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Power supply information

Rail complications #3 - minimum loading problems: too little current

Okay. You can have problems with too much loading and unbalanced loading. Well guess what? You can also get into trouble with too little loading. A PC PSU is a veritable minefield of limitations! Okay, it's not really that bad. You just have to get used to the fact that PC PSUs were designed for a range of loads you usually see in a PC - not for all possible loads. Occasionally you can set up a PC which has loads outside what the PSU expected to see and the PSU won't function properly. One of those rare cases is when you don't meet the minimum current requirement on a rail. PC PSUs are switching power supplies. They require a minimum load before their outputs are stable. Many switching PSUs have internal power resistors which draw the minimum load so that you don't have to worry about meeting any minimum requirements. But if the PSU isn't loaded above the minimum then it can either produce really bad output voltage or it can completely shut off. Most PC PSUs have minimum load requirements. This is one of the specs that the manufacturers actually do list so you can usually find it on their websites. Usually the minimum current load is one amp or less on each rail. You rarely have to worry about the limit on 3.3 or 5 volts because just about any motherboard with any RAM in it will exceed the minimum load requirement.

Normally you don't have minimum load problems with the 12 volt rail either. ATX12V 1.3 or older PSUs tend to have very small minimum load requirements for their 12 volt rails. It's usually less than 0.5 amps and often a lot less. So older ATX supplies don't usually have problems. But some newer ATX12V 2.xx supplies have higher minimum requirements of around one amp. Normally that's not a problem for a minimally loaded PC but there have been some cases of it. The problem arises when you have a motherboard which delays turning on the power to its CPU for a few seconds. Hard disks also often wait a little while before spinning up. If both of those things happen then the only serious load on 12 volts is the video card. If the motherboard is using integrated video or it has a low-power video card then the load on the 12 volt rail can be below the minimum loading requirements for some PSUs. When you have a setup which has this combination of things, the PSU cannot successfully power up. When you try to power up the machine, the PSU turns on for a few seconds or less and then turns off. This particular combination happened at the end of 2005 when you put an Antec NEO into certain motherboards which delayed turning on the CPU power. In that particular case a BIOS fix increased the 12 volt load enough so that NEOs could usually turn on. Antec also apparently decreased the minimum load requirement (presumably by increasing the load inside the power supply) and so the problem went away. After that rather high profile experience both PSU manufacturers and motherboard makers have been more careful about avoiding the situation.

If you think you may have this situation then you should first see if there is a newer BIOS for your motherboard. You may have an older BIOS which delays firing up the CPU. You can also try to solve it by increasing your 12 volt load. The easiest way is to connect fans (they are virtually always 12 volts) to the 4 pin peripheral power cables coming from the PSU. Those peripheral cables are powered as soon as you turn on the PSU. One high-speed fan is usually enough of a load. It may take more than one low-speed fan. I've seen cases in Internet forums of people who connected fans with speed controls to their peripheral power cables. If they turned the fans up to full speed then their machines booted properly. But if they turned the fans down to minimum speed then their PSUs would turn on for a moment and the turn off. I don't think it matters which 12 volt rail you connect the fan to but I'm not 100% sure. If you have a single 12 PSU then it clearly doesn't matter. If you have a current limited 12s then I would assume that adding the load to any rail would work because you really only have one 12 volt rail inside the PSU. But I've seen people claim it had to be on every 12 volt rail for their machines to run so I'd try it every way you can to be sure.

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Copyright © 2005 through 2007 by Mark Allen