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# What is an AC-DC Power Supply?

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• Written By: Andrew Portela
• Edited By: A. Joseph
2003-2017
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There was a time when some places were supplied with direct current (DC) power and others used alternating current (AC) power. When a power supply has the description AC-DC, this means that the equipment can operate on either alternating current or direct current. Converting AC to DC is the main function of an AC-DC power supply.

Alternating current is when the electricity flows one way and then back again over and over. Buildings and houses are supplied with AC power that comes in on a live wire and heads back out on the return wire to complete a circuit. Direct current is when electricity flows in the same direction and is always either positive or negative. Many appliances run on DC power and can not run on AC power.

A power supply delivers electricity to one or more electrical loads. The device traditionally converts one form of electricity to the other. Power supplies can be found on many things, such as for television sets, computers, video game consoles and even home phones. Power supplies are commonly found on devices that use a charged battery such as cell phones and other handheld devices.

The AC power that flowing into houses is too powerful for most DC appliances. This means that the voltage of the electricity must be reduced. Essentially, the AC power enters the AC-DC power supply and immediately runs through a coil and creates a magnetic field. The magnetic field creates an electric current on a second coil that is next to the first coil. This second coil contains less turns of wire to create a lower voltage of AC electricity.

An AC-DC power supply contains a rectifier that uses a diode to separate the positive and negative electricity. When the electricity is negative, the diode allows it to flow down the negative wire but blocks the positive wire. As soon as the electricity turns to positive, the diode switches and allows the electricity down the positive wire. There are different kinds of rectifiers, but they all do the same basic thing.

Some power supplies use a filter capacitor and select the appropriate regulator to generate a pure DC signal. The capacitors are used for blocking AC power and allowing DC power to pass. This makes the output of the power supply smoother when used on a device that uses radio frequencies.

In the past, all televisions and radios used vacuum tubes. These were devices that would control the movement of electrons in a low-pressure space in order to modify or create an electrical signal. The AC-DC power supply made it possible to operate these devices without using a transformer. This was an issue because transformers were both expensive and could not be used with DC power.