# Computer power supply

Again hello to all. I am an infrequent poster but long time lurker. This is by far the most informative group of people I have had the privilege of being a very small part of. Thank you all for your willingness to share your experiences both good and bad. I have a question about using a computer power supply to power an outdoor led lights. I have several old power supplies available, from 200 up to 600 watts. All function well and I (thanks to others) can get a 12 volt output. It would seem that this would work just fine for feeding perhaps 6 to 8 individual xml leds. I plan to build individual enclosures for each light from conduit boxes much like some of the projects that have been built before. Out of all that, do you think the power supply will work? Additionally do you think using a part of an old cpu heat sink will give me sufficient cooling? Thanks in advance David

Although I have not used one I cannot see why not, provided it is indoors.
However just a couple of things to be aware of, ATX power supplies have a tolerence of 10% when manufactured, that means they can be 11.8v~13.2v, also as they age the power output goes down and the regulation can deteriate, test under load before using for powering leds.
Most computer psu’s rate their wattage as total, meaning they add the 12v, 5v and 3v wattage together, have a look on the spec plate to find out the 12v wattage.
As the psu will be indoors and the leds outside the potential problem of voltage drop with dc over long lengths of wire rears it ugly little head, take the longest distance of wire you will be using, feed it 12v at one end and measure the voltage at the other, you might be suprised.
This is not a problem when you are aware of it, just use whatever voltage that you get.

Power required,
xm-l T6 2A ~ 3.24v = 692lm : 3A ~ 3.34v = 910lm
xm-l2 T6 2A ~ 3.24v = 773lm : 3A ~ 3.43v = 1048lm
Heaviest load 8*xm-l2 @ 3000mA, 3*3.43=10.29w per led, 8*10.29=82.32w, plus driver overhead, say 90w this could be 12v at 7.5A or 9v at 10A, this is what you need at the end of the wire run.

The above is basic as it depends on the voltage at the driver, what driver, and how the leds are wired, ie series or parallel but it should give you a general idea of working out the details yourself, have fun and please post, with pics, the end result :bigsmile:

Cheers David

Edit:- just noticed you said “6 to 8 individual xml leds” the above presumed all the leds together, no matter, the math still workes for more runs.

Thanks for your rapid response. All correct. I had given no thought to voltage drop. The ATX will be indoors. I plan to use the 2596 module to drop the voltage to the 3 volts that the led uses. Quite well may add especially if you are looking at the led when powered up. Take that you dummy it says in no uncertain terms. The 2596 driver? will be installed in led enclosure. I plan to go parallel along the pathway. Hopefully the run will be no longer than 120 feet or so. I may need more or less leds as I see how the output lights the areas on the run. If anyone has any suggestions please reply. I will probably ask many silly questions. The only stupid questions are the ones you don’t ask. Thanks again for your response. David

Heres a power supply built by JohnnyMac.

It may be of some help.

If you don’t want to worry about using the LM2596 module, you can try wiring the LED’s in parallel with the 3.3V rail.

6-8 XM-L’s will dissipate a lot of heat, make sure you have a nice hefty heatsink and perhaps active cooling as well.

I’m sure you know about jumping the green wire to the ground and using that as the switch.

Let us know how this project goes!

You can use a computer power supply, but……….
You should be driving the LED’s with a constant current source, not a constant voltage source. As an LED warms up, it will draw more and more current from a CV source leading to “Thermal Runaway” That is why all of our flashlights use CC drivers.

Here is a bench setup I did with 2 - XM-L’s in series driven from a 12V battery through an LM2596 board (CV).

The left most meter is reading the voltage off of a 12V battery. The meter to the right of it is measuring the current through the emitters. Next, the yellow meter is hooked to a thermocouple that is clamped to the back of the board at the thermal vias. The meter to the far most right is reading the voltage output of the board. Temperature is measured in degrees Farenheight.
Even though the current starts at a modest 1.6A, watch what happens as time passes.

I was very impressed with the voltage regulation of the LM2596, it is rock solid, even as the board got quite hot. Notice the current to the emitters though, it keeps rising. At the end, the temperature of the driver seems to stabilize, but actually it stopped increasing because the plastic clamp holding the thremocouple in place melted.

I wanted to be clear, the rising current through the emitters was NOT a result of the rising temperature of the CV driver but rather the increase in temperature of the LED’s. i did not measure that temp as it was not that high. In the end, i used this setup as is. The LM2596 has a builtin current limiter of around 3A.
My suggestion is that you consider one of them in conjunction with the computer supply.

as said you need 1 LM2596 each led

way better buy a cheap dc-dc step up driving all led in CV, I have that is solid and works fine just dont shortcut or invert polarity or is easily burned!
for example with 8 xml you can drive them in series on cv setting a voltage from 24v to 27v depending on your wish and led heat sinks, don’t wasting much heat(that is proportional to current) on cables

PS just 1 little cons the step up module is a bit less efficient of a step down exp 94% vs 97%

Thanks so much for that video. I would never have thought temps would rise so fast. Cooling I think is going to be an issue that will require addressing. Thanks so much for your input! Again thanks David

MixoMaxo Thanks for the idea on this driver. Wikk it drive all say 8 leds in series or stay in paralell? I saw a driver that featured both constant voltage and amperage but per normal Idid not save the link. I may look later for that again. Again thanks David

Ryansoh3 thanks for my unasked question. I intended to ask that question earlier and forgot. Those kinds of slips have the potential to be expensive. If I use this wiring, will the current be limited enough to prevent letting the smoke out? Again Thanks David

sry david my english is poor and dont fully understand your project

to drive them in series you are bound to keep all leds turned on/off in the same time and must connect each led to the others like here

ofc you can put in series how many leds you want also just 2 or 3 leds multiplying the VF for the number.
Here a VF table

so if you need to switch singular led or cant wire each led to the other, as said by Ryansoh3, is a good idea to use black and orange wire of you power supply, will give you 3.3v

anyway better use a multimeter to chk voltage first of connect a led

Many power supplies expect current flow to be primarily from the 3.3 or 5v and may not behave well with no current on those rails and higher current on the 12v.

I had spoke to my brother just last night and he mentioned that very point. I think you guys are correct about using the 3 volt output. that will eliminate the need for the other driver. This will mean cooling and current limiting will be my main concerns Thank you for your valuable input David

No need for any apology. The evidence shows that your command of Engkish is better than mine. Thank you for your valuable input. Thanks again David