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

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.

Reply to
Commander Kinsey
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A lot of cheap supplies regulate only one output, and rely on cross-regulation via the transformer to control the others. If the regulated output isn't loaded, it rises out of spec and so do the others.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Strangely, with no 5V load, I get 5.2V and 10.5V. A small rise and a large drop.

I can't understand why the following happens: No 5V load, 12V is out. Small (2A) 5V load, 12V is ok. Yet if I draw 30A from 5V, the 12V is still ok? How can zero load upset it, but 2A or 30A (big difference) both be ok?

Reply to
Commander Kinsey

The difference between nothing and 2A is a factor of infinity The difference between 2A and 20A is only a factor of 10

Actually the fisrt is more like the difference betweem 2mA and 2A because the internal feedback takes avout 2mA to run the LM431 and the optocoupler. So going to 2A loads the 5V output by 1000 times more, better than infinity. but not by much.

Diode voltage drop is logarythmic vs current so the voltage on the transformer needed to make 5V on the output is less with a 2mA load than it is with a 2A load.

--
  Jasen.
Reply to
Jasen Betts

If it's going to be so shit, they could have added a dummy load inside the PSU to make it work properly. They could even have it shut off if there was enough external load.

Reply to
Commander Kinsey

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Small (2A) 5V load, 12V is ok. Yet if I draw 30A from 5V, the 12V is stil l ok? How can zero load upset it, but 2A or 30A (big difference) both be o k?

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e PSU to make it work properly. They could even have it shut off if there was enough external load.

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That is absolutely stupid. A dummy load wastes power, it generates extra he at and the heat shortens the equipment's life. Those are dedicated power su pplies, not bench or lab supplies. They were designed to operate with a min imum load. If you don't like it, go to a four or five output supply with tr immers to calibrate each output. Be prepared to pay $250 to $400 for one.

You bitch about only getting 650 Watts output. Read the specifications. tha t 850W maximum is fully loading all outputs at the same time. Do yourself a favor and take some basic Electronics classes.

Reply to
Michael Terrell

I said "They could even have it shut off if there was enough external load."

A decent supply made by Corsair lets you take the full 850W from the 12V line. The designers of this supply are morons.

Reply to
Commander Kinsey

In a supply which powers a computer, where currents change all the time on different voltages, this is a very bad design. I guess I just buy the better ones in future.

Reply to
Commander Kinsey

All.

It's on the label on the side, but it's sold as an 850W supply, which it isn't, so they're lying. It's like selling a car which can go 150mph, but only downhill. Technically yes, it can go 150, but not in a useful way.

So pretty much everything in the computer.

No, 19V.

Find me a 1kW version I can get in the UK, and not from Amazon. I wouldn't trust those Charletons with a bargepole.

In fact nevermind, I've already got one from China. Unfortunately there's nothing nearer me so I'll have to wait a month for postage.

Reply to
Commander Kinsey

For automobile, boat, truck power, they use 12V, as I said. You're thinking about laptop computer charge power supplies.

MeanWell is a major manufacturer; your willingness to trust is... a problem only you can solve.

SLPower.com LU500S12T gives 500W, and you can bolt several together (they current-share). They won't come with cables and connectors like a PC uses, though.

Reply to
whit3rd

Well you did say "mobile-PC" not "truck".

Trust cannot be manufactured by me. I only trust those who give me what I pay for.

The site says 400W.

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When I tried a stock check, it didn't understand its own part number.

I don't need the connectors. I've bought this:

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12V 1kW version (now out of stock, I hope I get mine!)
Reply to
Commander Kinsey

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