Problems with 12V and 5V lines on a PC ATX supply

[Commander Kinsey]

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.

[Commander Kinsey]

Sorry for the mention of sorry the crosspost, that was for the (rather dead) electronics groups :-)

[Andy Bennet]

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.

[Commander Kinsey]

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

[Andy Bennet]

Can't be arsed. Have you heard of Google?

[newshound]

You already know the answer, put a load on the 5 volt line and all will be well.

[Commander Kinsey]

Yip, tried it, not the sort of thing that seems to have been discussed much. I found a discussion on stackexchange, but all anyone said was "The ATX standard requires the power supply to be within 10% of the correct voltage, at ANY loads". So clearly my cheap PSUs are not adhering to the spec, and assuming I'll take 5V from them aswell, but I want to know why this happens and why anyone would design a PSU in such a weird way. The mark space ratio to feed the 12V line and to feed the 5V line have to be seperate, to allow for varying loads on each as the computer runs. So how on earth could they be linked?

[Commander Kinsey]

I know the workaround, I want to know WHY this makes it work.

[Brian Gaff (Sofa)]

Maybe because its not well designed.Switch mode supplies tend to be designed with a minimum load in mind in my experience. Brian

[Brian Gaff (Sofa)]

AS I said, its all part of the fact that its switch mode, and the regulation is not a classic series regulator, its a feedback loop controlling the oscillation of the switch mode part, its all interconnected. I first encountered this weirdness many years ago when a pc I had had some tacky bright leds all down both sides of the front, making it look terrible and for me making my failing sight rebel. So I thought, simples, unconnect them. It was then that the pc decided it either would not start correctly or would not shut down an and lots of tother things went weird. Reconnecting the leds and all was well again. In the end I hid the leds in the case. Out of sight out of mind. I expect the spiders enjoyed it. Brian

[Commander Kinsey]

I can understand it going off with a high load, but zero or low load is just weird.

[Commander Kinsey]

That's weird. Surely the LEDs weren't the only thing drawing power at that voltage?

[Dean Hoffman]

There's a group sci.electronics.design that still has life. They might like the diversion from arguing the merits of the electoral college.

[Andy Bennet]

The scottish wanker loaded the same question to all the electronic groups at the same time. Playing off one set of groups against the other.

[Commander Kinsey]

Playing off? WTF are you talking about? Since I don't inhabit those groups, I don't know which have any traffic or anyone who can answer, hence I asked in all the relevant ones.

[Commander Kinsey]

I shall try it.

[Andy Bennet]

You are asking all the same f****ng questions as you have done here - and amazingly getting all the same answers. But still you persist.

[Commander Kinsey]

Of course I'm asking the same questions in different groups, to perhaps find someone who has the intelligence to answer them.

[Andy Bennet]

Did you get your "Does a parrot's foot conduct electricity?" post answered on the s.e.design group?

[Commander Kinsey]

I understand alright - that designers are morons.