Electrical and electronic fundamentals, components, and circuit analysis
This domain tests basic circuit laws, component behaviour, and safe measurement in DC and AC control circuits. Expect relay coils, indicator lamps, diodes, capacitors, series/parallel networks, and multimeter use.
Core rules
- Ohm's law: V = IR; power: P = VI = I²R = V²/R.
- Series: current common, voltages add, resistances add. Parallel: voltage common, currents add, reciprocal resistance sum.
- A diode conducts from anode to cathode when forward-biased; a flyback diode across a DC relay coil is reverse-biased in normal operation and clamps the inductive spike at switch-off.
- A capacitor blocks steady DC and passes changing voltage; it stores energy, so discharge it before testing.
- Measure voltage in parallel with the load, current in series, and resistance only on an isolated, de-energised circuit.
Worked example
A 24 V DC relay coil has 480 Ω resistance. Coil current is I = V/R = 24/480 = 0.050 A, or 50 mA. Coil power is P = VI = 24 × 0.050 = 1.2 W. If a PLC output rated 0.5 A drives the coil directly, the 50 mA load is acceptable, but the output must match the coil's current and the flyback diode must be fitted. The reasoning: calculate both current and power, then compare with the output and coil ratings rather than guessing from voltage alone.
Traps
Measuring resistance on a live circuit; assuming a continuity beep proves the circuit works under load; connecting an ammeter in parallel; fitting a flyback diode the wrong way round; ignoring AC RMS versus peak; and mixing series and parallel rules. These mistakes damage instruments or produce false diagnoses.