Power Supply

A power supply is the hardware component that provides electricity to power computers and other devices. It converts electrical current pulled from a power source, such as an outlet, battery or generator, to the correct format and passes it on to a device. It also regulates the voltage passed through to the machine to prevent overheating. Power supplies are sometimes abbreviated as PS or P/S, PSU (power supply unit).

Power supplies are rated in terms of the number of watts they generate. The more powerful the computer, the more watts it can provide to components.

Power conversion

Power supplies convert the formats of electricity pulled from power sources to match the format required by the machines they power. There are two main types of power supplies that differ according to the conversion. The most common type of power supply converts alternating current (AC) from the power source to direct current (DC), called AC-DC power supplies. DC-DC power supplies are less common. They are often used to plug electrical devices into battery power sources, such as car outlets, or other sources that supply DC current. Overall, the majority of power sources supply AC.


Power sources emit a consistent output voltage. However, some devices require more voltage to operate, while others that require a lower level of voltage may not be able to handle that output without overheating. Power supplies increase or decrease the voltage output to match a given machine’s requirements.

Components of a power supply

Power supplies consist of four main components that each carry out a specific function:

  • Transformers regulate the incoming voltage. They either step up or step down the voltage to match a device’s requirements.
  • Rectifiers convert the incoming AC to DC. These come as either half-wave, full-wave or bridge.
  • Filters smooth out the waves in the current after it’s converted to DC. The unregulated power outputted from the filter is much smoother but not completely flat.
  • Voltage regulators finish the job of the filter by reducing any leftover ripples in the voltage that could lead to drops in power or overheating.

Internal vs. external

Power supplies can be either internal or external. Most desktop computers have internal power supplies that connect directly to the motherboard. Smaller devices, such as laptops or external hard drives, use external power supplies to charge the batteries housed inside them.

Vangie Beal
Vangie Beal is a freelance business and technology writer covering Internet technologies and online business since the late '90s.

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