Rectifiers

PHYp–n Junction Diodes

A rectifier converts alternating current into direct (pulsating) current using diodes. The key examinable fact is the output ripple frequency: a half-wave rectifier passes one half-cycle per period, so its ripple frequency equals the input frequency, while a full-wave rectifier delivers a pulse for each half-cycle, doubling the ripple frequency.

Half-wave ripple frequency

f_{out} = f_{in}

f_{in}

= a.c. mains frequency; one output pulse per input cycle. For

50\,\text{Hz}

input, output ripple is

50\,\text{Hz}

Full-wave ripple frequency

f_{out} = 2 \, f_{in}

f_{in}

= mains frequency; two output pulses per input cycle. For

50\,\text{Hz}

input, output ripple is

100\,\text{Hz}

Ripple frequency (general)

f_{out} = m \, f_{in}

m = 1

for half-wave,

m = 2

for full-wave; compact form linking the two cases.

A half-wave rectifier conducts during only one half of each cycle, so output frequency equals input frequency.
A full-wave rectifier conducts on both halves (via two diodes or a bridge), giving twice the input frequency.
Higher ripple frequency is easier to smooth, so full-wave output is filtered more effectively.
The rectified output is unidirectional but pulsating; a capacitor (filter) is added to smooth it toward steady d.c.
A full-wave bridge uses four diodes; a centre-tapped full-wave uses two — both give $f_{out} = 2f_{in}$ .
Diodes conduct only when forward biased, which is what blocks the reverse half-cycle in half-wave rectification.
Ripple frequency depends only on the rectifier type and input frequency, not on the load.

Where the marks go

Assuming the half-wave rectifier also doubles the frequency — it does not; output stays at $50\,\text{Hz}$ .
Confusing the output ripple frequency with the input mains frequency for the full-wave case.
Thinking more diodes (bridge) changes the frequency from the centre-tapped value — both give $2f_{in}$ .
Mixing angular frequency $\omega = 2\pi f$ with ordinary frequency $f$ when quoting the ripple.

How the board asks it

Diagram / graphfull-wave bridge uses four diodes
Draw a labelled circuit diagram of a full-wave bridge rectifier and explain how the four diodes conduct in pairs during the two halves of the input a.c. cycle.
Diagram / graphunidirectional but pulsating output
Sketch the input a.c. waveform and the corresponding output waveform for (i) a half-wave rectifier and (ii) a full-wave rectifier, and mark the ripple period in each case.
Numericalripple frequency depends on rectifier type
The a.c. supply to a rectifier has a frequency of $50\,\text{Hz}$ . State the ripple frequency of the output for (i) a half-wave rectifier and (ii) a full-wave rectifier.
Distinguishhalf-wave vs full-wave conduction
Distinguish between a half-wave rectifier and a full-wave rectifier with respect to the number of half-cycles utilised and the ripple frequency of the output.
Give reasonshigher ripple frequency is easier to smooth
Give a reason why the output of a full-wave rectifier is easier to smooth into steady d.c. than that of a half-wave rectifier.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.