EnglandElectronicsSyllabus dot point

How does a low-power control signal safely switch a large electrical load?

High power switching systems: relays and the flyback diode, power MOSFETs, the thyristor and triac for AC loads, and pulse-width modulation for power control.

An Eduqas A-Level Electronics answer on high power switching systems: the relay with its flyback diode, the power MOSFET as a logic-driven switch, the thyristor and triac for switching AC loads, and pulse-width modulation as an efficient way to control power.

Generated by Claude Opus 4.813 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Quick answer

A relay is an electromagnetic switch that lets a small coil current control a large, isolated load; because the coil is inductive it needs a flyback (free-wheeling) diode across it to absorb the back-electromotive force when it switches off, protecting the driving transistor. A power MOSFET is a logic-driven switch with a very high input resistance and low on-resistance, ideal for switching DC loads efficiently. A thyristor (silicon-controlled rectifier) latches on when its gate is triggered and conducts in one direction, while a triac does the same for both directions and so switches AC loads. Pulse-width modulation (PWM) controls average power by switching the supply fully on and off and varying the duty cycle; because the switch is never partly on, it wastes almost no power, unlike a series resistor.

Jump to a section

What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

Eduqas wants you to describe high power switching: relays with the flyback diode, power MOSFETs, the thyristor and triac for AC loads, and pulse-width modulation for efficient power control. These let a small logic signal control motors, heaters and mains-powered loads safely.

The answer

Relays and the flyback diode

Power MOSFETs

Thyristors and triacs

Pulse-width modulation

Setting a PWM duty cycle for a target motor voltage

A motor is driven from a $12\ \text{V}$ supply by a MOSFET switched with PWM. What duty cycle gives an average of $9.0\ \text{V}$ , and what on-time is that at a switching frequency of $1.0\ \text{kHz}$ ?

step 1: Find the duty cycle

$V_\text{avg} = D \times V_\text{supply}$ , so $D = \frac{V_\text{avg}}{V_\text{supply}} = \frac{9.0}{12} = 0.75$ (75 per cent).

step 2: Find the period

At $1.0\ \text{kHz}$ the period is $T = \frac{1}{f} = \frac{1}{1000} = 1.0\ \text{ms}$ .

step 3: Find the on-time

On-time $= D \times T = 0.75 \times 1.0\ \text{ms} = 0.75\ \text{ms}$ (and off for $0.25\ \text{ms}$ each cycle).

Examples in context

High power switching connects the brains of a system to its muscles: a microcontroller switches a pump or heater through a MOSFET or relay, a triac dims mains lighting, and PWM controls motor speed in everything from drones to washing machines and sets LED brightness in displays. The flyback diode and the choice of switch (MOSFET for DC, triac for AC) are standard design decisions in the non-exam assessment when an output drives a real load.

Try this

Q1. State the purpose of the flyback diode across a relay coil. [2 marks]

Cue. It gives the inductive back-EMF a safe path on switch-off, clamping the spike and protecting the driving transistor.

Q2. State which device, a thyristor or a triac, is used to switch an AC load. [1 mark]

Cue. A triac (it conducts in both directions).

Q3. A PWM signal drives a $10\ \text{V}$ supply at 40 per cent duty cycle. Find the average voltage delivered. [2 marks]

Cue. $V_\text{avg} = 0.40 \times 10 = 4.0\ \text{V}$ .

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20215 marksA microcontroller output switches a

12\ \text{V}

relay coil through a transistor. Explain why a diode is connected across the relay coil, and state the consequence of omitting it.

Show worked answer →

Why the diode is needed (up to 3 marks): the relay coil is an inductor. When the transistor switches off, the current through the coil falls rapidly, and by $V = L\frac{\Delta I}{\Delta t}$ a large back-electromotive force is induced (a voltage spike that can be hundreds of volts). The diode (a flyback or free-wheeling diode), connected across the coil in reverse, provides a path for the decaying coil current to circulate and decay safely, clamping the spike to about $0.7\ \text{V}$ .

Consequence of omitting it (up to 2 marks): without the diode the induced voltage spike appears across the switching transistor and can exceed its breakdown voltage, destroying the transistor (or the microcontroller output).

Markers reward the inductive back-EMF on switch-off, the diode providing a safe current path that clamps the spike, and the destruction of the transistor if it is omitted.

Eduqas 20205 marksExplain how pulse-width modulation (PWM) controls the average power delivered to a load such as a motor or heater, and state one advantage over a series resistor.

Show worked answer →

How PWM works (up to 3 marks): the supply is switched fully on and fully off at a fixed frequency, and the fraction of each cycle that is on (the duty cycle) sets the average voltage and so the average power. A larger duty cycle delivers more power; a smaller duty cycle delivers less. The switching is fast enough that the load (its inertia or thermal mass) responds to the average.

Advantage over a series resistor (up to 2 marks): the switching device is either fully on (low voltage across it) or fully off (no current), so it dissipates very little power, making PWM highly efficient. A series resistor wastes the controlled power as heat.

Markers reward the duty-cycle control of average power, the load responding to the average, and the efficiency advantage (the switch dissipates little, unlike a dropper resistor).

Related dot points

Sources & how we know this

Eduqas GCE AS/A Level Electronics specification (A410QS) — WJEC Eduqas (2017)