My self-made lab-bench power supply

I build a lab-bench power supply from old computer parts

This is a fairly common build, since it’s probably the cheapest and easiest way to obtain a capable power supply. I used mostly salvaged parts and paid only 3€ for the amp/volt-meter display, as well as 11€ for a ready-made unit that will allow me to add a variable output with a current limiting feature. Buying a „cheap“ lab-bench power supply with these features will run you upwards of 80€ at least.

The last part has yet to arrive, so it’s not included yet.

This project is basically just an exercise in cable-management. I’m still learning.

What I did differently from all the other builds I’ve seen on YouTube and is that I mounted the fan on the outside of the case, to gain some more room on the inside – These things are built super compact, so this was a huge quality-of-life improvement when working on it. I don’t think I could’ve fit all the components into this tiny case without doing this. It will also greatly improve ventilation after I’ll add the board for the variable output. Last but not least, I simply enjoy the McGyver-ish look it gives the whole project. I love the unpolished, improvised look of self-made electronics.

I started with a used 550 watt computer power supply from bequiet!, which I got for free from my brother-in-law. Step one was figuring out how many of all the connections I needed and removing the superflous wires. I ended up having to solder another 5V-line back in in order to power the display, and will add at least two 12V-lines back in for the variable output as well, as the supply has two seperate 12V-outs and using both of them will allow me to draw more power should I ever need to.

I cut the front panel (which is really a side-panel based on the PSUs intended original orientation) with a knockoff-dremel and some cutting discs. Wearing eye-protection of course. Handling a rotary tool is a lot easier than I expected, but cleaning up the edges took *a lot* of filing. Do be careful to clean away any metal shavings that might fall onto the PCB, otherwise you risk the whole thing going up in flames when you switch it on.

This seems like a good place for the disclaimer:

These devices are powered by a 230V mains connection, which means that stupid mistakes can kill you. Power supplies also typically include very large capacitors, which can hold a charge for several minutes after you switched off the device. I don’t think that those hold enough electricity to kill you, but they hold their charge at up to 400 volts, and getting bitten by that really, really hurts.


…Why yes I did learn this the hard way.

The last bit about the capacitors is the reason why I think it’s non-optional to have a status-LED connected to the 5V standby-line: After switching off the power, this LED will draw current directly from those capacitors, which a) helps drain the caps a bit quicker than they would on their own, and b) gives you a clear optical warning while there’s still a charge remaining.

Schematic for the basic wiring connections

Some power supplies need a constant load or they won’t switch on; that’s what the Power Resistor in the schematic is for. That’s a screenshot from this video by the way, which I highly recommend you watch if you’re going to try this yourself. My supply can be switched on just fine without any load attached, so I left the resistor out. I also didn’t include the protective fuses – I accidentally shorted it a couple of times while testing (I need crocodile clamps with better insulation!), and the PSU seems to handle that just fine; Nothing happens except that it shuts off all the outputs and goes into standby-mode, so I think I’m good here even without the fuses.


As I already mentioned, I’m waiting for a buck/boost module based on a LTC3780. It’s 7.8 by 4.7cm and I hope I’ll be able to fit it inside the enclosure without blocking the ventilation too much. It will provide the two most important features: A variable output between 0 and 32 Volts, as well as current limiting.

The variable output will be powered by the PSU’s 12V-Line #1, which is rated for an output power of up to 26 amps. The actual output will be limited by two factors: Wire thickness and the cooling on the LTC3780-module. The latter can be improved by smart positioning inside the enclosure so that it gets efficiently cooled by the main fan, and/or by replacing its heat sinks.

The wire gauge can be „improved“ by simply using more than one wire for the connection. The video I mentioned above bundles three wires together for each of its outputs (which I think is overkill, but also did anyway).

The ultimate bottleneck which I can’t do anything about will be the volt/amp-meter. On paper it can take up to 10A, but somehow I doubt that. The common ground of all the outputs goes through this thing, because of course otherwise it wouldn’t be able to measure the amperage. It’s also a flimsy module with questionable QC that I paid less than 3€ for, so I expect this to be the breaking point.

I’m toying with the idea of connecting the variable output two a second set of binding posts with a ground connection of its own, bypassing the volt/amp-meter and all its questionable build quality. That would really allow me to draw full power from this thing, though I have no idea what I’d ever use it for.

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