Power supply

A power supply is an electrical device that provides electric power to a load. Its core job is to take current from a source and change it to the voltage, current, and frequency that the load requires, which is why these devices are also called electric power converters. Some power supplies are separate, standalone units, while others are built directly into the equipment they run—like the power supplies inside desktop computers and consumer electronics. Beyond basic conversion, a power supply can also limit the current the load draws to safe levels, cut off current during an electrical fault, condition the power to block noise or voltage surges from the input, correct the power factor, or store energy to keep the load running during a temporary power interruption (as in an uninterruptible power supply).

Every power supply has a power input connection that receives electric current from a source, and one or more output connections (or power rails) that deliver current to the load. The source might be the electric power grid (like a wall outlet), energy storage devices such as batteries or fuel cells, generators or alternators, solar power converters, or even another power supply. Input and output are usually hardwired circuit connections, though some power supplies use wireless energy transfer. Some also have additional inputs and outputs for external monitoring and control.

Power supplies are sorted in several ways. By function: a regulated power supply keeps its output voltage or current steady despite changes in load current or input voltage, while an unregulated one’s output can shift significantly with those changes. An adjustable power supply lets you set the output voltage or current via mechanical controls (like front-panel knobs), a control input, or both; an adjustable regulated supply does both. An isolated power supply has an output that is electrically separate from its input, unlike others that share a common connection between input and output.

By packaging: a bench power supply is a standalone desktop unit for tasks like circuit testing and development. An open frame supply has only a partial enclosure (sometimes just a mounting base) and is usually built into machinery. Rack mount supplies fit into standard electronic equipment racks. An integrated power supply shares a printed circuit board with its load. An external power supply—also called an AC adapter or power brick—sits in the load’s AC power cord and plugs into a wall outlet; a wall wart is an external supply built into the plug itself. These are common in consumer electronics because they transform the hazardous 120- or 240-volt mains current down to a safer voltage before it reaches the appliance.

By power conversion method: power supplies are broadly linear or switching. Linear converters process input power directly, with all active components operating in their linear regions. Switching converters convert input power to AC or DC pulses before processing, using components that mostly operate in non-linear modes (like transistors in cutoff or saturation). Power is lost as heat when components run in linear regions, so switching converters are usually more efficient because their components spend less time there.

By purpose: power supplies are sometimes grouped by intended use, such as “AC-DC front end for grid bias” or “DC-DC converter for point-of-load (POL) regulation.”

Types include DC power supplies. An AC-to-DC supply takes AC input and produces DC output; the output may contain some AC ripple, influenced by the input frequency and the supply’s operation. A DC-to-DC converter runs on DC input. In a linear power supply, the AC input goes through a power transformer, then rectification and filtering to produce DC. Filtering reduces the AC mains frequency component in the rectifier output—it can be as simple as a single capacitor or as complex as a pi filter. The needed filtering level depends on the load’s tolerance for ripple; in some cases, like certain battery chargers, ripple can be ignored, and the supply may be just a transformer, a diode, and a resistor to limit charging current. In a switched-mode power supply (SMPS), the AC mains is directly rectified and filtered to get DC. That DC is then switched on and off at high frequency by electronic switching circuitry, creating an AC current that passes through a high-frequency transformer or inductor.