PC PSU repurposing

Check the voltages. They can have weird cross-regulation.

when I use one I put 100 ohms on each output voltage.

That seems to work.

Else connect an old harddisk to provide a bit of load.

Today, PSU can operate without load ; they are regulated.even with no load (flyback). At minimum, they loose power in the leakage elements and the flyback circuitry.

I've been using one as a bench supply for some years with no ill effects and no minimum load. The one I have seems to only regulate the +5 line and the 12v lines are "unregulated" (set only by the turns ratio on the step-down transformer)

The +5 regulates the primary side of the transformer via a high frequency square wave and this provides line regulation to the 12 V but no load regulation.

Load the +5 one amp. That means a 5 o hm resistor, and it will be dissipating 5 watts of course.

You could try to get by with less load but that may vary from PS to PS.

I have some surplus general purpose switching supplies that came with load resistors on the output. Kind of odd, but one way of making sure it works. I guess the rest was cheap enough that adding power resistors, and wasting power, was still efficient.

Of course, PC power supplies have an intended use, an expectation of a certain amount of load, so it makes sense that they wouldn't bother with loading resistors.

Michael

Thanks for the replies.

I'm using the PSU's 12v as a source for this buck-boost converter:

Google hits on the question said that the 12v output needed a 1A minimum load and I didn't like the idea of a 12W resistor.

I'm going to put a 1/2A load on the 5v, cause it can't hurt and deal with a 12v load if I have to.

Thanks again, Bob

I solved this by having a 50 mA power on LED running from the 5 V line. You don't want a PSA without indication anyway.

Groetjes Albert

Normally, a good PSU is capable of running correctly without load.

I suspect this idea that PC supplies need a minimum load began because some did in the early days, but these days? No.

I looked at a schematic of one and the 5 volt supply was the only regulated supply. The others derived line regulation because the 5V set the standard, but the others had no load regulation at all - the voltage was strictly a matter of the turns ratio of those transformer windings.

There's always the chance that you may run into one of the old power supplies, but this idea that they all need some minimum load is simply not true.