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How does a transistor let a small signal switch a large current, and how is an output device such as a motor protected?

The transistor as an electronic switch driven by a small input signal, used to control output devices, with a diode to protect against the back-emf of inductive loads.

An SQA Higher Engineering Science answer on the transistor as an electronic switch driven by a small input signal, how it controls output devices such as relays and motors, and why a protective diode is needed across inductive loads.

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  1. What this key area is asking
  2. The transistor as a switch
  3. Protecting against back-emf
  4. Driving larger loads with a relay
  5. Examples in context
  6. Try this

What this key area is asking

The SQA wants you to use a transistor as an electronic switch: a small input signal at its base or gate controls a much larger current to an output device, and an inductive load such as a motor or relay coil needs a protective diode against its back-emf. The transistor switch is the standard output sub-system that connects a low-power processing stage to a real load.

The transistor as a switch

The key feature is that the input controls the output, not powers it. The processing stage (a comparator or a potential divider) only needs to supply the tiny signal that turns the transistor on; the large current for the load comes from the supply through the transistor. This is exactly the meeting point of electronic control and electrical power introduced in the contexts area: a small electronic signal switches a large electrical current.

  • Below the turn-on threshold: the transistor is off, negligible current flows, the load is not powered.
  • Above the threshold: the transistor is on, it conducts, and the load is powered.

flowchart TB V["+ supply"] --> L["output device (load)"] --> C["transistor collector / drain"] C --> T(("transistor")) S["small input signal"] --> B["base / gate"] --> T T --> E["emitter / source to 0 V"]

Protecting against back-emf

An inductive load (anything with a coil: a motor, a solenoid, a relay coil) stores energy in a magnetic field while current flows. When the transistor switches it off, that field collapses suddenly and induces a large reverse voltage, the back-emf, which can be many times the supply voltage. Without protection this spike can destroy the transistor.

The fix is a diode (a flywheel or freewheel diode) connected across the load, oriented so it does not conduct in normal operation but does conduct the reverse back-emf. When the load switches off, the diode gives the induced current a safe path, dissipating the stored energy gently instead of as a damaging voltage spike.

Driving larger loads with a relay

When the load needs more power than the transistor can handle, or must be electrically isolated from the low-voltage control side (for example a mains-powered device), a relay is used. A relay is an electrically operated switch: a small current through its coil closes (or opens) contacts that switch a separate, higher-power circuit. The transistor switches the relay coil, supplying the coil current the control circuit alone could not, and the coil's contacts then switch the high-power load. Because the relay coil is itself inductive, it too needs a protective diode across it.

Examples in context

A dusk-to-dawn lamp uses the full chain: an LDR potential divider feeds a comparator, whose output drives a transistor, which switches the lamp on when it gets dark. An automatic fan does the same with a thermistor and a motor, with a diode across the motor to absorb the back-emf. A mains appliance controlled by a low-voltage timer uses a transistor to energise a relay, so the dangerous mains circuit is switched and isolated by the relay contacts while the control electronics stay safely at low voltage.

Try this

Q1. State what controls whether a transistor switch is on or off. [1 mark]

  • Cue. The small signal at its base (or gate): above the turn-on level it is on, below it is off.

Q2. State why a diode is fitted across a motor switched by a transistor. [1 mark]

  • Cue. To safely conduct the back-emf produced when the inductive motor is switched off, protecting the transistor.

Q3. Give one reason a relay is used instead of switching a load directly with the transistor. [1 mark]

  • Cue. The load is too high-power for the transistor, or the load (for example mains) must be electrically isolated from the control circuit.

Exam-style practice questions

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

SQA Higher (specimen)4 marksA transistor is used to switch a motor on when a sensor circuit produces a high voltage. Explain how the transistor acts as a switch in this circuit, and explain why a diode is connected across the motor.
Show worked answer →

The transistor acts as a switch driven by the small input signal at its base (or gate).

When the sensor voltage is low, the transistor is off, so almost no current flows through it and the motor does not run. When the sensor voltage is high enough to turn the transistor on, it conducts and allows the larger motor current to flow, so the motor runs. A small input therefore controls a much larger output current.

The diode across the motor protects the transistor from the back-emf. A motor is an inductive load, so when it is switched off the collapsing magnetic field induces a large reverse voltage (back-emf). The diode is connected so it conducts this reverse spike harmlessly, preventing the high voltage from damaging the transistor.

Markers reward the off/on behaviour of the transistor (small input controlling a large current), and the diode safely conducting the back-emf produced when the inductive load is switched off.

SQA Higher (specimen)3 marksA control circuit can only supply a tiny current, but the output device is a 240 V mains lamp. Explain why a relay is used and the role the transistor plays.
Show worked answer →

A relay is an electrically operated switch: a small current through its coil closes contacts that switch a completely separate, higher-power circuit.

It is used here because the mains lamp runs at 240 V and a higher current, which the low-voltage control circuit cannot and should not switch directly. The relay isolates the low-voltage control side from the dangerous mains side.

The transistor switches the relay coil: the tiny control signal turns the transistor on, the transistor supplies the coil current the control circuit alone could not, and the energised coil closes the contacts that switch the mains lamp.

Markers reward the relay switching a separate higher-power (and isolated) circuit, and the transistor providing the coil current driven by the small control signal.

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