Power Electronics and Optoelectronics
What this domain tests
Thyristor family devices and controlled rectification, power conversion categories, plus LEDs, photodiodes, LDRs, solar cells and optocouplers. Expect firing-angle calculations and bias-condition questions.
Core rules
- SCR is a four-layer PNPN device with anode, cathode and gate. It conducts only when forward biased and a gate pulse is applied; once latched, the gate loses control and conduction continues until anode current falls below the holding current.
- Latching current is the value needed just after turn-on; holding current is the smaller value below which an already conducting SCR turns off.
- Single-phase half-wave controlled rectifier with resistive load: Vdc = Vm(1 + cos α)/(2π). Full converter: Vdc = 2Vm cos α/π.
- TRIAC conducts both halves (AC control); DIAC is a bidirectional trigger; UJT is used in relaxation oscillators to generate firing pulses. Chopper converts fixed DC to variable DC; inverter converts DC to AC; cycloconverter changes AC frequency directly.
- LED is forward biased and emits photons when electrons recombine across a direct band gap (GaAs, GaAsP); photon energy hν ≈ Eg controls colour.
- Photodiode is reverse biased and gives photocurrent; LDR resistance falls with illumination; solar cell needs no bias and delivers power, operating in the fourth quadrant.
Worked example
A half-wave controlled rectifier with resistive load has Vm = 100 V and firing angle α = 60°. Vdc = 100(1 + cos 60°)/(2π) = 100 × 1.5/6.283 = 23.9 V. Delaying the firing angle further lowers the average output; at α = 180° the SCR never conducts.
Traps that cost marks
- Claiming the gate can switch off a conducting SCR.
- Swapping latching and holding current values.
- Using the full-wave formula for a half-wave converter.
- Reverse biasing an LED or forward biasing a photodiode for normal operation.