Electronic Components and Passive Devices
The Electronic Components and Passive Devices domain tests whether you can identify, read and select components - resistor colour codes, capacitor markings, inductor and transformer behaviour - and judge tolerance and power rating. Practical papers ask you to decode a marking and decide whether the part is fit for the circuit.
Reading markings
- Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Third band = multiplier; fourth = tolerance (brown ±1%, red ±2%, gold ±5%, silver ±10%).
- Read from the band nearest the end; a fifth band is the temperature coefficient.
- Ceramic capacitor code: first two digits are significant figures, the third is the number of zeros in picofarads. 104 = 10 × 10⁴ pF = 100 nF = 0.1 µF.
- Electrolytic capacitors are polarised: the striped lead is negative. Reversing them causes gassing and bursting.
- A potentiometer is a three-terminal variable resistor; a rheostat uses two terminals.
- Thermistors (NTC/PTC) and LDRs change resistance with temperature and light respectively.
Worked example
Bands: yellow, violet, red, gold. Yellow = 4, violet = 7, red multiplier = ×100, gold = ±5%. Value = 47 × 100 = 4700 Ω = 4.7 kΩ. Tolerance = 5% of 4700 = 235 Ω, so the acceptable band is 4465 Ω to 4935 Ω. A meter reading 4.9 kΩ passes; 5.1 kΩ fails. Stating the tolerance window is the marked step.
Traps that cost marks
- Reading a resistor from the wrong end - the gold or silver band always sits on the tolerance side.
- Forgetting that a capacitor's code is in picofarads, not microfarads.
- Ignoring power rating: a ¼ W resistor in a 1 W position overheats and drifts.