Electrical and Electronic Fundamentals
This domain tests the theory behind the readings: Ohm's law, AC behaviour, motors, sensors, and control electronics. Questions often ask you to predict a value or explain why a measured value differs from the nameplate.
Core relationships
- V = I × R; P = V × I; for pure resistance P = V²/R = I²R.
- Series: current common, voltages add, resistances add. Parallel: voltage common, currents add, reciprocal resistances.
- Voltage drop is a diagnostic: a poor connection shows a large drop under load, not necessarily at rest.
- AC: 120 V and 240 V are RMS values; peak is 1.414 × RMS. Impedance combines resistance and reactance; capacitors and inductors shift current and voltage.
- Capacitors: microfarad rating, discharge before testing, and a failed capacitor causes a PSC motor to hum and trip.
- Motors: PSC, shaded-pole, universal, and ECM. Identify start and run windings by resistance; ECM modules need correct DC supply and signal.
- Semiconductors: diodes, bridge rectifiers, triacs, thermistors (NTC/PTC), thermocouples, and Hall sensors. A thermistor's resistance changes with temperature; a board reading is only valid if the sensor and harness are good.
- Control boards: separate line voltage from low-voltage signal; back-probing connectors is safer than piercing insulation.
Worked example
A dryer heater rated 4500 W at 240 V draws 18.75 A and has 12.8 Ω hot resistance. You measure 240 V at the element terminals but only 9 A. Since the voltage is correct, the element resistance must be higher than expected (about 26.7 Ω). A cold resistance check of 24–27 Ω confirms a degraded element; 0 Ω would indicate a short, and infinite resistance an open. You also check the element's series limits and connections, because a voltage reading taken with a high-impedance meter can appear normal through a poor connection.
Traps
- Measuring resistance with the circuit energized.
- Treating 0 Ω as "good" when it may be a short.
- Ignoring lead resistance on low-ohm readings.
- Using LoZ on a control board and disturbing the circuit.