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Power Supply Loading
PC power supplies use a technique called switching to generate their DC voltages. Due to the manner in which these sorts of power supplies function, they need to have a load, meaning something that draws power from the supply, in order to function properly. A power supply that is turned on with no load attached will either fail to function or will function improperly. Better-quality supplies will detect a no-load situation and shut down, but cheap ones can be damaged. This is why you should not "test" a power supply by just plugging it in with nothing attached to it.
The amount of load required by a particular power supply is often specified as its minimum load. Just as the total wattage of a power supply doesn't tell you enough about its output, neither would a minimum power requirement--you have to look at the minimum load requirements for each voltage level provided by the power supply. You will sometimes see these minimum current requirements listed as part of the power supply's output specifications. The amount of load required can vary considerably between different form factors, between manufacturers and even between specific designs from the same manufacturer.
In the early days of the PC, power supplies often had considerable load requirements, both for the +5 and +12 voltages. The +5 voltage requirement was easily satisfied by connecting the power supply to the motherboard, but the only devices that draw +12 V consistently are hard disk drives. (Some floppy disk drives also use +12 V, but only when they are actually spinning, which is not the case most of the time.) Thus, people who tried to troubleshoot power supplies without connecting a hard disk drive could be tricked into thinking the supply was bad. Also, systems with no hard disk drives would have a problem starting up unless equipped with a "dummy load". Essentially, this is a large resistor that plugs into a power connector to create a +12 V draw. Of course, this is rather wasteful of electricity, and heats the system needlessly.
Modern power supplies have drastically reduced the degree to which loading is an issue. Most newer power supplies have very small +3.3 V and +5 V load requirements, and many have no minimum at all for +12 V. The lower loading requirements make testing and troubleshooting much easier.
Tip: If you are setting up a
system that has no hard disk, use a power supply with a 0 A load requirement for +12 V.
Even a 0.5 A requirement for +12 V, which is pretty common today, will mean a hard disk is
required unless you can substitute some other +12 loads. A single cooling fan will not
generally draw enough, though several of them together might.