90 Watt LED Driver - Need a bit of help here :)

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LTC
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90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Sun Mar 26, 2006 3:13 am

I know a thing or two about electronics, but when it comes to the really complex stuff, it seems like a good idea to get the assistance of those in the know. Ok so maybe this is not THAT complicated but it is beyond me right now. You guys really know your stuff. Can somebody who knows more about this than I do give me a quick hand. This is a rather unusual project...

Here is my situation. I have a 200W PC Power supply. I have 18 3w 1000 mA LEDs. That's 63 amps @ 3.5v. I need to drive all 18 LEDs, each at 1000 mA. Three parallel lines of 6 in series (on second thought, 6 lines of 3 sound better). I have a constant VOLTAGE supply (the PC Power Supply). I do not have a constant CURRENT supply, which is what I need. As you all know, when the LEDs heat up, their resistance changes. Since I intend for these to run 24/7, if one were to short...we would have a problem on our hands. I would like to avoid that.

First solution - buy a LED driver. None of the "off the shelf" constant current regulators will do the job. They are QUITE expensive to boot - and wiring up two of them is going to cost too much.
http://www.advancetransformer.com/uploa ... luxeon.pdf

However, I figured out (with this boards help) that if I have a constant VOLTAGE then I can create a circuit to maintain constant CURRENT. CPU power supplies = constant voltage, and they are pretty cheap. Since I only need ~ 90 watts.... 200W supply should be plenty (as long as it can handle 100+ across the 12v lines).

Constant voltage to constant current example circuit:
http://www.onsemi.com/pub/Collateral/AND8109-D.PDF

My main problem though is this. I know the CPU power supply can drive 100+ watts no problem, and most should be able to pull enough watts across the 12v line. How do I set this circuit up so that it can 1) handle 100 watts (safety margin) 2) can constantly adust to provide a constant current, instead of constant volts, and 3) be fault tolerant - if a led goes out or overheats - I don't want the system to fail. That branch of LEDs would fail but the other 2 branches (or 5 if it is a 3 in series, 6 of those in parallel) would keep humming. Lastly, what happenes if a single LED fails but instead of opens the circuit, simply stays closed? I assume that the runaway current increases would be managed by such a circuit?

I have been an electronics enthusiast for many years but still a bit new to power regulation - so any help you could give me would be appreciated.

Schematic:
From: http://led.linear1.org/led.wiz

12v source voltage
3.5v forward voltage
1000 ma current
18 leds in array

Solution 0: 3 x 6 array uses 18 LEDs exactly
+----|>|----|>|----|>|---/\/\/----+ R = 1.5 ohms
+----|>|----|>|----|>|---/\/\/----+ R = 1.5 ohms
+----|>|----|>|----|>|---/\/\/----+ R = 1.5 ohms
+----|>|----|>|----|>|---/\/\/----+ R = 1.5 ohms
+----|>|----|>|----|>|---/\/\/----+ R = 1.5 ohms
+----|>|----|>|----|>|---/\/\/----+ R = 1.5 ohms

The wizard says: In solution 0:
each 1.5 ohm resistor dissipates 1500 mW
!! the wizard thinks the power dissipated in your resistors is a concern
together, all resistors dissipate 9000 mW
together, the diodes dissipate 63000 mW
total power dissipated by the array is 72000 mW
the array draws current of 6000 mA from the source.

So I would need at least 2 Watt resistors. I share the wizards concerns that I am dissipating a LOT of heat through those resistors (not to mention wasting a lot of power). Heat is already a big concern with the LEDs - I certainly don't want to add to the problem but 9 watts of resistor loss isn't TOO bad. 6 amps @ 12v should not be a problem for a 200W CPU power supply..... but what I am missing here? I still don't feel that this circuit as is is going to be as failsafe as it should be. I can (and would) add a fuse but what if 1 of the LEDs fail? Would it chain reaction the rest? What if the input voltage all of a sudden goes to 18v? This is what I need the assistance of some of you guys who are more seasoned than I!

I appreciate and and all help you are willing to provide.

<small>[ March 26, 2006, 03:45 AM: Message edited by: LTC ]</small>

dyarker
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by dyarker » Sun Mar 26, 2006 10:51 pm

The few failed LEDs I've had, failed to open not to short. So there shouldn't be a chain reaction (of course these big boys may behave differently than the 30mA max types).

I'd put all 18 in series.

