The silicon diode and its one-way (rectifying) behaviour: using a diode to protect a circuit against reverse polarity and against inductive spikes, and half-wave rectification to convert AC into DC.

What this topic is asking

WJEC Eduqas wants you to use the silicon diode's one-way behaviour for two practical jobs: protecting a circuit (against reverse-polarity connection and against the voltage spikes from inductive loads), and half-wave rectification to convert an AC supply into DC. You should be able to explain and sketch the half-wave output.

The diode and one-way conduction

This one-way valve action is the whole basis of diode applications. In the forward direction, once the $0.7\,\text{V}$ turn-on is reached, the diode conducts and behaves almost like a closed switch (with about $0.7\,\text{V}$ across it). In the reverse direction it behaves like an open switch, passing only a tiny leakage current. Connect a diode the right way for current to pass; connect it the other way to block.

Protecting against reverse polarity

This is a simple, common safeguard, especially in battery-powered products where a user might insert the battery backwards. The series diode means that wrong-way current simply cannot flow. The small cost is the $0.7\,\text{V}$ dropped across the diode when it conducts, which slightly reduces the voltage available to the circuit.

Protecting against inductive spikes

When current through a coil is interrupted, the collapsing magnetic field induces a large, brief voltage in the opposite direction, which can destroy a transistor switch. The protection diode (sometimes called a flywheel or freewheel diode) gives this induced current a safe path to circulate and die away, clamping the spike. It is fitted across the coil with its cathode to the positive supply, so it does nothing in normal use and only conducts the reverse spike.

Half-wave rectification

AC reverses direction every half-cycle. The diode passes the half-cycles of one polarity (when it is forward biased) and blocks the half-cycles of the other polarity. The result across the load is a one-way (DC) output: a string of positive humps separated by flat gaps. It is "half-wave" because only half of each AC cycle reaches the load. The output is DC in the sense that the current never reverses, although it is far from smooth; a smoothing capacitor (covered with timing circuits) would be added to flatten it.

Try this

Q1. State what happens to the current through a diode that is reverse biased. [1 mark]

• Cue. It is blocked (only a tiny leakage current flows).

Q2. State why a diode is connected across a relay coil. [1 mark]

• Cue. To absorb the voltage spike produced when the coil current is switched off, protecting the driving transistor.