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?

[Commander Kinsey]

If I don't load the 5V at all, the 12V becomes 10.5V. Connecting a car headlamp to the 5V rail (which draws about 1.75A) seems to make it behave.

I don't understand. Surely the mark space ratio has to be completely different for the 12V and 5V rails because any computer at any given moment could be drawing more from one than the other, then differently several seconds from now.

Link me to an example, because I looked at them before, and at the high powers I need (300W per card), it's massively cheaper to buy even the top of the range PC supplies from Corsair etc., I assume because they're made in larger quantities.

[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.

[gfretwell]

I suppose "that is just how they work" would be too simple an answer but it has to do with switch mode power supply design and you already know the work around. Put a big wire wound resistor on the 5v and get on with your life. I doubt you even need an amp. 25 ohms should do it. (200ma)

[Commander Kinsey]

That's not how I thought SMPSs worked. Surely since the current draw on 5V can vary from 1 amp to 35 amps, and the 12V from 1 amp to 56 amps, there have to be completely seperate ratios set up? Adjusting just the 5V could have the 12V going anywhere.

[Commander Kinsey]

I shoved a car headlamp on them, full beam from one PSU and dipped beam from the other. Drawing about 1.75A from each.

I might try smaller bulbs to reduce fire risk :-)

[Another Dave]

and select what you need. Plenty more.

Another Dave

[Commander Kinsey]

It needs more. I stuck an 18 ohm resistor on it, the 12V line went from 10.46V to 10.91V. With the bulb, I get 11.70V. I'll stick with the bulb (which is quite capable of taking that much heat since it's designed for 12V not 5V).

[Commander Kinsey]

Incorrect, this PSU is happy with me taking 1.75A from 5V. It's rated at up to 35A on 5V. So clearly it will be ok anywhere from 1.75A to 35A on 5V. Explain how that is possible to vary what I take from 5V without affecting the 12V regulation, yet 0A on 5V upsets it.

I'll stick to Corsair PC PSUs. That one you linked to is Chinese noname rubbish, I find those tend to overheat and break if you load them over 50%. Yours is £21 for 360W. I can get a Corsair of the same power rating for £30 that I know will work. Or 550W for £35.50.

[Commander Kinsey]

No cheaper than Corsair PC PSUs, which I know will work and last.

[Commander Kinsey]

But PCs could vary the 5V and 12V considerably. For example when you start playing a game, the 12V changes dramatically for the CPU and GPU, perhaps by a factor of 10. Fuck knows what still uses 5V, but it won't be anything like constant.

[Commander Kinsey]

What does it vary with then?

[Commander Kinsey]

Yet it does when the current is zero? Or as I've just tested, less than 1.5A?

Not on Corsairs, they don't have the above problem. The ATX specs (which they bother adhering to) require that all voltages be within 10% no matter what current is drawn from anywhere.

[Commander Kinsey]

I've bought PSUs from China before. They get too hot to touch running at 50% load!