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Description
DC adaptors come in two main varieties: the plug-in type and the in-line adaptor. A plug-in, or ̶0;wall-wart̶1; adaptor has a chunky design, usually made of black plastic, and has an AC plug built directly into its body. They measure from 1 inch to 2 or 3 inches on a side and have a DC power cable molded into the body. In-line adaptors, also called ̶0;power bricks,̶1; have a main body, somewhat smaller than a paperback book, made of black plastic. A traditional modular AC power cord plugs into one side of the power brick; a slimmer DC power cable comes out the other side and plugs into the power jack of an electronic device. Both types of adaptors contain electronic components that convert AC to DC at a reduced voltage.
Linear Supply
A linear DC adaptor has a more traditional electronic design, consisting of an iron transformer, a rectifier, filter capacitors and a voltage regulator. The transformer reduces the voltage, the rectifier converts AC to DC, and the capacitors and regulator remove electrical noise and hold the voltage to a steady value. Linear supplies are simple and inexpensive, but they waste 50 percent or more of the incoming power as heat; an adaptor that draws 20 watts from the AC outlet may provide only 10 watts to the electronic device.
Switching Supply
Switching power supplies are a newer, more complex design that uses a high-frequency electronic circuit to convert AC to DC. Because it uses a high frequency, the transformer is smaller, and the circuit is much more efficient, generally 80 percent or better. A switching power adaptor that consumes 20 watts provides 16 to 18 watts of useful DC power.
Plug-In Adaptor
Plug-in DC adaptors are smaller than their in-line counterparts. Their smaller components carry less current and consequently provide less power than in-line supplies. The smallest plug-in adaptors output voltages between 6 and 12 volts with 300 milliamps of current. Multiplying volts by amps to find watts, 12 times .3 equals 3.6 watts of power. The largest plug-in supplies provide about 1.2 amps of current, or 14.4 watts for 12 volts.
In-Line Adaptor
Plug-in DC adapters power smaller electronic devices such as cell phones and portable DVD players. In-line adapters usually have high-efficiency switching designs to power larger, more power-hungry equipment such as laptop computers, video monitors and televisions. The smallest units supply 30 watts, a typical in-line adaptor provides 60 to 90 watts of DC power; larger adaptors output 150 watts (Reference 3).