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I have a PSU that I would like to verify and see if it is outputting all of the correct voltages.

It's an old computer PSU from 1990 with a specific form factor that cannot be easily replaced with a new one.

The PSU will only switch on with a certain load attached to it. Usually it is sufficient to hook up a hard drive or an old floppy drive in order to get a computer PSU to start properly. Without load it will either not start, or try to switch on but make a clicking noise. Without load it is also not possible to measure any output voltages.

I am always a bit reluctant to hook up the actual intented load on to it (in this case a mainboard + hard drive + floppy drive), in fear of risking to damage the mainboard should there still be an issue with the PSU.

What is the best way to simulate such a load ? I was thinking about hooking up some resistors on certain power lines (The PSU also exposes -12V) but I am a bit unsure on how to calculate the resistor values (actual resistance but also wattage)

What would be a safe / meaningful way to simulate some load and make the PSU think it is hooked up to the mainboard, so I can verify that the voltages are OK before hooking up the PSU to the mainboard.

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  • \$\begingroup\$ Do you by any chance have a datasheet or a link to the PSU? If it'll only work with a specific load, it may not be that a simple resistive load will work. It could be actually looking for something in particular. Without knowing more about the PSU, it's difficult to give advice \$\endgroup\$ – MCG Jan 20 at 12:39
  • \$\begingroup\$ Unfortunately I don't have the datasheet and haven't found anything online. It's a 1990 Olivetti 286 PC. I tried hard drive + floppy drive + a 12V fan and although The fan did run, I could hear the PSU switching on/off constantly. Only with the motherboard hooked up to it did I get clean output voltages. Would it hurt to try with a resistive load (10 ohm resistors with sufficient wattage)? \$\endgroup\$ – ddewaele Jan 20 at 14:50
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I'm not too familiar with computer PSUs, but I can offer a few things that I think might help.

1) Look at the specifications for your intended load. In a datasheet or specsheet there should be a power draw spec or at least an operating voltage and a range of possible current draws. This allows you to look at the worst case(most power absorbed by load) and best case(minimum power absorbed by load). After doing so, you could create a resistor network on the output terminals you're interested in that draw the same power. It's important that you choose resistors suited for that power draw, otherwise they're going to overheat.

2) If you want to do it empirically, you could connect a network of potentiometers (each with large values) in series, and you can connect them so that they are all at the highest impedance at first. This would essentially simulate an open circuit, so you would have close to zero power draw. After doing so, you can very gradually, one by one, lower the resistance of each pot (from highest impedance to a short) until you can see that the power supply is switching on. It's important that you never try to draw more power than allowable by the PSU though, as this will damage it, so make sure to check the spec on max power for your PSU.

I'm making these assumptions based on my usage with general power supplies on lab benches. I'm not sure about the specifics of computer PSUs, but it's strange to me that the power supply refuses to turn on without a load, seeing as an open circuit shouldn't damage the supply in any way I believe. This is a reach, but it sounds like your power supply is somehow shorting without a load (maybe internally) that's causing it to not turn on, maybe as a short circuit protection mechanism. This probably isn't the case seeing as it works with certain loads. Hope this helps.

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  • \$\begingroup\$ I do not have a datasheet, but was thinking about getting some 5W or 25W resistors and hook them up between the 5V and GND and 12V and GND. So a 10 ohm resistor on the 12V rail would give a 1.2amp current at 14.4 watts. A 10 ohm resistor at 5V would generate 0.5amp current at 2.5 watts. Would that be an ok way to verify the PSU, providing it is capable of handling those currents ? Or should I pick other resistor values ? \$\endgroup\$ – ddewaele Jan 20 at 11:57
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my experience is with ATX pc power supplies, different brands (models even) might vary. but the load doesnt matter. if its enough for it to turn on, it cant really tell what exactly its trying to power, it just knows SOMETHING is pulling current so it should stay on. ive got one that i use to power a 3d printer that i trick into staying on by using a 5v usb led strip that i cut the plug off and twisted the wires together. also older supplies do their sensing on either the 5v or 3.3v line, some newer ones on the 12v but shortly after they stopped needing minimum loads for the most part, so you could probably just hook up an old cell phone or ANY old device that has usb input to the 5v line and short the black and green wires

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