Power Supply Function and Operation
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The basic function of the power supply is to convert the type of electrical power available at a wall socket, 120 volt, 60 Hz, into current the computer can use, +5 and +12 volt DC current. Usually the digital electronic components and circuits in the computer, such as the motherboard, adapter cards, and disk drive logic boards, use +5 volt power, and the fan and disk drive motors use +12 volt power. For the computer to run properly, a steady supply of both types of current must be supplied at all times.

Although the power supply generates both +5 volts and +12 volts, it also generates -5 volt and -12 volt signals. These signals are not used by any of the motherboards Gateway has sold. The -5 volt signal was originally used by analog data separator circuits found in older floppy disk controllers. This is why the -5 volt signal is supplied to the bus. The signal is routed to pin B5 on the Industry Standard Architecture (ISA) bus and is not used in any way by the motherboard. The -12 volt signal was originally used for motherboards that included built-in serial ports and was routed to pin B7 on the ISA bus. All of Gateway motherboards have serial port circuits that run on 5 or 3.3 volts.

The main function of the +12 volt power is to run disk drive motors. Although a +12 volt signal is routed to the motherboard, the motherboard logic does not use this signal. However, this voltage is available for use by any adapter card on the bus. Besides supplying power to disk drive motors, the +12 volt signal is also used to run any cooling fans present in the computer.

In addition to supplying power to run the computer, the power supply also ensures that the computer does not run unless the power being supplied is sufficient to operate the computer properly. By doing so, the power supply actually prevents the computer from starting up or operating until all the correct power levels are present. The power supply completes its own internal checks and test before allowing the computer to start. The power supply sends a "Power-Good" signal to the clock timer chip on the motherboard when the correct power levels are present. If this signal is not present, the computer does not run. The effect of this is that when the AC voltage drops and the power supply becomes over-stressed or overheated, the Power-Good signal ceases and the clock generator circuit stops the processor by stopping the clock signal, forcing the computer to reset or shutdown completely.