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Power Supply Loading

PC power supplies are of a switching rather than a linear design. The switching type of design uses a high-speed oscillator circuit to convert the higher wall-socket AC voltage to the much lower DC voltage used to power the PC and PC components. Switching type power supplies are noted for being very efficient in size, weight, and energy in comparison to the linear design, which uses a large internal transformer to generate various outputs. This type of transformer-based design is inefficient in at least three ways. First, the output voltage of the transformer linearly follows the input voltage (hence the name linear), so any fluctuations in the AC power going into the system can cause problems with the output. Second, the high current-level (power) requirements of a PC system require the use of heavy wiring in the transformer. Third, the 60Hz (hertz) frequency of the AC power supplied from your building is difficult to filter out inside the power supply, requiring large and expensive filter capacitors and rectifiers.

The switching supply, on the other hand, uses a switching circuit that chops up the incoming power at a relatively high frequency. This enables the use of high-frequency transformers that are much smaller and lighter. Also, the higher frequency is much easier and cheaper to filter out at the output, and the input voltage can vary widely. Input ranging from 90 volts to 135 volts still produces the proper output levels, and many switching supplies can automatically adjust to 220v input.

One characteristic of all switching-type power supplies is that they do not run without a load. This means that you must have the supply plugged into something drawing power for the supply to work. If you simply have the power supply on a bench with nothing plugged into it, either the supply burns up or its protection circuitry shuts it down. Most power supplies are protected from no-load operation and shut down automatically. Some of the cheap clone supplies, however, lack the protection circuit and relay. They are destroyed after a few seconds of no-load operation. A few power supplies have their own built-in load resistors, so they can run even though no normal load is plugged in.

According to IBM specifications for the standard 192-watt power supply used in the original AT, a minimum load of 7.0 amps was required at +5v and a minimum of 2.5 amps was required at +12v for the supply to work properly.

Because floppy drives present no +12v load unless they are spinning, systems without a hard disk drive often do not operate properly. Some power supplies have a minimum load requirement for both the +5v and +12v sides. If you fail to meet this minimum load, the supply shuts down.

Because of this characteristic, when IBM used to ship the original AT systems without a hard disk, they plugged the hard disk drive power cable into a large 5-ohm, 50-watt sandbar resistor, which was mounted in a little metal cage assembly where the drive would have been. The AT case had screw holes on top of where the hard disk would go, specifically designed to mount this resistor cage.


Several computer stores I knew of in the mid-1980s would order the diskless AT and install their own 20MB or 30MB drives, which they could get more cheaply from other sources than from IBM. They were throwing away the load resistors by the hundreds! I managed to grab a couple at the time, which is how I know the type of resistor they used.

This resistor would be connected between pin 1 (+12v) and pin 2 (Ground) on the hard disk power connector. This would place a 2.4-amp load on the supply's +12v output, drawing 28.8 watts of power (it would get hot!) and thus enabling the supply to operate normally. Note that the cooling fan in most power supplies draws approximately 0.1–0.25 amps, bringing the total load to 2.5 amps or more. If the load resistor were missing, the system would intermittently fail to start up or operate properly. The motherboard would draw +5v at all times, but +12v would normally be used only by motors, and the floppy drive motors would be off most of the time.

Most of the power supplies in use today do not require as much of a load as the original IBM AT power supply. In most cases, a minimum load of 0–0.3 amps at +3.3v, 2.0–4.0 amps at +5v, and 0.5–1.0 amps at +12v is considered acceptable. Most motherboards easily draw the minimum +5v current by themselves. The standard power supply cooling fan draws only 0.1–0.25 amps, so the +12v minimum load might still be a problem for a diskless workstation. Generally, the higher the rating on the supply, the more minimum load required; however, exceptions do exist, so this is a specification you want to check when evaluating power supplies.

Some high-quality switching power supplies have built-in load resistors and can run under a no-load situation because the supply loads. Other high-quality power supplies, such as those from PC Power and Cooling, have no internal load resistors. They require only a small load on the +5v line to operate properly. Many of the cheaper clone supplies, which often do not have built-in load resistors, might require +3.3v, +5v, and +12v loads to work.

If you want to bench test a power supply, make sure you place loads on at least one but preferably all of the positive voltage outputs. This is one reason you should test the supply while it is installed in the system, instead of testing it separately on the bench. For impromptu bench testing, you can use a spare motherboard and hard disk drive to load the outputs.

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