground (as it is now).

The 334 drives the PNP on hard enough to raise the gate voltage enough to turn the FET on enough to pull the rated LED current and satisfy the '65 mV rule' for regulation.

I haven't done it (stability might be an issue, the R/C snubber may need a tweak), but it should work fine. Plenty of similar circuits (including some cited above) abound.

Anyway, glad to hear it's working for you.

Doug Owen


----------



## LEDboy (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

Thanks a lot for help Doug,
I have built the circuit described above and used the same 2N3906 for the PNP driver, BSS295 FET (because thats all I have to hand), PNP collector resister 3K9, kept the snubber RC as is and reduced the sense resistor to 0.18. Result I get 342mA perfect.

James


----------



## Doug Owen (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

You're welcome, James. Glad you tried it and had success. I don't know the FET you used, but it shouldn't matter all that much. The key is keeping the PNP stable (which it should be with the values used), it needs to be checked. I'd be careful if the PNP gets close to saturation (could be an issue if Vg has to be fairly high for the FET you use), but as long as Vg is less than Vbat you should be fine.

Have you checked current against battery and temperature changes?

Doug Owen


----------



## LEDboy (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

I have been running this now for around 2 hours and Vg is around 2V. The FET is a logic level one and is in a TO92 package rated at 1W with a Ron of 0.3, and should be dissapating around 400mW.
I am using 4 Ni-mh bat pack, which started out at 5.5V and has droped to 5.05V now. I plan to run until 3.6V.
If all is well I will run again at different temperatures, then work on a low voltage disconnect.

James


----------



## Doug Owen (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

Cool, thanks.

FWIW, my math is a bit different than yours, I make it closer to 500 mW (1.5 Volts times 342 mA), a fair bit more earlier on. Still under a Watt for sure, but close. Remember that one watt is in free, 25 degree air. As the flashlight warms up, you need to use the derating number to cut the power back. I'm not comfortable with anything like a Watt in the TO-92 inside something hot....

And another thought about using a FET this way, you may well want a resistor (say a few dozen K) from B to E on the PNP. The 3906 can get some awesome gains at lower currents, more so when hot. You're looking for what half a mA for Ic? If beta's up around 300 that's less than two micoamps of Ib. The 334 isn't going to do well that low down, as it's on the ragged edge. Forcing it to go to at least a few microamps 'can't hurt, might help'. Think of it in terms of chicken soup.....

Doug Owen


----------



## LEDboy (Jun 1, 2004)

*Re: LM334 Circuit for Driving Luxeons*

I was'nt paying attention and the batt voltage has dropped to 3.2V. Everything seems fine. I will re-run in the morning, but use my logger this time.

James


----------



## LEDboy (Jun 2, 2004)

*Re: LM334 Circuit for Driving Luxeons*

Hello again,
I have detailed logging results, if anybody is intrested, I will email them to you. This is for a standard 20 Degs test. I will perform a temperature test on my next run.

Cheers
James


----------



## Doug Owen (Jun 2, 2004)

*Re: LM334 Circuit for Driving Luxeons*

James,

I'm interested, PM sent.

Thanks.

Doug Owen


----------



## reviewum (Aug 21, 2004)

*Re: LM317 Simple Constant Current Circuit*

Okay guys... since I started this thread I can resurrect it! 

I'm abandoning the luxeon for now and am going to use the LM317 with my 5mm Nichias in my wife's car.

I've got about 11.5 volts from the battery when the car is off. I want to run about 6 LEDs at about 20 mA each in the dome light. 

I'm wondering what the best config would be. 3 strings of 2 LED's in series, each with their own LM317? 

Thanks in advance!


----------



## evan9162 (Aug 21, 2004)

*Re: LM317 Simple Constant Current Circuit*

Only 11.5V from your car battery?? Where are you measuring? A car battery at rest should read 13V+.


----------



## reviewum (Aug 22, 2004)

*Re: LM317 Simple Constant Current Circuit*

I'm measuring from the dome lights while the car is off before starting it up. While running it is at about 14v and after turning it off it stays at about 12+ for a while but settles back down to 11.5 or so.

But the configuration I need would work fine for anything 11 volts on up right?

So, 3 strings of 2, or another way?


----------



## Doug Owen (Aug 22, 2004)

*Re: LM317 Simple Constant Current Circuit*

[ QUOTE ]
*reviewum said:*
I'm measuring from the dome lights while the car is off before starting it up. While running it is at about 14v and after turning it off it stays at about 12+ for a while but settles back down to 11.5 or so.

But the configuration I need would work fine for anything 11 volts on up right?

So, 3 strings of 2, or another way? 

[/ QUOTE ]

First off, the 'somewhat low' voltages don't bother me. I assume you're measuring the bulb when on? There's sure to be some voltage drops from the battery (and back). If you're really interested, get a long enough lead so you can measure from each side of the battery to each side of the bulb. You'll probably find more loss on the plus side, but some on both.

Anyway, to your question, yes I'd go with three strings of two. If I was using TO220 package devices I'd probably go with one regulator and say 100 ohms in each string with all three strings in parallel. With TO92 packages I'd go three separate strings (might even consider switching two of them off with a 'high/low' switch).

Doug Owen


----------



## evan9162 (Aug 22, 2004)

*Re: LM317 Simple Constant Current Circuit*

Sure, 3 strings of 2 sounds like the best way to work with the drop to 11V. Since you're dealing with so little current (20-30mA), you don't need a heatsink on the 317. 

Are you measuring the voltage with the dome light bulb still in and running, or is this with the bulb removed?


----------

