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I have a reasonably-decent old 20-pin PSU which I want to use on a newer 24-pin ATX motherboard.

I see that the ATX page on wikipedia mentions that 24-pin ATX is backward-compatible, and I can find 20-to-24 pin adapters to buy for a couple of dollars/pounds at lots of places, but I can't find any mention of restrictions on the use of these.

Will this work on any motherboard, or is it a per-motherboard compatibility question?

Are there any other restrictions like the level of power available (and hence the additional 4 pins with +12, +5 and +3.3V lines which are already on other pins)?

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up vote 5 down vote accepted

I have no exact information to provide you here, but my gut feel would be that you should only consider trying this if you have a relatively low-power system on the motherboard. Something with not too much memory, ideally no more than 1 hard-disk, and probably not too high-end a dual-core processor... if you have any kind of modern graphics card installed I would probably not even consider going there.

Note that what the 24-pin supply does, is add one additional supply line of +3.3V, +5V and +12V each. If you use the adapter you suggest it will copy one or more of the existing lines to supply these pins... which means that they need to share the additional load.

From having a look at the Wikipedia page you reference, it looks like the standard 20 pins have a decent number of +5V and +3.3V lines already, but there is only one +12V line, which gets duplicated by the extra 4 pins. I do not know offhand what hardware uses the +12V, but that would be my main stability concern when using the splitter attachment.

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The PCI Express port uses +12V. I would assume that as long as he doesn't use that port it'd be fine. – jonescb Apr 2 '10 at 18:21

It is not recommended as exceeding the current draw available will cause the PSU to either lower voltages (which could damage components) or simply shutdown in overload. My recommendation is to buy a new PSU. Really, they're not expensive.

However, if the motherboard has a Molex socket, then use that as well as the 20-pin plug and you'll be fine. There also used to be adapters from a Molex socket to the 4-pin extension, which would also be fine.

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A motherboard could work with just a 20-pin connector plugged in. Likely failure, if it fails, is the ATX silver box will shut down. Although, you need to accept the risk as solely your own when you do this.

In order to answer the question, we need to know what the motherboard specifications are for current draw from the power connector for each voltage rail, the specifications of the power supply, and what else you have powered by the power supply.

  • An ATX silver box power supply, PSU, will have specifications rating how much current it can supply on each power rail (12V, 5V, 3.3V, -12V, 5Vstandby). Sometimes 12V is shared or split into different rails with different ratings.
  • Motherboard manufacturer's typically don't break out their requirements to the detail necessary, though. You'd need the max requirements for each power rail as well for comparison.
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I used a 20 pin power supply on a motherboard with a 24 pin socket for years with no problems at all.

I had forgotten all about it until this last weekend when I replaced the PSU (failing fan) and had to remove a sticker that the manufacturer had placed over the last 4 pins (with a little graphic indicating that they were needed only with a 24 pin PSU).

This was with an AMD Athlon64 X2 4800+ CPU, mobo video, 4GB RAM in 2 sticks, 3 hard disks, DVD writer and commonly running 7 USB devices, 6 of which are powered from the computer.

In retrospect, I am a little surprised.

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With the potential exception of the CPU none of those are high draw devices. if you had a high end GPU or two each trying to draw 75W from the PCI bus you'd be more likely to get into trouble. The power limits are designed around godbox level systems. Only using the 20 pin plug might cause a slight voltage sag but a drop of .05v on a rail with a 5% tolerance isn't a problem unless you're already right at the margin which is rare unless the PSU is starting to fail or is running near max capacity. – Dan Neely Apr 5 '10 at 20:57

There has been (which was starting when this question was written but has become even more so in the years since) a trend in PC hardware design away from 3.3V and 5V and towards 12V. The modern low voltage (often less than 1V) electronics can only practically be supplied by point of load converters and it's more efficient to run those power converters off a 12V input than a 5V input.

Unfortunately the ATX power connector was designed in the days when 3.3V and 5V dominated. The 20 pin ATX power connector only has one 12V pin, the 24 pin connector has two.

The CPU has it's own 12V connector so we don't have to worry about that but we do have to worry about general motherboard loads and any expansion cards. IIRC a PCIe graphics card is allowed to draw up to 75W from the motherboard (beyond that they use a supplementry connnection to the power supply).

Add a PCIe graphics card to the various loads on the motherboard and you are up to arround 100W or so.

A mini fit JR is supposed to be good for 9A. The newer mini fit HCS (which is pin compatible but higher rated and is reccommended by the ATX12V 2.0 standard) is rated a little bit higher. 9A at 12V works out to 108W. Unfortunately however PC vendors don't tend to use genuine molex connectors. They use cheap copies which often can't handle the current.

So in summary.

  • In a machine without a graphics card a 20 pin PSU will almost certainly be fine.
  • In a machine with a single graphics card it's borderline, a 20 pin PSU should be ok if all the parts are high quality and in good condition but there is a real risk of burnout.
  • In a machine with SLI/Crossfire using a 20 pin PSU would be a very dumb idea.

The adaptors you mention aren't strictly needed, you can just plug the connector straight in. The advantage of using an adaptor is that if you do burn out a pin it's a pin on an adaptor rather than a pin on the motherboard which is easier to deal with.

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