1. 48V, 2A transformer; 3A fullwave bridge rectifier block, aluminum case power resistor, mylar filter cap.

OR

2. 115VAC thru one of those caps that are yellow on the outside (or 250V poly), 3A fullwave bridge rectifier, aluminum case power resistor, mylar filter cap. (all exposed metal bonded to green wire!) (The series cap is a more efficient way to drop line to like 70V level than resistor)

The filter cap would be more for line spikes than ripple filtering. At 120Hz the blinking should not be visable.

----------------------------------------
Any way you go, I suggest 18 holes in cast aluminum box, and insert LEDs from inside. That will give the LEDs some heatsinking.

Just a couple idea, good luck,

<small>[ March 27, 2006, 01:36 AM: Message edited by: Dale Y ]</small>
Dale Y

LTC
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Mon Mar 27, 2006 3:02 am

The LED heatsinks are not electrically neutral. Makes heatsinking a pain. Why go 48v or 120v ac? Why not just use a 12v source? 6 rows of 3 = 18 with very little power loss. 9 Watts.... 87.5 % efficiency atm, not counting ac->dc losses. I agree though, most leds fail open. The gold wires act as fuses. Too much current and they simply stop carrying. The MTTF on these is crazy, but no reason to put them all in series - they would need approx 66.7v. I would think its better to use regulated 12v dc?

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by dyarker » Mon Mar 27, 2006 3:53 am

Simplicity. One transformer, one FW rectifier, one 5 Ohm 10W resistor disipating 5W; all in one package with the LEDs.

Oh, the LEDs are SMD, that does make things harder re: enclosure and heat sinking.

The old PC supply will need a dummy load on +5.

Better than 12V? Maybe, maybe not; I just put a couple ideas to think about.

Cheers,
Dale Y

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abelk
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by abelk » Mon Mar 27, 2006 4:01 am

Hi,

You can check the computer PS for a rating. It will usually say:
12v x.x amp
-12v x.x amp
5v x.x amp

You will have to see if the PS is up to the task, first. If you do run 6 sets of 3, why not run a fuse in series with each set if you are worried. You are drawing ~1amp on each line, so maybe a 1.5amp slow-blo fuse would do.

The other question is do you really need to put 1000mA through each one? You could increase the resistor value on each set. That will decrease power dissipation of the resistors, reduce load on the PS, and maybe increase the life of the LEDs. The light output will be less, though.

Good luck.

Kent

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by dyarker » Mon Mar 27, 2006 4:20 am

Kent reminded me to ask something. Is 1000mA (1A) the max rating, or the recommended continuous current rating? Since you want to run them 24/7, 75% to 80% of max would be better for longevity.

Curiosity question, what are using them for? 63 Watts of LEDs must be equivilent to 300 to 400 Watts of incandescient!

<small>[ March 27, 2006, 04:21 AM: Message edited by: Dale Y ]</small>
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LTC
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Mon Mar 27, 2006 7:47 am

1000 mA is the normal and maximum rating. They can be run at 700 mA - but less light output (of course). Your transformer example sounds like a good idea but what happens when the AC input current were to change? Not likely but certainly possible if it spiked. What happens to the current output when the leds warm up and lower their resistance? What if one led opened up and as a result one string went out and the others stayed lit?

I would prefer the $20 PS blow than damage 18 $8 LEDS. Plus a PS gets more voltage protection (?). Not as simple but the nominal cost difference does not seem to be too huge. How much power loss for the transformer circuit? Seems to me that the AC-DC loss would be fairly minimal (around 10%) and since the 12v = nearly the perfect voltage for 3 leds in series - power loss to the resistors are minimal. Overall power loss seems on the order of ~ 20% for the resistors in series driven with a power supply example due to the voltage rquirments being close to the output already.

Very helpful posts though!

Yes, you are correct - I will need to put voltage across the sense on the PS or it will not function. I believe that is pin 11 but will have to check.

This project is for a high lumen output / low power use / high cost project. Always seems to be 3 things, pick 2 doesn't it.....

<small>[ March 27, 2006, 07:54 AM: Message edited by: LTC ]</small>

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Mon Mar 27, 2006 7:53 am

As to ensuring the power supply is up to the task.

