The big idea: A passive component can only limit, store or divert what is already in the circuit — a resistor, a capacitor, a switch, a relay.
An active component controls the flow — a diode, an LED, a transistor. A transistor can make a small signal control a large current, which is the thing no passive component can do.
Each component with its circuit symbol and the job it does — the same symbols you will meet again on a circuit diagram.
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| Component | What it does | Where you meet it |
|---|---|---|
| Fixed resistor | Limits current and sets voltages, converting some energy into heat. Its value in ohms is fixed at manufacture | In series with every LED, and as the pair dividing a supply down to what a sensor needs |
| Variable resistor | The same job with a value that can be changed — a potentiometer for the user, a preset for a one-off calibration | A volume knob, a dimmer, a joystick, the trim that sets a thermostat exactly |
| Capacitor | Stores a small charge and releases it; blocks steady voltages and passes changing ones | Smoothing a power supply, setting a timing delay, coupling an audio signal |
| Switch | Makes or breaks a circuit. A push switch is momentary, a toggle latches | A doorbell push, a kettle toggle, a limit switch telling a machine it has reached the end |
| Relay | An electrically operated switch: a small coil current moves contacts carrying a much larger one, with the two circuits electrically separate | A microcontroller switching a mains heater, a car starter, anything where 5 V must control 230 V |
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| Component | What it does | Where you meet it |
|---|---|---|
| Diode | Allows current one way only, so it protects against a reversed battery and converts alternating current into direct | Reverse-polarity protection, mains rectifiers, and a flyback diode across every relay coil and motor |
| LED | A diode that emits light: a fraction of the power of a filament lamp, tens of thousands of hours of life, and always needs a series resistor | Indicators, displays, torches, backlights, and most domestic lighting |
| Transistor | A tiny base current controls a much larger one — so it works as a switch or as an amplifier | Switching an LED or motor from a sensor, amplifying a microphone, and billions inside any processor |
Two things that are always needed and always forgotten: An LED always needs a series resistor. Connected directly it draws whatever the supply allows and destroys itself within seconds.
Any coil — a relay or a motor — always needs a diode across it. When the current stops, the collapsing magnetic field produces a large reverse voltage spike that will destroy the transistor switching it.
How this is tested — explaining the purpose of passive and active electronic components. It comes up two ways:
Paper 1 — multiple choice
- Identify a component from a description of its behaviour.
- Choose the component that performs a stated job.
Paper 2 — analysing a product
- Explain the purpose of a named component in a given circuit.
- Explain why a particular component was included.
The trap: Describing what a component IS instead of what it DOES in that circuit. "A resistor resists current" earns nothing; "it limits the LED current to 20 mA so the LED survives" earns the mark.
A microcontroller switches a 230 V immersion heater through a transistor and a relay, with a diode across the relay coil. Explain the purpose of each of those three components.
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