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 the common electrical outlets of one geographical region for the outlets of another area.[1] Most single phase alternating current electrical outlets in the world have a voltage range of 210 to 230 volts or a range of 110 to 120 volts. Converters usually can only double the voltage or halve the voltage but some are set up to do both.

Often these are sold with plug end adaptors that provide conversion of the size and shape of the plug end. If voltage conversion is not needed the plug adaptor can be used without a voltage converter.

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 volts, 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 volt Direct Current 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 volts to 230 volts and step down will convert from 230 volts to 110 volts. Most voltage converters convert both ways.

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