Power Supplies and Regulators
What this domain covers
Power Supplies and Regulators covers the chain from mains transformer to regulated DC: rectification, filtering, regulation and protection, including IC regulators and switched-mode supplies.
Core rules
- A regulated supply has four blocks: transformer (step down and isolate), rectifier, filter (smoothing capacitor or LC), and regulator.
- Ripple factor r = V_ripple(rms)/V_dc; smaller is better. For a full-wave rectifier V_ripple ≈ I/(f·C), where f is the ripple frequency.
- Percentage regulation = (V_no-load − V_full-load)/V_full-load × 100.
- The 78xx series gives fixed positive outputs (7805 = +5 V, 7812 = +12 V); 79xx gives negative outputs. Dropout voltage is the minimum input-output difference for regulation, about 2 V for 78xx.
- A series-pass transistor regulator uses negative feedback; an SMPS switches at high frequency for efficiency but generates noise.
Worked example
A 7805 regulator is fed from 12 V and delivers 0.5 A. Power dissipated in the regulator = (V_in − V_out) × I = (12 − 5) × 0.5 = 3.5 W. The package and heatsink must handle this, so a heatsink is required.
Traps that cost marks
Reversing the 7805 pinout (input, ground, output); feeding the regulator too little input and ignoring dropout; using a filter capacitor whose voltage rating is below the peak rectified voltage; and assuming an SMPS output is clean without filtering.