ATX Power Supply for LED strips

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I recently purchased about 100m of white 12V LED strips to replace the fluorescent lighting in my shop, and I was thinking of using an ATX power supply to power them. I’ve done some reading about using ATX supplies as bench supplies, so I think I’m roughly familiar with the considerations I need to make for this project, but I’d just like to check here and see if I have my bases covered.

First of all, load resistors. From what I’ve read, the ATX spec defines the minimum load for the 5V rail at 0.3A, so that’s 16.66 ohm or less. 3.3V rail is 0.5A, so that’s 6.6ohm or less. So, napkin math, if I use a 10 ohm and 5 ohm load resistor, respectively, that’s 2.5W and 2.2W. So, [this and [this?

Second, switching. I’d like to switch the lights on using a standard light switch, without having to mess with anything on the PSU, so can it just be rigged so that it powers on whenever it is plugged in? I know the power supply is enabled by shorting the /PS_ON pin to Gnd, but doesn’t that also turn it off if it’s shorted for more than a few seconds, or is that function caused by the PC motherboard? Can I just short that pin permanently to ground, and then have the lightswitch controlling the electrical outlet that it’s plugged in to?

Third, PCB current capacity. My LED strips are rated at 2.88W/m, so if I round up to be safe, I’m talking something in the ballpark of 300W to power all of these LED’s. How wide do my PCB traces need to be to handle 25A? With a double-sided PCB, I can use both sides to run multiple traces. Also, since I’m only using the 12V rail, I could use a large polygon fill instead. Would that be better or worse? I’ve also seen some designs that omitted the soldermask over the power rails so you could apply solder to the rails in order to increase the current capacity. Would that be a good idea?

Finally, wiring. I’m thinking of just using standard solid-core house wiring to run the main power to each block of lights, each of which will probably be around 15W worth of LEDs or less. The individual blocks of lights will wire into a junction of some kind with the main power lines using standard 18awg stranded wire. Is this sufficient, or do I need to rethink my wiring?](AC05000005008JAC00 Vishay Beyschlag/Draloric/BC Components | Resistors | DigiKey)](AC05000001009JAC00 Vishay Beyschlag/Draloric/BC Components | Resistors | DigiKey)

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Your reasoning seems sound, but here are some thoughts.

  • 2 W of heat needs to be dissipated in each resistor. I like that you’ve chosen 5W resistors, but you still might want to double the resistance and use two in parallel.

  • There tools out there for calculating PCB trace width. I don’t have a link handy but I’m sure Google will find them.

  • A 300 W ATX power supply (100*2.88) won’t be able to deliver all those W on 12 V. It’s built to deliver multiple voltages. So you’re going to need 700W supply or maybe more. A dedicated 12 V supply might be a better solution.

  • You not only need to get power to the strips, you also need to be aware of the resistive drop as you go down the strip. Even 5 m strips may show sag at the far end. You may need to power from both ends or use shorter strips. This will have the added benefit less current in each power feed line.

  • Calculate the current capacity of you wire (gauge, length, acceptable drop) then bump to the next higher gauge for safety.

Good luck with your project.

  • Chip

I do not work for SparkFun

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I’m aware that not all of the rated wattage is on the 12V rail, my PSU actually has the wattage for each rail listed on it, so I’m good there. A dedicated 12V supply at that wattage is fairly spendy, whereas I have an entire stack of old PC supplies I can use for free.

Also I’m not planning on running the strips full-length. That’s why I mentioned separate blocks of lights each tapping into the main power lines. Those blocks are going to be about 4’ long, and probably 4-8 strips in parallel. I’ll be sure to wire them up in parallel to minimize the resistance.

I’ll look into the cost difference between doubling up the resistors vs just getting higher-wattage ones. I figured 5W would be enough, but on second thought it certainly couldn’t hurt to go higher.