Use ATX power source to power Raspberry Pi

Question

I'm setting up several Raspberry Pi's and Arduinos connected together, and I'm looking for a way to power them.

Would it be possible to adapt an ATX stock power source with female USB plugs using the 5 V wires to provide more than 500 mA?

• Jules: Both arduinos and pis can take power from jumper pins as well as their USB connectors, which might be easier, as soldering USB connectors is a total pain...

Answer 1

You need a power resistor across either 3V3, 5V or 12V - this varies, apparently with the build year of the PSU. Wire colors also vary. Usually red, yellow and orange are 5V, 12V, and 3V3 respectively. The 'unique' colours (in my case grey, purple, white IIRC, on a Corsair 850W PSU) are for e.g. ON/OFF (purple), STATUS_ON (white, with a LED for example, 5V), and grey must be loaded.

I had to load 12V for some reason, not 5V. All I had to do was poke around with my DMM, so it's not rocket science. It is prudent to install fuses and/or current limiters (with e.g. an cheap LM317, but I didn't, of which I repent.

You really should install at least fuses that blow at 2-3A, or a short on a circuit board will vaporize whatever it crosses... 20A will melt things and cause burns. It's an important caveat. I expected a short to kick a built-in polyfuse, but don't rely on that.

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I mounted some outputs on the front, and have a nice supply.

I will have to build a few current limiters however - I should have done that from the start and mounted them inside. An 'ON' indicator is helpful to remind you to shutdown the PSU when unused.

The answer is 'yes' - Your PSU will happily feed your the max required 12W for pi3.

Answer 2

I see no reason why this shouldn't work. ATX power supplies generate a relatively well regulated +5V rail with lots of power available. I am of course assuming your plan is to connect to the micro-USB power port (as opposed to the USB host ports).

The only thing to remember is to add a minimum load on the +5V rail to ensure proper regulation. A 22Ω 2W rated resistor to ground should be sufficient to ensure proper regulation.

• Jules: An ATX power supply is required to supply a maximum of 5.25V at all times (even with no load, although it is permitted to shut down entirely under such circumstances), so unless your PSU is misbehaving, the resistor shouldn't be a requirement. See: formfactors.org/developer/specs/atx12v%20psdg2.01.pdf (pages 12 and 27 are the relevant ones).
• Stack Exchange Broke The Law: @Jules You said it yourself, the resistor might be necessary to prevent the PSU shutting down entirely.
• Jules: @immibis - true, but I've done this with plenty of ATX power supplies in the past and have never yet encountered one that actually does this.
• Tom Carpenter: @Jules but equally, I've come across many a cheap ATX PSU that doesn't regulate properly with no load - one I came across got up to 6V! (granted I discarded that one)
• Grasshopper: Just out of curiosity, why not use host ports?