Driving XM-L's off 3.3v rail on CPU PSU

Hey all,

I am making a fish tank light, it will consist of a few XM-L's running at either 2.1 amps or 3.15amps
ill be using the AMC7135 (1050ma) from DX that i got a few months ago,
and i would like to run them off the 3.3v rail of a computer PSU

is it as simple as it sounds, i think the PSU will put out more than 3.3v but still in the range of the AMC's
im not sure of the amperage rating on the PSU but i imagine it is somehting above 35+ amps?

i would like 4 XM-L's in my 4foot fish tank and probably 3 on my 3foot if it is going to be pretty simple.

is there anything i am missing here?

and before its mentioned i already have 8x 300mmx350mm heatsinks from heatsinksUSA from another build.

so im thinking of thermal pasting an LED at each end of 2x heatsinks for spacing, and just a very small PC to keep it super cool

http://www.cree.com/products/pdf/XLampXM-L.pdf

Bump.

I dont see a problem with this, if there is a supple > Led voltage.

But is there some funny business with using computer power supplys?

I'd give it a try. With such low voltage, the loss in the wiring might actually underdrive the LEDs. You will have to test and take measurements. It might be better to use the 5 volt line and current sharing resistors at each LED.

thanks for the reply, it gives me a bit more confidence.
underdriving wont be so bad, as long as its not down around 1amp as i have XRE's for that.

Ok well i will have to get some load onto this PSU and see what sort of voltage i am getting.

another bonus of running a computer PSU is i can just plug a CPU fan into one of the 4pin connectors

I am using meanwell NES-25-12 to drive 4 XML in serial, eliminating the needs for AMC7135. Just slowly adjust the voltage output so the XML have enough juice to run @ 2.1Amps .

thats a pretty good idea, i would probably still use the AMC chips to prevent thermal runaway if a fan fails or something and they drop their voltage.
although i dont think the cpu can be adjusted if the LED required 3.2v but was being fed 3.3v+

DodgeyDIY said:

thats a pretty good idea, i would probably still use the AMC chips to prevent thermal runaway if a fan fails or something and they drop their voltage.
although i dont think the cpu can be adjusted if the LED required 3.2v but was being fed 3.3v+

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Yeah. Meanwell PSU is adjustable but not the normal CPU power supply. Feeding XML with 3.3V from the computer PSU is fine but you need to take care of the extra current since most of the computer PSU support up to 15+Amps on the 3.3V rail .

Another way is to use those 1.5amps wall wart AC-DC adapter which works fine with 3~4 XML.

well im hoping i can rig up the AMC7135 chips to limit current under maybe 2.8amps or thereabouts.
according the side of the PSU it says 3.3v 28A i shouldnt go over 12 amps though.

hooked an XML up to the PSU just now and the most current i can get out of it is 1.3amps when i first connect but then drops nearly instantly to 960mA and stabilises.
it seams to regulate the 3.3v to 2.85v when it has a big load on it.

is this normal? any ideas on how to stop it.

the voltage before LED load is 3.34v

I wouldn't direct drive an LED from ANY power supply without control of the power supply. And, you have no control of the computer PSU.

Computer power supplies tend to do their own thing. Many newer PSU's are eco-greenie so you might not be able to use it as planned.
You need read up on the specs of that power supply and what the power supply is looking for. As already mentioned, you might be forced to use the other rails and step down the voltage.

http://en.wikipedia.org/wiki/ATX +3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows for remote sensing of the voltage drop in the power supply wiring.

deadrx7conv said:

http://en.wikipedia.org/wiki/ATX +3.3 V sense should be connected to the +3.3 V on the motherboard or its power connector. This connection allows for remote sensing of the voltage drop in the power supply wiring.

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Thanks for that, so from my understanding, without the Sense cable it cant "monitor" voltage.
well i hooked up the sense cable parallel with the 3.3v wire and the only change was a drop in current and voltage.

when i tried an XPG instead of an XML i got an increase in voltage through the LED of about 3.1V (1.3amps for the XPG)
but i still have the same voltage drop issue with the XML (3.8-3.9V)

A 7135 has 0.1 volt minimum drop across it, and with 3.2 v across it an XM-L will only draw 2 amps.

You could use a 5 volt supply, but for every 10 watts the LEDs dissipate, the 7135s will be dissipating 5 watt.

MikeAusC said:

A 7135 has 0.1 volt minimum drop across it, and with 3.2 v across it an XM-L will only draw 2 amps.

You could use a 5 volt supply, but for every 10 watts the LEDs dissipate, the 7135s will be dissipating 5 watt.

Click to expand...

I would be happy with 2 amps and 3.2 volts haha.
Yeah I wouldn't bother using the 7135 on 5volt

If anything I will try 4 in series on the 12v rail with maybe a 1ohm resistor.

I do have the driver off DX the is 32-36vdc at 3amps but i don't need 10 xml's at full power for my fish tank haha.
Although I did read about the dimming mod for the same 100w driver, I should look into that more and check on the efficiencies at the dimmed levels.

Just a little update for those interested.

I tried the "Voltmod" for the 3.3v rail. was quite simple. from my understanding it tricks the 3.3v sense wire into thinking it need to output more voltage thus increasing voltage.

so i tested it with an LED and could boost the voltage up to 3.48v. any further and the PSU shuts itself off, maybe from overvoltage or too much current being drawn.
i only had an XRE Q5 to test as my XML got packed away somewhere and i didnt feel like looking for it.

but with the LED hooked up at normal voltage XRE sat at 3.34V and drawing 0.98 amps and as i slowly upped the voltage ov the PSU (via potentiometer) i could get the LED to draw 1.62amps before hitting the PSU saftey function.

So this evening im going to either search for my XML or just glue a new one on a new heatsink and do some testing with the raised voltage.

if all else fails i will attempt the "VOLTMOD" on other rails to suit a series array.

alright,

I tried the XML with the voltmod but to no avail, it just would NOT let me increase the voltage enough to get any real current.
is there some other safety feature these PSU's have that i am unaware of?