Basic Electricity and Electronic Components
Basic Electricity and Electronic Components
This domain tests Ohm's law and Kirchhoff's laws, AC quantities, passive components, colour codes, ratings and protective devices.
Core rules
- V = IR; series elements share current, parallel elements share voltage. Power is I²R or V²/R.
- A 230 V RMS mains supply has a peak of about 325 V. Use RMS for heater and transformer ratings unless told otherwise.
- Capacitive reactance is 1/(2πfC); inductive reactance is 2πfL. Reactance limits AC without dissipating heat like a resistor.
- Read four-band resistors from the end opposite the tolerance band. Yellow-violet-red-gold is 4.7 kΩ ±5%.
- Derate capacitor voltage; an electrolytic used near its rating ages faster. Observe polarity.
- A fuse needs a voltage and interrupt rating suitable for the circuit, not just the correct current.
- An NTC thermistor falls in resistance when hot; a varistor clamps surges; a relay coil needs a flyback diode.
Worked example
A 4.7 kΩ resistor is connected in parallel with 10 kΩ. Find the equivalent resistance and the power in the combination at 12 V. Product over sum gives (4700 × 10000)/(14700) = 3197 Ω, about 3.2 kΩ. Total power is V²/R = 144/3197 = 45 mW, so 0.25 W resistors are adequate. Checking with branch currents: 12/4700 = 2.55 mA and 12/10000 = 1.2 mA, total 3.75 mA; 12 V/3.75 mA = 3.2 kΩ, confirming the result.
Traps
Starting the colour code at the gold band; ignoring capacitor ESR; assuming an inductor behaves like a resistor at high frequency; forgetting meter loading; choosing a resistor by resistance only and ignoring wattage.