Power Supplies, Rectifiers, Filters and Voltage Regulators
What this domain tests
Power Supplies, Rectifiers, Filters and Voltage Regulators covers transformers, half-wave and full-wave or bridge rectifiers, capacitor and LC filters, ripple, zener shunt regulators, three-terminal IC regulators, and SMPS basics.
Core rules
- Half-wave: one diode, ripple frequency equals supply frequency. Full-wave or bridge: ripple frequency is 2× supply frequency.
- A bridge conducts two diodes at a time, so no-load DC ≈ Vpeak − 2Vf; half-wave ≈ Vpeak − Vf.
- Vpeak = 1.414 × Vrms for a sine wave.
- Capacitor filter: larger C and lighter load reduce ripple; ripple ≈ Iload/(f_ripple×C).
- 78xx/79xx need input headroom (dropout about 2 V) and decoupling capacitors; heatsink for dissipation P=(Vin−Vout)×I.
- Zener regulator: series resistor drops excess voltage; the zener must have sufficient knee current but not exceed its power rating.
Worked example
A 15 V DC input feeds a 9 V zener with a 20 mA load and 10 mA zener current. Total current is 30 mA. R=(15−9)/0.03=200 Ω. Zener dissipation is 9×0.01=0.09 W; resistor dissipation is 6²/200=0.18 W. Use a resistor rated at 0.25 W or higher and check the zener rating.
Traps
Using Vrms as peak overestimates or underestimates rectified DC. Forgetting two diode drops in a bridge is a common error. A regulator cannot regulate with insufficient input or no load. Ignoring ripple-current heating shortens capacitor life. The 79xx pinout is not the same as 78xx, so check the data sheet.