A voltage converter changes the voltage of an electrical power source and is usually combined with other components to create a power supply. The term "voltage converter" is sometimes used as a generic term for a power supply. A voltage converter or a power supply may be called a "transformer" even if it does not contain an actual transformer as the term is used in electronics.

A common use of the term voltage converter is for a device that allows appliances made for the voltage of one geographical region to operate from the power outlets of another area. Most single phase alternating current electrical outlets in the world supply power at 210-230 V or at 100-125 V. Converters usually can only double or halve the voltage, but some can do both.

Often these are sold with plug end adapters that provide physical compatibility of normally-incompatible plugs and sockets.

There are a number of methods for converting voltage. For Alternating current (AC) a transformer can be used to decrease or increase the voltage. The common power supplies for small electronics usually have a transformer that drops the voltage down to relatively small voltage ranging from 4.5 to 12 V, a full-wave rectifier to convert the AC to a pulsed direct current and some additional components to flatten the wave. Some devices have only a transformer in the external power supply with any rectifier or additional power conditioning provided inside the appliance.

Voltage converters sold for use in cars with 12 V DC outlets almost always have no transformer and instead use a Zener diode to drop the DC voltage with a relatively large power loss as heat. Converting a low DC voltage to a high DC voltage requires conversion to AC, the use of a transformer to increase the voltage and then a rectifier and conditioning circuit to convert it back to DC. Going through an AC step can also be used to drop voltage. For example one might use an inverter to produce 110 AC current from a car's electrical system and then use the conventional power supply that came with the device.

There are two types of voltage converters, step up and step down. Step up converts from 110 V to 230 V and step down will convert from 230 V to 110 V. Most voltage converters convert both ways.