Electronic Fundamentals
What you must know
This domain covers semiconductor devices and their aircraft applications: diodes (rectifier, zener, light-emitting), transistors as switches and amplifiers, thyristors, and integrated circuits. You need the PN junction behaviour (forward bias conduction, reverse bias blocking), half-wave and full-wave rectification, ripple and smoothing, and the function of filters and voltage regulators. Familiarity with op-amp basics, oscillators, and the difference between analogue and digital signals is expected.
Traps to watch
- Diode direction: the arrow points in the direction of conventional current (anode to cathode); forward bias needs anode more positive than cathode.
- Zener versus normal diode: a zener is operated in reverse breakdown to regulate voltage; a normal rectifier must not be.
- Rectifier output frequency: a full-wave rectifier on a 400 Hz supply produces 800 Hz ripple, not 400 Hz.
- Transistor terminals: base, collector, emitter; a small base current controls a larger collector current (current gain).
- Smoothing capacitors reduce ripple but do not regulate voltage; a regulator is a separate stage.
Worked example
An aircraft 400 Hz AC supply is full-wave rectified. Each half-cycle produces one output pulse, so the ripple frequency is 2 x 400 = 800 Hz. A smoothing capacitor is then sized to reduce ripple; if ripple must be small, a larger capacitance or higher load resistance is needed (time constant τ = RC should be long compared with the 1.25 ms period).
Rule of thumb
Half-wave rectification keeps the supply frequency in the ripple; full-wave doubles it. Always check whether the question asks about supply frequency or ripple frequency.