Explain how a relay lets a small current switch a large or isolated load and why a flyback diode is connected across the coil

What this dot point is asking

SEAB wants you to explain how a relay lets a small control current switch a large or mains-powered load, and why a protective diode is connected across the relay coil. The central insight is that a relay is an electrically operated switch: a small current through a coil moves a magnetic contact that switches a completely separate, higher-power circuit, giving both power handling and safe isolation.

The answer

What a relay is

A relay is a switch operated by an electromagnet. It has two parts kept electrically separate:

• A coil on the low-power control side. When current flows through it, it becomes an electromagnet.
• A set of contacts on the high-power side. The magnetic field pulls a movable contact, opening or closing the high-power circuit.

Because the two sides are only linked magnetically, a small control current can switch a much larger load.

How a relay switches a load

A transistor (driven by a sensor or logic stage) passes a small current through the relay coil. The coil energises, becomes magnetic, and pulls the contacts closed. The closed contacts complete the separate load circuit - which may be a high current or even the mains - switching the load on. When the transistor turns off, the coil de-energises, the contacts spring open, and the load switches off.

The advantage of isolation

The control side and the load side share no direct electrical connection; they are linked only by the magnetic field and the moving contacts. This isolation is a major safety advantage: a low-voltage electronic circuit can switch a dangerous mains load while the user-facing electronics stay safely separated from the high voltage. A relay can also switch much larger currents than a transistor alone.

Why a flyback diode is needed

The relay coil is an inductor. When the transistor switches the coil current off, the current falls very fast, and a coil opposes any sudden change in current by producing a large voltage spike, called a back-emf. This spike can be far larger than the supply and would damage the transistor. A diode connected across the coil, called a flyback or protection diode, gives this current a safe path to die away gradually, clamping the spike and protecting the transistor. The diode is fitted so it does not conduct in normal operation, only when the spike appears.

Examples in context

Example 1. A central-heating controller. A low-voltage thermostat circuit switches a transistor that energises a relay, and the relay contacts switch the mains-powered boiler pump on. The relay lets safe low-voltage electronics control a powerful mains load, with the two sides kept electrically apart. The flyback diode protects the transistor each time the pump is switched off.

Example 2. A car horn or starter. A small current from the steering-wheel button energises a relay coil, and the heavy contacts then carry the large current needed by the horn or starter motor. The thin button wiring never carries the big current; the relay does the heavy switching. This is why relays are everywhere in vehicles.

Try this

• Cue. State the safety advantage of using a relay to switch a mains load. Isolation: the low-voltage control circuit shares no direct electrical connection with the dangerous mains circuit.

• Cue. Explain why a diode is placed across a relay coil. To clamp the back-emf voltage spike produced when the coil current is switched off, protecting the driving transistor.

• Cue. A relay coil of $300\ \Omega$ runs from $9\ \text{V}$. Find the coil current. $I = V/R = 9/300 = 30\ \text{mA}$.