Each LED = 3.75w (3.75 V @ 1000 mA)

Assume I have 6 rows of 3 leds, each serial row takes up approx 12v + 1 mA. Since there are 6 rows, each taking up 1mA @ 12v, the total power use is 6 amps @ 12v, not 18 amps @ 12v. (6*12=72 watts) (18*12=216 watts) The circuit seems to me to be taking up 72, not 216, and since I am running the diodes in serial and parallel - the current to each LED winds up being 3.75v @ 1000 ma (and the resistors suck up the extra .75v difference between 3x3.75=11.25v).

Do you follow that? Seems to make sense to me. Most power supplies can easily do 10amps across the 12v line (3.3v and 5v are a different story).

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by KamPutty » Mon Mar 27, 2006 8:27 am

Whats your plan for the heat build up? I'm playing with smaller LED's ([email protected] / 42 Lumens), not the 1000mA you have, and that single LED builds up alot of heat.

~Kam (^8*

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Externet
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by Externet » Mon Mar 27, 2006 9:41 am

Hi LTC.
Use strings of 3 leds in series to a 12V (constant voltage) PC power supply.

Yes, leds can run at constant voltage.

3 x 3.75 V = 11.25 V. Either trick or adjust the voltage in the power supply to come down to ~11.25 or drop the excess with a silicon diode. No resistors.
Or use a modern power supply that supplies 3.6V to newer CPUs

Many pc power supplies run shy of the 12V rating, chances are you will find one with only 11.25 V real output if you do not want or can't trick it.

Still agree with comments above, do NOT run your leds at full maximum specifications for a 24/7 application. You will have failures. You want more light, get more leds and drive them under limits.

Concerns about some miliwatts wasted by the resistor ? So what ! Isn't the power supply itself wasting much, much more ?

Miguel
- Abolish the deciBel ! -

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Chris Smith
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by Chris Smith » Mon Mar 27, 2006 10:32 am

No two LEDS are identical which is why they recommend one resistor for each Led, unless reliability and lack of brightness maintainability is second fiddle on your list.

Run all LEDs through a individual resistor, and maintain the voltage rail accurately.

For true accuracy match each resistor to the actual draw of each LED. This is especially important on LEDS that draw such heavy currents.

In the end your brightness will be matched to each other and the life span of the LEDS will be increased to their actual life span predictions, and blackening wont occur so early in their expensive life.

You will find most foot notes for leds not recomending a parallel or series configuration for maximum life span.

For cheaper Leds of lesser draw its not as important.

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Mon Mar 27, 2006 1:43 pm

Originally posted by KamPutty:
Whats your plan for the heat build up? I'm playing with smaller LED's ([email protected] / 42 Lumens), not the 1000mA you have, and that single LED builds up alot of heat.

~Kam (^8*
18 1" by 1" heatsinks and a fan. No doubt this thing will get warm but should give off less heat than a regular lightbulb.

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Mon Mar 27, 2006 2:31 pm

Originally posted by Chris Smith:
3 x 3.75 V = 11.25 V. Either trick or adjust the voltage in the power supply to come down to ~11.25 or drop the excess with a silicon diode. No resistors.
Is that not a really BAD idea? What happens when the LEDs heat up? Their resistance changes, and they start pulling more current, then they get hotter and draw more current(same voltage?) as their internal resistance drops further.... then the gold wire supplying the power to the diode experiences thermal breakdown and you've got a dead LED. Oh, because you used no resistors - the entire circuit has heated up and blown as well..... I think the resistors are a good idea to limit the current per LED unless I am missing something about how current and volts work? *confused*

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Chris Smith
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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by Chris Smith » Mon Mar 27, 2006 5:13 pm

One resistor one led, one voltage source.

I think you mixed me up with some one else like externet?

Monitor each LED with a meter, and calculate the proper resistor, same as all LEDs should be done, for a long life and no early blackening.

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Re: 90 Watt LED Driver - Need a bit of help here :)

Post by LTC » Mon Mar 27, 2006 5:59 pm

Oops you are correct. I meant to quote Externet. The only problem is that I cannot easily put more than 20A across the 3.5v line in a normal power supply. If I am to drive 70+ watts, this seems like a bad idea. I could drive @ 5v and use a resistor to minimize the current but that also seems to be a bad idea - lots of wasted power and heat. What is so bad about putting 3 leds in serial and then putting those in parallel with a constant voltage? Am I missing something or is that a perfectly ok way of doing it? If the voltage spikes it might be a problem - perhaps just fuse it?

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