Understanding and Troubleshooting the PC Power Supply

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The Closed Metal Box

The power supply is usually depicted as the small, closed, metal box, which sits inside the computer chassis. Discussions are usually limited to the physical shape of the box (form factor), the type and number of motherboard connectors, the number of connectors for peripherals, and sometimes the electrical considerations.

The box is closed so that the line voltage is controlled, thereby lowering the risk of harm. In fact, by closing off the voltages, only low, non-threatening voltages are accessible. According to Winn L. Rosch in Hardware Bible 5th edition, “you can grab a board inside your PC even when the system is turned on.” Of course, you wouldn’t do this since low voltages can destroy electronic equipment.

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The disadvantage to a closed box is the apparent ‘voodoo’ or magic which occurs inside the box. Fortunately, manufacturers place a label on the outside of the box so that the voltages are known. We’ll discuss this more in the section on Electrical Considerations.

Physical Construction – Form Factors

Power supplies come in many different shapes and sizes. These are generally discussed in terms of the motherboard form factor; e.g. AT, ATX, and ATX12V.

The AT power supply was common for the IBM PC, XT, and early AT models. Early models had a large, red power switch attached to the metal box. It was clearly marked as the means to power on a system. Because this was inconvenient to reach, newer models replaced the red power switch and used a four-wire cable attached to the front of the computer case to power on the system. Replacing AT power supplies using the four-wire ‘remote’ switch was always an exercise. Bridging these wires incorrectly led to many throws of circuit breakers. This job wasn’t for the faint of heart.

Just as important, AT power supplies came in different sizes; e.g. Baby-AT, mini-AT, full-size AT. They also came in different shapes: some power supplies were square, others rectangular, and some had ‘cut outs’ so that they didn’t interfere with the motherboard. Therefore, replacing a power supply always meant purchasing the correct shape as well as size.

The common features between all of these various AT power supplies were the 5-volt and 12-volt signals vital to proper operation of the unit, and the two separate sets of wires plugging into a motherboard (P8 and P9). Older technicians will remember instructing neophyte Do-It-Yourself Assemblers to make sure that the black wires of the two sets stayed together, because improper installation would destroy the motherboard, as well as other peripherals.

Soon after its introduction in 1995, the ATX form factor started to replace the AT form factor. The ATX power supply built on the success or the AT and eXtended its usefulness with a few design changes. Presently, ATX is the dominant form factor.

The ATX power supply provides a direct 3.3-volt supply as well as introduced the 5-volt standby supply. Instead of two separate sets of wires plugging into a motherboard (P8 and P9), ATX power supplies have a single connector with 20-wires. The ATX design also changed the system airflow. Rather than using positive-pressure cooling, ATX uses negative pressure – in which the air is exhausted out the rear of the power supply.

The ATX specification helped eliminate many of the ‘physical’ issues confronted by an AT design, however, there are some differences that should be noted. Some ATX power supplies have an overflow switch while others do not have them. Some of them have an extra connector for power management, some are designed for a desktop chassis, and others are for full-size towers. In addition, some ATX power supplies have an exterior fan that fits ‘under’ the power supply. This helps circulate the air near the processor, however, these power supplies may not fit since the fan takes the same space as the processor. Therefore, always ensure that the power supply physically fits into the chassis.

Just in case you are becoming comfortable with the power supply, the Intel Pentium 4 systems will use a newer power supply, ATX12V. This power supply has additional motherboard connectors so that power is properly distributed across the motherboard.


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