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I'm trying to use an old 20 pin ATX power supply as a substitute for the failed power supply in a Dell Optiplex 7040† I managed to find the pinouts for the Dell power supply which only has two output connectors. One of them is a standard 4 pin CPU connector. The other one is a non-standard 8 pin connector. I compared the pinouts with the old 20 pin ATX, and everything matched: BLACK Gnd; GREEN PS_ON; GRAY PWR_OK except for the purple wire. Every pinout I found for the ATX power supply indicated PURPLE +5VSB. The problem is that the "Dell" pinout showed the purple standby wire as +12VSB.

My own uneducated guess tells me that the ATX standard would not allow anything other than the specified +5VSB The other factor which fed my suspicions was that the "Dell" pinout wasn't from an official source, but was posted by someone in a user forum and the diagram didn't have a professional appearance the way I would expect a diagram would look if Dell had made it. I think it's likely that a mistake was made transcribing the pinout I found posted in the forum. It was a long and frustrating time I spent before finding the "Dell" pinout, so I thought I would try asking here before searching for another pinout that agreed with the first one. (And if those two pinouts didn't agree, then this unpleasantness would continue at least until I found a third pinout to break the tie!)

I hope this question isn't excessively low in quality and/or particularly annoying to the reader, since I can imagine someone pointing out that the only way to get a reliable answer is to hunt down the actual documentation from the manufacturer. I really went nuts just to get the "Dell" pinout above so I'm willing to settle for a less than a definitive answer if it were something like, "I've been a professional in the field since the first IBM PC and, while I can't say I'm absolutely certain that no PC has ever had the purple standby wire carry +12VDC, I think you'll probably be OK assuming that it's really +5VSB" I'd even be OK with a 1 in 10 chance of disaster if the odds came from a seasoned pro who was comfortable in making such an estimate of nominal accuracy.

I hope I haven't made a faux pas by including the previous paragraph. In the event that it was a blunder, can I pivot to a less subjective question? Assuming that the pinout is true and +12VSB is actually correct, what are my options going forward? What functionality depends on the purple standby wire? Would my PC be inoperable without it? How critical are the tolerances involved? Is there any/enough flexibility in the design specifications to allow modification? Would it be possible to (gulp!) splice the purple wire to a 12V switch-mode "wall-wart" which came with my USB external hard drive (or any such typical 12VDC adapter)?

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† The old ATX is too large to fit in the PC enclosure, so I plan on using it as an external power supply and running the wires into the PC where they've been spliced onto the connector taken from the failed unit.

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EDIT: the original version of this question implied that I had verified WHITE +12V . . . But I learned from the answer I got that this color was used for a different voltage and not even used at all anymore. I didn't want to have bad information sitting there looking like it was true (even if it is just in this little question), so I edited it out of the question and added this acknowledgment of what I did so that the answer and comments don't appear to be making references to something that doesn't exist. Yes I know that I'm going overboard to preserve context when it's most likely that nobody would ever notice or (more to the point) even be reading this in the first place! Obsessive waste of time on my part, yes. Well... that's what I do!

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  • \$\begingroup\$ TL;DR, if it's not an ATX standard supply, it does not have to have +5V, it can have manufacturer specific standby voltages and colors. 12VSB is certainly reasonable and can be converted to any 5V or 3.3V standby voltages locally where needed (such as always-on USB charging port). \$\endgroup\$
    – Justme
    Commented Jan 18, 2023 at 22:40

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TL;DR; Unless the device you have specifies that it conforms to the ATX standard, you can't assume anything about them using the same wire colours for the same purpose as an ATX device.


The ATX specification uses purple for +5VSB. That use of purple is only defined for power supplies which use the ATX standard.

Dell like to make up their own pinouts and specification that are not compatible with ATX (I would speculate this is intentional). Therefore it is not suprising that one of their power supplies uses wire colours differently. Having their power supply provide a +12VSB is perfectly reasonable. Just because ATX uses +5VSB doesn't mean Dell had to. (*)

I see also you said "WHITE +12V" - on an ATX supply, +12V is yellow not white. The white wire used to be -5V before it was removed from the spec.


In terms of making a suitable +12VSB from your new ATX power supply, you could use simple step-up DC-DC converter to step up the ATX +5VSB to +12VSB your pinout info says is needed. You can get these readily and cheaply in module form either for mounting on a PCB or with screw/solder terminals if you so desire.

Alternatively you could just buy the appropriate replacement power supply from Dell or a reseller.


As an aside, I wouldn't be suprised if that 4-pin connector uses a different pinout from the ATX P4 connector too. I had one dell machine where the 4-pin connector used had the 12V/GND reversed when compared to an ATX P4.


(*) Anecdotally it most likely is +12VSB as I've seen adapter cables which take the purple +5VSB line from a 24-pin ATX connector and feed it through a mysterious PCB wrapped in heatshrink before connecting to the 8-pin Dell connector.

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  • \$\begingroup\$ As a final alternative which I'm not adding to the answer as it's shopping related, there are also companies selling products with names like "Dell OptiPlex 7040 PSU Main Power 24 Pin to 8 Pin Adapter Cable" which seem to do what you want according to this Dell support forum post. \$\endgroup\$ Commented Jan 18, 2023 at 18:14
  • \$\begingroup\$ Thanks for your answer and also for catching the mistake I made about the white wire. I don't know how I could have made such a careless error while the ATX was right in front of me. Had I been paying attention it would have been easy to see that there wasn't a single white wire to be seen anywhere on the ATX! Also, I should have noticed the Dell's +12V wires were the standard color yellow on the 4 pin connector but white on the 8 pin (shoulda, coulda, woulda!) I saw those adapter cables you mentioned, but I thought they were just wire and connectors which I have all the \$\endgroup\$ Commented Jan 18, 2023 at 21:14
  • \$\begingroup\$ parts to make. I'll probably add a +5V to +12V booster to the +5VSB line. I already have some DC to DC converters, if I can't find them or they're the wrong type (voltage range or cut instead of boost) then I'll order some and hafta wait. But since I'm impatient, I'm still toying with the idea of using a separate +12V adapter to feed the +12VSB pin on the connector. Because I came up with this hack, it just seems like it must be a bad idea. But... IS it really a bad idea? There's so much I don't know that it wouldn't surprise me if it really is a bad idea. Do I risk frying my PC? \$\endgroup\$ Commented Jan 18, 2023 at 21:50

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