Linear Power Supplies and Voltage Regulators

A linear power supply converts alternating-current mains voltage into a steady direct-current voltage suitable for powering electronic circuits. The conversion usually follows several stages. A transformer first steps the mains voltage up or down and provides electrical isolation. A rectifier, built from diodes, then changes the alternating waveform into a pulsating direct current. A reservoir capacitor smooths this pulsating output, leaving a direct voltage that still contains some residual ripple.

The output of an unregulated supply varies with the load current and with fluctuations in the mains. To hold the voltage constant, a regulator stage is added. The simplest form is a zener diode regulator, in which a reverse-biased zener diode clamps the output near its breakdown voltage while a series resistor absorbs the difference. For larger currents this basic shunt arrangement is combined with a series pass transistor, producing a series regulator that adjusts continuously to keep the output fixed. Three-terminal integrated regulators package this function in a single device.

Linear regulators are described as “linear” because the pass transistor operates in its active region, dropping the surplus voltage as heat. This makes them simple, quiet and free of switching noise, which suits sensitive analogue and audio circuits. The trade-off is efficiency: the energy lost in the pass element rises with the difference between input and output voltage, so linear supplies run warm and are less efficient than switching designs. Related circuits regulate current rather than voltage, holding a constant current through a load such as a battery being charged.

Frequently asked questions

What is the difference between a regulated and an unregulated supply?

An unregulated supply produces a DC voltage that drifts with load and mains changes, while a regulated supply adds a circuit that holds the output voltage steady despite those variations.

How does a zener diode regulate voltage?

When reverse biased into breakdown, a zener diode maintains a nearly constant voltage across itself, clamping the output while a series resistor carries the excess current.

Why are linear regulators inefficient?

The series pass element dissipates the voltage difference between input and output as heat, so a large drop or high current wastes significant power.