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  • Author Author: dougw
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Convert a PC PS to a Bench Power Supply

dougw
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I have a collection of old PCs from various scavenging activities. Somehow I just couldn't seem to pass up opportunities to acquire "free" PCs. They are probably all working, but I don't actually have a use for them as computers. I have "intended" for a long time to scavenge whatever might be repurposed off some of these computers - disk drives, power supplies, heat sinks, etc. But even for those items, I don't have a pressing need. I did make several attempts to design a conversion to create a bench power supply from the PC power supplies, because in the past I always seemed to need more power supplies, and they can be pretty expensive to buy.

I bought connectors and binding posts and even designed a couple of versions of PCB to get started. However I always stalled when trying to come up with a reasonable (cheap) way to package the bench supply. This Projec14 Upcycling theme is a good excuse to finally put a complete supply together.

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These old PC power supplies have a bunch of output power connectors and the main connector comes in a couple of variants.

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Note that the V2 connectors have all the same pins in the same places as the V1 connector so they are backward compatible, except the V2 power supplies are less likely to have a -5 V output.

Additionally, some of these power supplies need a load before they will regulate properly. The supply in the picture below is like this so I have provision for a load resistor on my PCBs.

I bought both types of connector, just in case, but the bigger one will always work, if plugged in at the right end.

I also bought a cheap commercial version (also shown in the picture below - the green card), but its tiny binding posts are at a non-standard spacing and cannot accommodate banana plugs.

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Note that this particular power supply uses a V1 connector and it has a -5V output. (The LEDs from left to right indicate +12, +5, + 3.3, -12, -5)

The larger card (that is powered up in the above picture) was the first one I designed - when I had a different idea about how to package it.

The smaller card takes no more room than the power supply front face, so this is what I used in the build.

The supply I used in the build (different from the one pictured above) has a V2 connector and it does not need load resistors to regulate properly, but it has no -5 output.

Here is a video showing how these parts fit in the case I designed:

I still have enough parts to build at least 2 more complete bench supplies plus more supplies and partial kits of parts, if I can find the time or if a need arises.

These supplies do take up space, but not as much as the full PC box they came out of.

I am happy that I didn't need to order parts (because I bought them years ago) and I am happy that I can at least mostly cross this project off my backlog/bucket list of projects.

It turned out to be quite a bit more work than I had envisioned way back, but I am happy with the final look of the supply, given all the awkward constraints it needed to accommodate.

I could have picked a more photogenic color, but black was what was in the printer.

In the end it does look like a bench supply with a nice handle and it has high quality binding posts.

Update:

I traced the power switch issue to a faulty switch and fixed it:

 

A question arose in the comments below about how much power the 5VSB (5V stand by) pin could supply. I extracted another PC supply from yet another old PC to test it.

I connected my little load box which can switch in a load of up to four 10 ohm resistors in parallel and measure the voltage and current...

The 5VSB could supply 0.5 amps more or less okay, but 2 resistors (= 5 ohms) brought the voltage down below 3V...at which point the meter didn't have enough voltage to work.

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No load = 5.01V                                                      10 ohm load -> 4.39V, 0.46A

My power supply doesn't do anything with this pin.

 

Relevant links:

Project14 | Recycle & Retrofit: Reduce Waste in Style By Upcycling Vintage Electronics!

A later project to convert a PC Power Supply

  • in reply to ajcc

    The handle was made to be comfortable for my hands.

    Most handles in the world seem to try to be the minimum size they can get away with, which I find generally uncomfortable - not enough room for all fingers or knuckles and so skinny they dig in painfully, especially on something this heavy.

  • in reply to Gough Lui

    Most of my PC supplies were built in the last century.

  • in reply to ajcc

    It's highly dependent on the supply. I've got some older ATX supplies rated as low as 1.5A on the 5VSB line. As they age, with degraded capacitors especially on the cheap/generic units, it wouldn't surprise me if the output was a bit low. The line definitely droops while approaching full load. Originally the 5VSB functionality was mainly to power the soft-off power management IC which controls the power button and provide some power to the onboard RTC which would take load off the CR2032 cell when mains was available.

     

    Newer supplies usually are rated 2.5A to 3A on the 5VSB line - this is because more modern computers have increased functionality that depends on the 5VSB to operate. This includes refreshing the RAM banks when you're in S3/Suspend to RAM mode, powering the LAN/WLAN chipsets for WoL/WoWL functionality, powering modems for Wake-on-Ring (not a thing anymore) and supplying current to USB ports which are configured to be powered off 5VSB (e.g. for charging while the system is off).

     

    There is still the chance to overload the 5VSB line under certain configurations - I found that out the hard way when I destroyed the rectifying Schottky diode on a power supply by inadvertently leaving a USB bus-powered hard drive attached to a port powered by 5VSB and suspended the computer. It was the load that "overloaded" the supply (as it was WoL/S3 at the time with other loads attached too) and even though the specs usually say that the supply should shut down prior to any harm occurring, instead I got a burning smell and by the time I tracked it down, it was already toast.

     

    - Gough

  • in reply to dougw

    That's far from the recommended 2 A, thanks for checking that. Wikipedia says USB should be within 5.00+0.25-0.60 Volt, so it's even less than half an amp image

  • in reply to ajcc

    The 5VSB couldn't supply 2A - see update above.