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I am currently trying to boot an Intel D915GAG motherboard fed by a Corsair HX450W. This PSU has two 4-pin processor power supply connectors available. I tried booting with either one of these plugged into the Pentium 4 power supply connector without success. Then, I used a Molex -> P4 adapter to supply juice to the CPU and I got the motherboard to boot fine. (The RAM stick I use is fine, and the only devices connected to the motherboard are a monitor, a PS/2 keyboard and a front panel power switch.)

However, I read somewhere that the 4-pin connectors may be inverted, either on the motherboard or on the PSU itself. Since supplying power to the CPU via the Molex -> P4 worked on two motherboards (this Intel one and a MSI), it is most likely that the inversion be on the PSU. (Or that the two (brand new) PSU's 4-pin connectors are dead.)

So, my question is : what happens if I invert one of the PSU's 4-pin CPU connector, so that the two ground and +12V lines are swapped ? Will it fry the motherboard, or damage the PSU ?

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2 Answers

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You may well fry the motherboard.

Test the output of the power connector with a volt meter and compare it to what you get through the molex->p4 adaptor. If the p4 power reads negative when the molex->p4 reads positive then they may be inverted. If the p4 reads nothing at all, return the PSU to the supplier and ask for a working one.

http://pinouts.ru/Power/atx12v_pinout.shtml has the pinouts so you know what you should be getting.

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I tested the output of the two 4-pin CPU connectors under load : 0.00V, compared to 11.95V with the Molex -> P4 adaptor. The PSU is therefore going back to where it came ! –  Bo Real Mar 31 '11 at 16:47
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Of course killing your motherboard is a possibility if you make the wrong connections, that’s why you’ll need to verify the pinouts and wires before making any changes. You’ll also want to do some additional tests to confirm before messing with what should be a standard (I believe that the inversion you are referring to is a propriety pinout that certain distributors like Dell use—or used).

This page has all the information you could need about PSU connectors. It details (with nice, closeup photos) all known (standard) connectors, wires, and pinouts.

The main things to note are that there are two different 4-pin connectors in the ATX standard. Both basically just provide more power for newer, more power-hungry CPUs, video-cards, devices, etc. but they are very different. One is an addition to the normal 20-pin and has 3.3V, 5V, and 12V rails and a ground, while the other is often labeled as a “standby” line (to support ACPI S3) in motherboard manuals and has two 12V rails and two grounds. As you can see, they are quite different, and plugging the wrong one into the wrong socket would be very bad.

Look at your board and check the power connectors; there should be a 20 or 24-pin socket and probably a 4-pin socket. Then look at your PSU’s wires; there should be either a 20-pin cable, a 24-pin cable, or a 24-pin cable where a 4-pin block can snap off. (Checking the manuals for both is advisable.)

Also, look at the colors of the wires. They should make it easier to determine what’s what. If they don’t correspond to the standard, then it may be a proprietary PSU and you may want to consider getting another one.

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Thanks, I checked my cables and connectors for inversions, and found none. –  Bo Real Mar 31 '11 at 16:50
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