Can I use a standard PC power supply to power a Raspberry Pi, e.g. iMicro IM400W 400W ATX12V Power Supply (or just take one from an old unused computer)? Amazingly, these things are sold starting from 1$.

Most people seem to be looking for small and lightweight power sources, but since we are contemplating to build a PCR machine, size and weight are not an issue.

So far I only see advantages (but maybe that's because I don't really understand electronics):

  • Lots of power (like 20-30A): We are thinking about using a peltier element for heating and cooling (e.g. TEC1-12706 Thermoelectric Peltier Cooler 12 Volt 92 Watt). This one requires 12V and draws apparently up to 6A (and probably shouldn't be on the same circuit as the Pi, right?)
  • Different Voltage outputs (3.3/5/12 Volts).
  • Lots of cables. So I'm thinking we just pluck a 5V cable for the Pi, and a 12V cable for the heating element (right?).
  • Nice on/off switch
  • Nice casing
  • Nice inbuilt fan

Any reason why this wouldn't work?

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    I've heard that some PC power supplies need a minimum load to keep operating. I'm not sure if the Pi by itself will be sufficient. – joan Mar 31 '15 at 16:23
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    @joan, this is an important and valid point. I took the liberty to write another answer to cover this fact. – Ghanima Mar 31 '15 at 19:35
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    One thing that other answers don't address besides the voltage stability at low loads and noise is efficiency. Raspberry PI is <2W power drawing device. PC power supplies come in typical ranges of 400W-1000W. Newer models have quite good efficiency, but only at peak power. You will be wasting (as heat) a considerable amount of power at low loads. If you need extra power for devices that you controlling though ATX power supply is great. – Joop Apr 1 '15 at 8:22

Actually, yes - and as luck would have it, you are unlikely to get into minimum load issues either, because there is a 5V rail designated as the standby, so it's made to handle low-power applications (typically up to 3A/15watts or so), however most computers idle in the 1W range (really, they are only listening for a wake-up packet over the network or for the "power on" to be pressed)

I wouldn't connect the Pi to the heating element's 5V (who knows if it's a power rail or only a small 5V rail), but instead directly to the PSU

One thing to beware of is that cheap PSU's are often unsafe (as in lacking proper protection circuitry), I wouldn't get anything that doesn't at-least have the 80 PLUS bronze rating (since that ensures it works up to a certain degree).

One other thing to consider is that PSUs in this class often have cheap fans that could be a source of noise - an alternative would be a laptop-style brick, there are several models that have a Molex output (12V and 5V), and others that have a 12V laptop setting + a USB output - while less powerful, they should still suit your needs and won't have any fans (and are also in the same price range).

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  • Interesting, I didn't know such laptop bricks existed. The one I just looked at now only provides 4 A... But a very good idea. – Reto Höhener Apr 4 '15 at 21:28

As suggested by @joan's comment the minimum load of such a power supply could be an issue. Back in the olden days when people used PC power supplies as simple and cheap yet powerful lab power sources they connected high-power resistors and even light bulbs to provide a base load.

The issue at hand is the fact that not all switching-type power supplies regulate properly without a load - the open circuit voltage could differ greatly from the values to safely operate any connected devices. Tomshardware states that the issues of no-load operation range from shutting down of the supply to destruction. No-load operation includes operating at currents below a certain minimum load - and 1A for the RPi could be too low for some power supplies out there. It might also be necessary to apply a certain load to not just the +5V rail but also the +3V3 and +12V.

So if going for this kind of an ATX power supply definitely make sure that "No minimum load" is explicitly specified for the selected supply or that the minimum load requirements are met.

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I'd say that something like that might work, even if it's somewhat overkill. Just be certain that you're using a 5V output to power the Pi, and that the output is relatively clean. I'm not entirely an electrical engineer - I usually work on the software side - however you can power the Pi using the GPIO pins.

I suggest you check this site out to know exactly where to put your power. Power should be going in on one of the 5V pins (red, top right on the Pinout), and connect the ground on any pin marked black. I'd suggest physical Pin 4 and 6, respectively.

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  • Thanks for the pin info (that would actually have been my next question). I assume you mean pins 4 and 6? I have no idea what a "clean" power output is or how to check for this property, but if the output is good enough for a normal mainboard, shouldn't it be good enough for a pi board, too? – Reto Höhener Mar 31 '15 at 15:37
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    I meant 4 and 6, yes. Also, chances are if it's good for a typical desktop mobo then it should be fine for the Pi... hopefully. I'm not 100% certain about it. – sctjkc01 Mar 31 '15 at 15:39

It should work just fine.

Be aware that if you feed power into the GPIO header, you are bypassing the on-board fuse ("poly-fuse"). You may want to include your own external fuse, or just be careful about what other loads you put on the Pi.

For example, if you plug something into a USB port which draws excessive current, you could overheat the device (perhaps burn the traces between the GPIO header and the USB ports).

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    or just skip the gpio pins, use the micro usb power port, and keep the polyfuse in the loop. its not that hard to get the pinout of a usb cable, and cutting one and connecting it to the pc psu isn't any harder than connecting the psu to the gpio pins. preserves option of using a HAT, too – Matthew Najmon Apr 1 '15 at 1:19

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