Electronic Components, Devices and Circuits
What the domain tests
This domain covers component behaviour and simple electronic circuits: resistors, capacitors, diodes, LEDs, bipolar transistors and rectifier/filter stages. Expect questions on symbols, polarity, biasing, rectification, smoothing and using a transistor as a switch.
Core rules
- Resistors limit current and set bias; capacitors block DC and pass AC, and smooth rectified outputs.
- A silicon diode conducts at about 0.7 V forward drop and Blocks reverse current until breakdown. LEDs need a series resistor.
- A BJT is current-controlled: IC ≈ hFE × IB in the active region. For switching, drive it into saturation with enough base current.
- A half-wave rectifier gives one pulse per cycle; a full-wave bridge gives two. A smoothing capacitor reduces ripple.
Worked example
A transistor switches a 100 mA relay from a 5 V logic output. The transistor has hFE = 100 and VBE ≈ 0.7 V. Minimum base current for saturation: IB = IC/hFE = 100 mA/100 = 1 mA. Base resistor: RB = (5 − 0.7)/1 mA = 4.3 kΩ. Choose a standard 3.9 kΩ resistor to ensure enough base current. The relay coil needs a flyback diode to clamp inductive spikes.
Traps that cost marks
Omitting the base resistor can destroy the transistor. Do not assume hFE is a fixed design value; it varies widely, so use conservative saturation design. Check diode polarity and capacitor polarity in electrolytic types. For rectifiers, do not confuse RMS and peak secondary voltage, and remember the diode drop.