Power Supplies
The Power Supplies domain covers converting mains AC into the regulated DC that circuits need - transformer, rectifier, filter and regulator stages - plus the SMPS and UPS systems used in real equipment.
Stages and rules
- The transformer steps 230 V down and provides isolation; its secondary rating is RMS.
- Half-wave rectification gives one pulse per cycle; full-wave and bridge give two, so ripple frequency is 2f (100 Hz on 50 Hz mains). Each conducting silicon diode drops ≈0.7 V.
- A reservoir capacitor smooths the pulses. For a full-wave supply, ripple ≈ I/(2fC).
- 78xx fixed positive and 79xx fixed negative linear regulators; dropout is about 2–2.5 V, so a 7805 needs at least ≈7.5 V input.
- Linear regulators are quiet but inefficient - heat = (Vin − Vout) × I. SMPS units switch at high frequency for 80–90% efficiency, at the cost of ripple and EMI.
- UPS: offline switches over on failure; online double-conversion gives continuous clean power.
Worked example
A 12 V RMS secondary feeds a bridge and 1000 µF into a 1 A load. Peak = 12 × 1.414 = 16.97 V; subtract two diode drops = 15.6 V. Ripple = 1/(2 × 50 × 0.001) = 10 V, so the output would swing between about 15.6 V and 5.6 V - unusable. For 1 V ripple, C = I/(2fVr) = 1/(100 × 1) = 10,000 µF. The reasoning, not the formula alone, is marked.
Traps that cost marks
- Treating the RMS secondary voltage as the DC peak.
- Forgetting the two diode drops in a bridge.
- Reversing electrolytic filter capacitors.