Electronic Components and Devices
Electronic Components and Devices
This domain checks that you can identify components, state their characteristics and ratings, and calculate their operating point in a circuit.
Core rules
- Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; third band = multiplier; gold ±5 %, silver ±10 %. Power rating must exceed I²R.
- Capacitors store charge Q = CV; capacitive reactance Xc = 1/(2πfC) falls with frequency. Electrolytics are polarised and must not be reverse-connected.
- Inductive reactance XL = 2πfL rises with frequency; an inductor opposes change of current.
- Diode: conducts when forward biased above the barrier voltage (≈0.7 V silicon, 0.3 V germanium); a zener regulates in reverse breakdown; an LED needs a series resistor.
- BJT: IC = βIB, β = α/(1 − α); the device sits in cutoff, active or saturation according to bias.
- FETs are voltage-controlled; SCR, triac and diac switch power; varistors and thermistors are protective or sensing parts.
Worked example
An LED with a forward drop of 2 V at 20 mA is to run from a 9 V battery. Series resistor R = (9 − 2)/0.02 = 350 Ω. The nearest preferred value is 390 Ω, giving I = 7/390 ≈ 18 mA - acceptable brightness with longer life. Resistor dissipation P = I²R = 0.018² × 390 ≈ 0.13 W, so a ¼ W resistor is adequate but a ⅛ W part is marginal.
Mark-losing traps
- Forgetting the diode drop when calculating a series resistor.
- Fitting an electrolytic capacitor with reversed polarity.
- Confusing current gain β with the voltage gain of a stage.
- Treating a zener as an ordinary rectifier in forward bias.