Introduction

What this section sets up

This is an orientation section, not an examinable derivation. It recalls from Std XI that a p-n junction diode conducts freely when forward biased and blocks current when reverse biased, and points out that this one-way switching action is exactly what is needed to turn AC into DC.

Why rectification is needed

• Generating AC at a power station is more cost effective than generating DC, and transmitting AC is more economic than transmitting DC. So the mains supply is AC.

• In India the mains AC voltage is $230\ \text{V}$ and its frequency is $50\ \text{Hz}$, varying sinusoidally.

• Many electronic gadgets — a TV, a mobile charger — need a DC supply. The AC mains must therefore be converted to DC. This is done with junction diodes.

The diode as a switch

• Forward biased: the diode behaves as a closed switch and current flows in the circuit.

• Reverse biased: the diode behaves as an open switch and only a negligibly small current flows.

Block diagram of a simple DC power supply

Described in words, left to right:

• The AC mains supply is connected to the primary of a transformer. The secondary delivers a stepped up or stepped down sinusoidal AC voltage.

• The secondary is connected to a rectifier circuit. The diode rectifier converts the sinusoidal AC into a DC voltage, drawn as a pulsating wave.

• The rectifier output still contains an AC component. This AC component present in the DC output of a rectifier is called ripple.

• The ripple is removed by a filter circuit. The output of the filter is almost pure DC, though it can still contain a little ripple.

• A voltage regulator circuit after the filter restricts the output to the desired value. The voltage at this final stage is pure, regulated DC.

Standard results

• Mains in India: $230\ \text{V}$, $50\ \text{Hz}$.

• Order of blocks: transformer, rectifier, filter, voltage regulator.

• Ripple is the AC component in the DC output of a rectifier; the filter removes it and the regulator fixes the output value.