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.

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Commander Kinsey
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\ There's often a good reason for it. The 5 volt supply is regulated and the others are not (generally speaking). The feedback path is from the 5VDC output back to the mains side of the controller.

The others get line (but not load) regulation via the 5V supply because they share a common transformer. They are also switching supplies that work at a high frequency so the transformers have fewer turns of wire and more volts per turn which results in excellent transformer "regulation."

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So on a cheap shit supply, the 5V is guaranteed to be very close to 5V, but the 12V will drop under heavy load? And on a decent supply like Corsair, they must regulate both seperately? I still don't understand why the regulation goes to pot when under 1.5A is taken from 5V. It still regulates that 5V perfectly with no load, but the 12V goes wildly wrong. Why does the regulation need current to be flowing through 5V?

Reply to
Commander Kinsey

I suppose there may be some that regulate the 12 for both line and load but I haven't seen it. Not having separate regulation for the coarse 12 supply isn't necessarily a design flaw. The 12V is less critical,

Some (particularly older) designs do require some minimal loading of the 5V to keep it from drifting above 5, or above 5 AND causing the crowbar over-voltage protection from kicking in and shorting it to ~1V.

5V is more critical since a lot of components start smoking at 5.2V or higher and those are the logic components.

The 12V runs fans, platter motors in CDROM, disk drive motors, etc. and may be used for RS232 protocols, and it can be regulated on the board close to the load if the load is critical. The -12V is often pretty wimpy power-wise and it's function is for bias and/or communications protocols.

The PS should work for the purpose intended, but if you are trying to use it as a general bench power supply you may encounter issues with poor regulation.

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If you look at the 12V line on a Corsair supply under any load, it will always be between 11.8 and 12. A cheap PSU like CIT, with the 5V loaded normally, the 12V can be 11V to 12V depending on its own load. Buy an Alpine supply and it will literally go bang if you exceed 50% load for more than an hour. I went through 10 Alpine supplies, costing the shop a fortune, before I told them enough was enough and got my money back.

The supplies I'm having bother with are not that old, probably 5 years. But they were the second cheapest. They also lie on their specs. They're sold as 850W supplies, but you can only draw 650W of that on the 12V line, which is where 99% of the power goes in a modern PC.

Nowadays, aren't all the chips running at about 1V and powered by their own VRMs, fed off the 12V line?

I am using it for PC components, but only for one component, the graphics card, which only has a 12V input.

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Commander Kinsey

You mean to say "you get what you pay for?"

You could just get a nice bench supply if you only need 12V,

Yeah, a lot of the newer stuff particularly those processors made for smart phones, minimalist single board computers, TV box, Tablets, "Eee box" style ones, etc., do use 3.3 and 1.2 volt supplies on the board.

Most of my desktop computers are "off-lease" second hand ones.

I notice that my super-compact desktops use a single ~20V supply but that's probably just to keep the wire gauge small between the supply and computer.

How many graphics cards are you running from this supply at the same time? Just get a dedicated 12V supply(s). I'm using some ebay ones in projects, and have had good results. They even have a pot that lets you adjust the voltage +/- 10%. They go from 1 amp to 60 amp sizes.

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I can get more reliable supplies using PC supplies from Corsair, for not much more money. Plus they have the extra voltage lines if I need some.

Laptops do the same. Trouble is you end up with a thin wire which the user always breaks. Surely a thicker wire wouldn't cost much compared with the whole laptop? Maybe it also means thinner wires inside the laptop on the motherboard, less tracks, less space.

I can understand them wanting the voltage to be higher than that of the battery to make charging easier, but 20V is a bit much.

One each. The supplies are CIT 850W (650W on the 12V rail). The graphics cards are AMD Radeon R9 280X (250W TDP). I can actually run two per supply, but since I've been having bother, I'm letting them have one each to be sure, since I have a surplus of those supplies sat on a shelf. If I get short, I'll double them up.

I was thinking of car batteries with a charger or three. But that would provide 13.8V....

I might try that next time I need to buy one. Last time I looked they would have cost more than PC supplies for anything over about 200W. But that was a few years ago.

Reply to
Commander Kinsey

Commander Kinsey prodded the keyboard with:

In the old days I remember fitting load resistors in nice perforated metal boxes to PC power supplies. 12 ohms and 4.7 ohms. Some machines only needed the 5V one. Some the 12V one, a few needed both.

I still have a couple of them kicking about.

Best Regards: 
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