Passive Components and Their Applications
2. Passive Components and Their Applications
This domain tests the identification, coding, behaviour and circuit use of resistors, capacitors, inductors and transformers - components that cannot amplify or generate power.
Resistors
- Fixed types: carbon composition, metal film, wire-wound; variable: potentiometer, preset; special: thermistor (temperature), LDR (light).
- Colour code: Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, Blue 6, Violet 7, Grey 8, White 9. The third band is the multiplier (power of ten); the fourth is tolerance: gold ±5%, silver ±10%, none ±20%.
Capacitors and inductors
- Capacitance C = Q/V in farads; energy stored = ½CV². Capacitors block DC and pass AC; X_C = 1/(2πfC).
- Inductance L in henries; energy = ½LI²; inductors pass DC and oppose change of current; X_L = 2πfL.
- Series and parallel rules for C and L are the reverse of those for R.
- Series resonance at f = 1/(2π√(LC)), where X_L = X_C and impedance is minimum.
Worked example
A ceramic capacitor is marked 104. Code: 10 × 10⁴ pF = 100 000 pF = 100 nF = 0.1 µF. At 50 Hz, X_C = 1/(2π × 50 × 0.1 × 10⁻⁶) ≈ 31.8 kΩ, so it behaves almost as an open circuit at mains frequency but as a near short at radio frequency.
Traps
- Reading 104 as 104 pF or as 104 µF.
- Treating an electrolytic capacitor as non-polarised; reversed polarity destroys it.
- Applying resistor series/parallel formulae to capacitors.
- Expecting a transformer to work on steady DC - it needs a changing flux.