Power Electronics and Control
What this domain tests
This domain tests semiconductor devices (diode, SCR, TRIAC), rectifier and inverter action, choppers, PWM, and simple control circuits such as relays and sensors.
Core rules
- A diode conducts in one direction only; a half-wave rectifier gives average output Vm/π and a full-wave rectifier gives 2Vm/π.
- An SCR conducts one way and is turned on by a gate pulse; it turns off when current falls below the holding value. A TRIAC conducts both ways, so it suits AC lamp dimmers and fan regulators.
- An inverter converts DC to AC; a rectifier converts AC to DC; a chopper converts fixed DC to variable DC.
- PWM switches a device on and off rapidly; the average voltage depends on duty cycle D, where Vavg = D × Vdc.
- A relay lets a small control current switch a large load; a flyback diode protects the driver transistor from the coil's back EMF.
Worked example
A PWM signal of 12 V DC has a duty cycle of 25 percent. Find the average voltage across a lamp and the effect on brightness. Vavg = 0.25 × 12 = 3 V. The lamp sees an average of 3 V, so it glows dimly; increasing the duty cycle to 75 percent raises the average to 9 V and the lamp brightens. This is the principle behind a PWM lamp dimmer.
Traps that cost marks
Using an SCR for a two-way AC dimmer instead of a TRIAC; calling an inverter a rectifier; omitting the freewheeling diode across a relay coil; mixing up Vm/π and 2Vm/π; thinking duty cycle changes frequency rather than average voltage.