Old 02-27-2014, 11:35 PM #1
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Default Powering a high power LED with a computer PSU?

Hello all. I just got a nice little 10 Watt LED from ebay in the mail today, and I thought I would ask someone with more experience about this.

The LED's spec sheet says it takes 9-12 volts @ 1050 Amps (Which would be 12 Watts actually). I have an old computer PSU that has a 12 Volt 500 mA output on it, and I was wondering if it would be safe to run my LED off of it to test it, or would I still need a ballast of some kind?

If I do need a ballast, could I use something other than a resistor? Like say a computer fan? (I've seen people use motors as ballasts before). I'm not trying to run this for a long time, just to play with it a little.

And yes, I did order a proper LED driver that matches the specs for my LED, I just ordered them separately so it didn't arrive at the same time.

And yes I'll stick it to a heatsink.

Thanks!


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Old 02-28-2014, 01:04 AM #2
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Default Re: Powering a high power LED with a computer PSU?

You can only draw as many watts as you have available. So that psu wouldn't be able to fully power the led. You could have 2 power supplies in parallel.
Yes you would need a resistor to limit the current, unless the power supply is the exact amperage you need.

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Old 02-28-2014, 01:30 AM #3
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Default Re: Powering a high power LED with a computer PSU?

I wasn't planning to fully run it, just to test if it lit up. Tried it, but it turns out the 12 Volt 500 mA output on the PSU is dead. All the others are 6 and 10 amps so I won't be using any of them. I'll just wait until the LED driver gets here.
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Old 02-28-2014, 01:57 AM #4
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Default Re: Powering a high power LED with a computer PSU?

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Originally Posted by oahu99 View Post
You can only draw as many watts as you have available.
Not true in this case, the computer power supply could do 1 of 2 things (or possibly both if it's crap): Go into overload protection and shut down, or 2. Keep going, supply a higher current, and set itself on fire. Computer power supplies aren't current limited, they just (usually) have overcurrent protection. The 500mA rail can safely supply 500mA, it's up to you to keep that draw under 500mA.

Putting switchmode power supplies in parallel isn't so straightforward, you need diodes to prevent them from interfering with each other, and there's no point since you'd still need a ballast.

You'll need a driver, or a bulky resistor and run it off one of the real 12V rails.

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Old 02-28-2014, 02:06 AM #5
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Default Re: Powering a high power LED with a computer PSU?

I was just suggesting that in his case he wouldn't be able to get the 12 watts out of the psu he needs for the led.

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Old 02-28-2014, 05:50 AM #6
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Default Re: Powering a high power LED with a computer PSU?

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All the others are 6 and 10 amps so I won't be using any of them.
At what voltages? I don't think you understand how power supplies work. They output their rated voltage, but they cannot force the rated current through the load, you see. It's not "ohm's guideline."
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Old 02-28-2014, 12:59 PM #7
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Default Re: Powering a high power LED with a computer PSU?

@ 1050 amps , that's a big LED xD

Simple answer is get a constant current driver rated for the 1050 mA the led needs , For simple testing , a 12 volt psu would be fine but current May vary as temperature does a little bit
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Old 02-28-2014, 01:49 PM #8
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Default Re: Powering a high power LED with a computer PSU?

If you look up the datasheet of the LED, you can determine what current it will run at 12V. Flashaholics run 3A rated Cree XM-L and XM-L2's at 6-7A, so you might be able to be flexible with it.
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Old 02-28-2014, 02:49 PM #9
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Default Re: Powering a high power LED with a computer PSU?

I mixed up spec sheets for my LED :P the one I'm trying to use right now needs 12 Volts @ 900 mA. My ATX power supply has a 12 V 500 mA, 12 V 6 amp, and 12 V 10 amp output. The 500 ma output doesn't work (and as I've been told, computer PSU's don't limit current)
so I would need a pretty hefty resistor to limit a 6 amp current down to 900 mA.

I did get a proper driver. I'm just impatient waiting for it to arrive and want to play with my LED now XD
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Old 02-28-2014, 05:38 PM #10
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Default Re: Powering a high power LED with a computer PSU?

The ATX doesn't work like that , its a voltage source type PSU , it will give out a fixt voltage and current draw will vary depending on the load .

The 12 volt rail will run the LED at 12 Volts and it will draw a current of ( I would guess 900ma ) but it all depends on the exact forward led voltage .

As the PSU is a voltage soruce it dosent matter if the output is 12 volts at 1000 amps , a load will only draw a given current for a given voltage , and as the PSU is fixed voltage , if the load is specked at 10 amps say at 12 volts , then it will draw 10 amps at 12 volts regards of the PSU max rating ie 1000 amps @ 12V
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Old 02-28-2014, 07:28 PM #11
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Default Re: Powering a high power LED with a computer PSU?

