ATX power supply tips

Hello guys, I have two questions on you.

I'm working on a bench power supply from an old ATX power supply. I don't have enough money for a proper lab power supply - I know that it won't do miracles, but I just need something to power my experiments. I want to make a power supply with 1x variable output, 2x 12V output, 2x 5V output and 2x 3.3V output + 2x USB connectors for charging. I've added Arduino for measuring voltages, LM317 circuit for the variable 1.25-10.8V line and some thermal sensors for measuring the temperature. This part is done, it works. But here are my two questions:

  1. Every time I read about ATX PSU conversion to bench PSU there is something about a dummy load on the 5V line. But I don't know if I even need it. I've tested my ATX PSU. It outputs stable 12.06V, 5.09V and 3.4V. When I attach some load (9V DC electric motor - no matter which line I use) - it works. Voltage drops only slightly, current is around 1-1,5A - and voltages across all the other lanes are the same. Does it mean that I don't need a dummy load?

  2. I unsoldered the 3.3V sense wire, because I want to have two 3.3V outputs, not just one - I thought that I might solder back two another wires and attach one of them to each 3.3V output line. Will it work? Or it won't get accurate information when I will draw different currents from different outputs? Or it will be better to attach that sense wire only to one output? Or should I keep it removed?

Thanks for your answers :slight_smile:

  1. I have 2 old ATX supplies that I use all the time. Neither have dummy loads.
  1. ATX supplies are not designed to drive no or light loads. So they might do funny stuff then. But it all depends on the supply. I added some load on both 5V and 12V, just to be safe and not have to worry about it.

  2. Will kind of work. But those two lines are probably just connected to the same rail on the PCB. (Check it!) So by connecting the sense wire to both terminals you essentially just parallel the two wires. So I would just connect both 3V3 wires and the sense wire to the first output and just a short but relative thick wire to the second output.

Easy "load" for both 5V and 12V outputs is a 12V car tail light incandescent bulb. AND it tells you the power is on.. The 5V output is a lower glow but a good load.

After thinking about the sense wire - I think it will be better to have him removed, because sensing a voltage drop at one output would result in increase - and because all the 3.3V wires comes from only one node, bad stuff can happen on the second output.

And by the way, I also want to use some switches. I have two types - first is rated for 250VAC 3A and second for 125VAC 6A. Does this mean that they are both rated for 750W, so at the 5V line they can run 150A? Or I can use only the shown current (3/6A)?

Or I can use only the shown current (3/6A)?

Only use the shown current.

After thinking about the sense wire - I think it will be better to have him removed,

Uhm, I doubt you can run it without... With the sense not connect the PSU just thinks the 3,3V is low and will try to compensate it. So you need to connect it somewhere. If you do it close to the PCB there is no change of a voltage being higher then 3,3V, only lower because of losses.

Does this mean that they are both rated for 750W, so at the 5V line they can run 150A?

That's not how current works... Current is current. But they give you a AC value. AC is easier to switch so I would not even try to switch 3A or 6A DC with it, I would at best half it.