A power supply provides electrical power to a circuit or device. It may convert wall power, battery power, solar power, or another source into the voltage and current the electronics need.
What a power supply can include
A power supply may include a transformer, rectifier, filter capacitors, voltage regulator, protection circuit, connector, or switching converter depending on its design.
Why the right power supply matters
A device needs the correct voltage, enough current capacity, correct polarity, and suitable connector. Using the wrong supply can cause unstable behavior or damage. Related basics include voltage and current.
In plain language, the power supply is what feeds the circuit.