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

Commander Kinsey · 2020-02-20T22:16:53+00:00

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?

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John Miles, KE5FX 6 years ago

I'm working on a box right now that uses a 250W redundant ATX supply (at the customer's request):

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My application only needs +12V, and when I took one of the PSUs apart for evaluation, I saw that only the low-current +5 standby and +12 rails are actually generated by the ATX supplies. +5, -12, and +3.3 are generated from +12 by outboard regulators on a separate board.

What's frustrating is that while this particular supply works just fine with no load on any of the rails, the manual calls for 1A minimum loads on +3.3, +5, and +12, and 500 mA on +5 standby.

This is for $$$, not a homebrew hack, so I have no choice but to follow the manufacturer's specs and bolt three big resistors to the chassis to burn almost 10W for no reason.

The other weird thing about redundant power supplies is that there is no way for the supply to signal that one of the supplies has failed. There's a loud alarm buzzer, of course, and LEDs to indicate the status of both supplies, but the power-good signal stays high when you unplug one of them, and the server motherboard has no way to know anything is amiss.

So I did hack it to the extent of replacing the status LEDs with connections to the controller. I feel like I'm overlooking something obvious here, because most of these supplies end up in data center racks that are hundreds of yards away from the nearest human. Do Google and Amazon just send somebody down the aisles every so often to listen for buzzers...?

-- john, KE5FX

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John Miles, KE5FX 6 years ago

Actually, looking back at the data sheet, there's supposed to be a separate TTL signal cable, which for some reason the supplies I received didn't have.

WTF, that's kind of annoying.

-- john, KE5FX

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Commander Kinsey 6 years ago

Someone running the SETI project told me he has a 1600W EVGA power supply (for 7 watercooled graphics cards). Apparently it runs perfectly. I'd never even heard of a computer PSU over 1000W. I've always stuck to Corsair for supplies, but I'll be adding EVGA to that list.

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jrwalliker 6 years ago

There are plenty of old HP server power supplies on eBay at very low prices. They are all 12V at up to around 1.5kW and have built-in fans. The lower power ratings are more common, for example 750W or 1.2kW. They all have a standard edge connector. Multiple identical PSUs can be connected in parallel. Certain pins must be linked to turn them on - the info is out there. I can post the part number of the mating connector if anyone is interested. The connectors are available from Mouser.

John

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Commander Kinsey 6 years ago

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ve a standard edge connector. Multiple identical PSUs can be connected = in

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sted.

Are they happy giving out only 12V? As in no load on the other voltage = lines?

I didn't know older supplies would be that powerful. Wow, =A317.50 for = a 2450W including postage in the UK. I wish I'd seen that. I'm waiting= on a 1000W supply from China....

Changing the connectors ain't a problem.

Are they really ok in parallel? If one is trying to give out 12.05V and= the other 11.95V (even identical models will never be really identical)= , then surely only the first will supply any current, and will overload.= Not that it matters with 2450W, I'd have several smaller loads so they= can be split between supplies.

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amdx 6 years ago

This looks handy!

Mikek

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Michael Terrell 6 years ago

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ine,

One review said those are not banana jacks. Just cheap binding posts to wrap wire around.

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amdx 6 years ago

Well that does suck! If I were interested, I have a drawer with plenty of banana connectors I could do a replacement.

Mikek

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Michael Terrell 6 years ago

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line,

s to wrap wire around.

I have a few hundred of various styles, to replace worn out ones on test eq uipment. I just wanted to point it out, before someone bought one, then the y had to order real combo binding posts/banana jacks.

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Commander Kinsey 6 years ago

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0 for a 2450W including postage in the UK. I wish I'd seen that. I'm w= aiting on a 1000W supply from China....

5V and the other 11.95V (even identical models will never be really iden= tical), then surely only the first will supply any current, and will ove= rload. Not that it matters with 2450W, I'd have several smaller loads s= o they can be split between supplies.

+12V.

Pah, earths aren't all they're cracked up to be. In fact I've taken the= earth off my microwave as my pet sits on it and chews wires. No ground= , no circuit.

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Commander Kinsey 6 years ago

What is it with California?

"WARNING: This product can expose you to chemicals including Nickel which is known to the State of California to cause cancer and which is known to the State of California to cause birth defects and/or other reproductive harm."

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

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