I was just given an old (from what I can tell) IBM AT power supply. It's pre-ATX and has a physical switch that connects the wall power to the internal circuitry.
Output is:
12 V  8 A
5 V  20 A
-12 V  0.3 A
-5 V  0.3 A

Now, the 12V rail sits around 10V unloaded, and if any load is connected, the voltage drops to basically 0. However, if the 5V rail is loaded (tested at around an amp), suddenly the 12 V rail maintains around 11.2 V 0.7 amps.

I'm wondering whether this is expected behavior or if something is seriously wrong with my power supply?

  • 3
    Yes, expected behavior by design. – DrMoishe Pippik Jul 26 '19 at 22:20

This sort of behavior is pretty common for older power supplies, which used relatively crude regulation technology by today's standards. The AT power supply had a minimum load of 7A @ 5V and 2.5A @ 12V; some configurations of the IBM AT actually shipped with an internal 5Ω power resistor to maintain that minimum load! (source)

Any PC power supply made within the last 5 years will have no minimum load, as Intel Haswell CPUs (which were introduced in 2013) introduced very low power idle states. Older ATX power supplies may have minimum loads, although they will generally be much lower than the ancient unit you're working with.

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