Electronic Components and Devices
Electronic Components and Devices tests recognition, ratings, polarity and simple application of passive and active components: resistors, capacitors, inductors, diodes, BJTs and MOSFETs. The exam favours practical identification and safe substitution over deep semiconductor physics.
Core rules
- Resistor colour code: first two bands = digits, third = multiplier, fourth = tolerance. Gold = ±5%, silver = ±10%, brown = ±1%.
- Capacitor code 104 means 10 × 10^4 pF = 100,000 pF = 100 nF = 0.1 µF.
- Electrolytic capacitors are polarised: stripe = negative; reverse connection can vent or fail.
- Diode: band = cathode; silicon forward drop ≈ 0.7 V, germanium ≈ 0.3 V; an LED needs a series resistor.
- BJT: NPN turns on with base-emitter forward bias; collector current is controlled by base current. A MOSFET is voltage-controlled at the gate.
- Never exceed voltage, current or power ratings; resistors have a wattage rating, capacitors a voltage rating.
Worked example
A four-band resistor reads brown, black, red, gold. Brown = 1, black = 0, so digits 10; red = ×100; value = 10 × 100 = 1000 Ω = 1 kΩ, tolerance ±5%. Its possible range is 950 Ω to 1050 Ω. If it carries 20 mA, P = I²R = (0.02)² × 1000 = 0.4 W, so a 0.25 W resistor would overheat; choose at least 0.5 W.
Traps
- Reading the tolerance band from the wrong end; start from the band nearest the end.
- Treating 104 as 104 pF rather than 100 nF.
- Assuming an LED limits its own current; it does not.
- Confusing BJT base current with MOSFET gate current; a MOSFET gate draws negligible steady current.