Power Supplies and Voltage Regulators
This domain tests the conversion of AC mains into a stable DC supply: transformation, rectification, filtering and regulation. Expect questions on ripple, diode drops and three-terminal regulator limits.
Core rules
- A step-down transformer reduces mains voltage and provides isolation.
- A half-wave rectifier uses one diode; a full-wave centre-tap uses two; a bridge uses four. A bridge conducts through two diodes per half-cycle.
- Peak secondary voltage Vp = √2 × Vrms. No-load filtered DC is approximately Vp minus the conducting diode drops.
- Ripple frequency is 50 Hz for half-wave and 100 Hz for full-wave on a 50 Hz supply. Larger filter capacitance or load resistance reduces ripple.
- A Zener regulator holds output near VZ while the Zener remains in reverse breakdown and the series resistor drops the excess voltage.
- A series IC regulator such as 7805 gives a fixed +5 V; 7905 gives −5 V. Input must exceed output by the dropout voltage.
- Line regulation describes change in output for input change; load regulation describes change for load-current change.
Worked example
A transformer secondary is 12 V rms. A bridge rectifier feeds a capacitor filter. Vp = 12 × 1.414 = 16.97 V. Two diodes conduct, so approximate no-load DC = 16.97 − 1.4 = 15.6 V. This is sufficient input for a 7805 regulator, but not necessarily for a 7812 if dropout is about 2 V and ripple is large.
Marks-losing traps
Do not use rms where peak is needed. Do not subtract four diode drops in a bridge; only two conduct at a time. A 78xx series is for positive supplies and 79xx for negative. Do not reverse the regulator pins or forget heatsinking. A filter capacitor reduces ripple but does not regulate against load changes.