A voltage regulator is a device that maintains a relatively constant output voltage even though its input voltage may be highly variable. There are a variety of specific types of voltage regulators based on the particular method they use to control the voltage in a circuit. In general, a voltage regulator functions by comparing its output voltage to a fixed reference and minimizing this difference with a negative feedback loop.
Passive voltage regulators are a very simple design that can only be used when the input voltage will always be greater than the output voltage. It contains a resistor that reduces the output voltage to the desired level. The resistor simply dumps the excess voltage as heat. Circuits that may require an increase in voltage will need an active voltage regulator.
A basic voltage regulator relies on a simple electromechanical design. A wire connected to the circuit is coiled so that it forms an electromagnet. As the voltage in the circuit increases, so does the strength of the electromagnet. This causes an iron core to move towards the electromagnet which is connected to a power switch. When the moving magnet pulls the switch, it reduces the voltage in the circuit.
Negative Feedback Loop
The iron core is held back from the electromagnet by some force, such as a spring or gravity. When the voltage in the circuit decreases, the electromagnet gets weaker. This allows the iron core to move back towards its resting position which turns the switch back on and increases the voltage of the circuit. This produces a negative feedback loop, meaning that the voltage regulator reduces the voltage when it's too high and increases the voltage when it's too low.
The sensitivity of a voltage regulator can be increased substantially with a design that allows the iron core to move along a range of resistances or windings. As the position of the iron core changes, it contacts the circuit at different points, which alters the circuit's voltage as required. This design allows the voltage regulator to respond to very small changes in the circuit's voltage.
A mains regulator is a more specific term for a device that controls the voltage on an AC power distribution line. An AC voltage stabilizer typically uses a continuously variable autotransformer to regulate the main voltage in a home. A DC voltage stabilizer frequently controls the raw voltage from a battery by using a shunt that only conducts electricity at a specific voltage.