On Wed, 19 Feb 2020 17:01:52 -0000, Andy Bennet wrote:

On 19/02/2020 16:46, Commander Kinsey wrote:
Why do (cheap? expensive ones may be better) PC ATX power supplies need
current drawn from the 5V line to make the 12V line work correctly?

I have a PC with 3 graphics cards running scientific applications. I
acquired three old graphics cards that take about 300W each, and have
loads of cheap (CIT) PSUs that are rated at 650W on the 12V line, which
is what those cards use. So I run each card off its own supply. But
the 12V line at no load, or even at 300W, is only giving out 10 to
10.5V. If I attach a small dummy load of an amp or so to the 5V line,
the 12V line suddenly becomes 12V.

Why are the two lines related in any way?

Sorry for the crosspost, I'm not sure which of these groups are active.

On el cheapo SMPS the primary supply is the 5V whose feedback controls
the conversion transformer. The 12V supply is derived from the same
transformer so if the 5V is not breaking into a sweat not enough current
is available in the transformer secondary to get the 12volts up to
regulation.

Ok, I do have a degree in electronics, but it was a very long time ago. Can you go into more detail?