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

torsdag den 20. februar 2020 kl. 23.16.57 UTC+1 skrev Commander Kinsey:

urrent drawn from the 5V line to make the 12V line work correctly?

uired 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 tho se 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 sm all dummy load of an amp or so to the 5V line, the 12V line suddenly become s 12V.

they are all on the same transformer so they are related, and the feedback is a combination of 12V and 5V

Not sure how a shared transformer can work. Since I can draw for example 5 times as much current from one line so the other would be adversely affected surely? They have to adjust each individually or it would go completely out of whack. I take more 5V current, so the whole thing gets boosted, now 12V becomes 14V.

d current drawn from the 5V line to make the 12V line work correctly?

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 bec omes 12V.

ack is a combination of 12V and 5V

5 times as much current from one line so the other would be adversely affe cted surely? They have to adjust each individually or it would go complete ly out of whack. I take more 5V current, so the whole thing gets boosted, now 12V becomes 14V.

Time-averaged output voltage(s) of an e.g. ideal flyback converter with either single or multiple secondaries in continuous conduction mode is a function of the input voltage, duty cycle, and turns ratio only it isn't directly dependent on any currents.

That is to say in the non-ideal case with a heavy load on the 5V and a light load on the 12 the 12 will not go out of regulation nearly that bad in a well-designed converter; the duty cycle is not supposed to be swinging wildly over light load to full load in CCM (until maybe it drops into DCM at very light load.)

Sounds like a clever design which fails under certain circumstances. Plenty folk have a computer with two or more graphics cards, so they need a 2nd power supply. Nowhere does it state the supply has to be loaded on the 5V line.

d current drawn from the 5V line to make the 12V line work correctly?

acquired three old graphics cards that take about 300W each, and have l... I run each card off its own supply. But the 12V line at no load, or even a t 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.

ack is a combination of 12V and 5V

The transformer puts proportional (AC) voltages on its windings, so the DC outputs are always proportional as long as resistive and rectifier losses are similar to the t est condtions. If you draw zero current through one of the rectifiers, but normal current through another, there's a discrepancy to be expected. Usually, 12V power has a high tolera nce, it's often the one to be left out of the regulation feedback loop.

Yes it "fails" when they cheap out on everything else in the supply, surely. How much you pay for them again?

In modern PCs just about everything runs of the 12V but almost nothing runs off it directly, it's a bulk supply to feed other converters on the mobo.

LEDs, fans, hard drive spindles get it raw I guess.

That's why I'm surprised they can get away with making PSUs that can't run properly with no 5V load. It's quite likely your PC sometimes draws almost nothing from 5V. I have to take almost 2 amps from 5V to make this PSU work correctly.

Not regulating the 12V properly is cheaping out, and from what I've read in these groups a lot of them are like that.

They're CIT supplies. They don't explode like Alpine when you dare to use more than 50% of the rated current, but they're not as stable as a proper Corsair.

bitrex wrote in news:CnF3G.207815$ snipped-for-privacy@fx43.iad:

Hard drive spindle motor drive circuits get it raw, but the spindle motor itself gets a quite controlled feed from that circuit.

"Commander Kinsey" wrote in news:op.0gbbg6ebwdg98l@glass:

Best PC PSUs I have used are by EVGA.

Those guys know how to make circuit boards and propely manage what is on them and where the juice is moving and what parts are producing heat. Their MOBOs are pretty good too, thought the gamingtard mobo makers have won out now due to so many gaming dopes building their own machines. Once you get all that money you can start doing better and better engineering in your products. Look at AMD. They *might* beat out Intel for a bit, but don't count out the big boys that got us all here yet.

No, it's your fault for using a supply designed for obsolete systems.

I think some PCIe cards like GPUs still use 3.3 off the PCI bus for something. The really honking ones have a connector elsewhere on the PCB for a higher current connection to the 12.

Pretty much all PC CPUs and GPUs need a direct 12 volt connection for power . It's been that way for over 10 years now. They used to use 5 volts for the CPUs, but the power levels got too high and 5 volt connections had too much current flowing. So they switched to 12 volts for the CPU on board PS U. These days GPUs use pretty much the same power levels.

--

  Rick C. 

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Nonsense; it's not a fault to find a way to repurpose an otherwise obsolescent part for such a use, it's a talent. The 'cheaping out' was an economy measure, and DID make the parts less expensive, so that was a job well done, too.

Nope. The coupling between windings is high, so they track pretty accurately--drag one output down and the others follow. Cross-regulating like that is quite common. In the limit of tight coupling (k=1), low resistance, and zero diode drop, the cross-regulation becomes perfect.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

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This, (k=~1) is what seems magical about transformers to me. (Right you can tell me it's Faraday's law, voltage integrated around closed line is change in B-flux through surface... etc..)

George H.

I largely agree.

However, I've found that salvage parts often have a down side that makes them sub-optimal for re-use. It's a matter of is the down side a blocker for a specific re-use.

In this case, it sounds like there must be current on a different voltage rail. Depending on the power levels, this doesn't sound like too big of a down side to me.

--
Grant. . . . 
unix || die