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Revision as of 08:01, 31 October 2004
A voltage regulator is a mechanical, solid state or vacuum tube device designed to regulate DC voltage levels.
Mechanical regulators
Early automobile generators and alternators had a mechanical voltage regulator using three or two relays and ballast resistors to stabilize the generator's output on 6 or 12 V, independent of the engine's rpm. Later designs used 'solid state' technology to do the same.
These regulators fall into the class of 'shunt' regulators, which work by diverting ('shunting') unwanted current from the load, through a ballast resistor, to ground. The diverted current causes the ballast resistor to get hot, and therefore wastes power.
Mains regulators
In mains-powered devices a transformer is used to bring the mains AC power down to a lower AC voltage, then a rectifier (diode bridge) to convert AC to DC, followed by a linear regulator or Zener diode to stabilize it to the required voltage.
Solid-state linear regulators
Linear regulators exist in shunt (see above) and series types. Shunt types, because of their inefficiency, are used only to supply very small currents, of the order of microamperes or a few milliamperes. With a series regulator, the current through the regulator is approximately equal to the current through the load, and there is no ballast resistor. The regulator itself dissipates heat, causing some power to be wasted.
Solid state regulator semiconductor chips come in either fixed or variable types. Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages), and common fixed voltages are 5 V (for transistor-transistor logic circuits) and 12 V, e.g. in personal computers.
In fixed voltage regulators the reference pin is tied to ground, whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider (such as a potentiometer) allows the user to adjust the regulated voltage.