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Originally Posted by Things View Post
Putting switchmode power supplies in parallel isn't so straightforward, you need diodes to prevent them from interfering with each other
All I need is blocking diodes to parallel PSUs? Neat-O!
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Old 02-28-2014, 07:36 PM #12
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Default Re: Powering a high power LED with a computer PSU?

Yeah but LEDs don't self limit their current draw, so if I connected the LED to a high amp source would it not keep drawing current until it destroyed itself?

Or would the voltage drop across each sub-led (the LED is actually comprised of 9 tiny LEDs) also limit how much current it could pull?
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Old 02-28-2014, 07:52 PM #13
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Default Re: Powering a high power LED with a computer PSU?

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Originally Posted by Blarg King View Post
Yeah but LEDs don't self limit their current draw, so if I connected the LED to a high amp source would it not keep drawing current until it destroyed itself?

Or would the voltage drop across each sub-led (the LED is actually comprised of 9 tiny LEDs) also limit how much current it could pull?
LEDs work the same as laser diodes , for a given voltage , they draw a given current , or for a given current , they drop a given voltage .

The Led will be multiple led emitters in series , if the led is specked 12V @ 900ma , then when driven at 12 volts it will draw around 900ma .

But as with laser diodes , leds are the same in this respect of that a small change in voltage can cause a larger change in current . That is the reason we set laser diodes and leds with constant current drivers , so that as temperature changes and the junction voltage drop changes , there is always the rated current to past though it and power .

Long story short , if the led is run at 12V then No the led won't keep increasingly draw current so long as its cooled properly and kept within operation temperatures , it will draw around 900ma as that's what its specked to do at 12 volts , But if driven from a voltage source the current may be above or below ( 100ma or so id guess ) the recommended driver current from the data sheet due to the variation of the voltage drop as a lot of multi emitter leds will have a min and max voltage drop range

( for this case ) The current rating doesn't matter on the 12V rails , if the outputs are 12V @ 0.5A , 6A , 10A , its still 12 volts , so the led will draw the same current from any of them ( or what the 0.5A rails max is ) , the current rating is purely there to state what the PSUs max current output can be , nothing more
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Old 02-28-2014, 08:23 PM #14
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Default Re: Powering a high power LED with a computer PSU?

Ahh I see now. The way I was always told was that LEDs would keep pulling current until they destroyed themselves.

So if I properly heatsink the LED then there shouldn't be too much risk of destroying it if I run it off the ATX power supply.
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Old 02-28-2014, 11:38 PM #15
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Default Re: Powering a high power LED with a computer PSU?

Try a 9V battery, *might* be enough voltage to at least verify that the chips are connected.

If you can see 9 LEDs and this is supposed to be a white LED, chances are it is a 3S3P array of "1W" LEDs, which usually are meant to be driven at 350mA (times 3 strings, 1050mA total for the whole LED). Not that this matters, or that knowing this will help you with driving this off an unregulated 12V, but just for your info ;-)

Best to just wait until the matching driver you ordered shows up, though.
Edit: if you are hell-bent on driving this off your PSU, you will probably want to throw a resistor in-line with it. it would help greatly to know the Vf of the LED at the desired current, but to do that, you'd need a CC source, or you'd need a variable voltage PSU and ammeter, and the reason why you want to drive it off a PC PSU is because you don't have this stuff.

So, let's assume worst case scenario of very very efficient LED chips with a Vf of 9V.
Let's also assume you want to drive this LED @ 1A, and that your PSU puts out voltage-regulated 12V.

From the 12V supply, the LED will drop 9V across itself @ 1A. Therefore, your resistor needs to drop the remaining 3V (at 1A, since it will be in series with the LED, and every device in series sees the same current).

To determine the resistance you need, you can take the above info and plug it into Ohm's Law: V=I*R
V=3V
I=1A
R=unknown

V=I*R
3V=1A*R
R=3V/1A
Therefore, R= 3 Ohms

However, you also need to know how much power the resistor is dissipating, so that you can choose a 3 Ohm resistor that won't fry itself at 1A.
Power = Current * Voltage;
P=I*V
P=1A*3V
P=3W

Resistor needed: a 3 Ohm resistor capable of handling 3W.

So, while you only need a 3 Ohm resistor, it will need to be able to handle 3W of heat, meaning you will be looking for one of those beefy ceramic wire-wound resistors, most likely.

Keep in mind that LEDs and LDs are "dynamic." You can't ask "What is the Vf of this LED?" without also mentioning what current is passing through it. That said, a back of the napkin calculation like above can work in a pinch.

Last edited by BShanahan14rulz; 02-28-2014 at 11:49 PM. Reason: been a while, hope I did this right....
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Old 02-28-2014, 11:48 PM #16
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Default Re: Powering a high power LED with a computer PSU?

I thought so - you can use either of the other 12V rails. Use a 2.2ohm 3W+ resistor for ballast - Good enough.
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