Basic Power Supply Schematic


Power supply is a reference to a source of electrical power. A device or system that supplies electrical or other types of energy to an output load or group of loads is called a power supply unit or PSU. The term is most commonly applied to electrical energy supplies, less often to mechanical ones, and rarely to others.

Basic components[]

The basic components make up the purpose of a power supply that is to transform (via a transformer), rectify (via a bridge rectifier), smooth out (via a capacitor), and regulate (via a voltage regulator) rough, dirty, AC voltage into clean, smooth, DC voltage.

Uninterrupted power supply[]

An Uninterrupted Power Supply (UPS) takes its power from two or more sources simultaneously. It is usually powered directly from the AC mains, while simultaneously charging a storage battery. Should there be a dropout or failure of the mains, the battery instantly takes over so that the load never experiences an interruption. Such a scheme can supply power as long as the battery charge suffices, e.g., in a computer installation, giving the operator sufficient time to effect an orderly system shutdown without loss of data. Other UPS schemes may use an internal combustion engine or turbine to continuously supply power to a system in parallel with power coming from the AC mains. The engine-driven generators would normally be idling, but could come to full power in a matter of a few seconds in order to keep vital equipment running without interruption. Such a scheme might be found in hospitals or telephone central offices.

Overload Protection[]

Power supplies should have some type of overload protection. Overload protection is important to protect the electronic equipment hooked up to the power supply and to also prevent overheating, which could potentially lead to an electrical fire. Fuses and circuit breakers are two of the more frequent mechanisms used for overload protection.


A piece of wire is connected between two metal ends. The two metal ends of the fuse are connected by either a tube of glass or plastic which surrounds the wire. If too much current flows, the wire overheats and melts. This interrupts the power supply, and the equipment stops working until the problem that caused the overload is identified and the fuse is replaced.

There are two types of fuses, slow-blow and fast-blow. In a fast-blow fuse, the wire inside the fuse will melt if the current exceeds the rated current, even if it is just for a fraction of second. This concise process is important in electronic equipment where even a small spike in the current could damage the equipment. A slow-blow fuse is designed to only melt when there is a continuous overload. Slow-blow fuses are ideal for motor systems.

Circuit Breakers[]

One benefit of using a circuit breaker as opposed to a fuse is that it can simply be reset instead of having to constantly replace the blown fuse. A circuit breaker works once an overloaded current causes some element to heat and trigger a spring which shuts the circuit down. Once the element cools, and the problem is identified the breaker can be reset and the power restored.