Power Supplies and Oscillators
What the exam tests
Power Supplies and Oscillators tests the chain from transformer to regulated DC, and the conditions for sustained sine-wave oscillation. Expect calculations of filter ripple, regulator dissipation, oscillator frequency and feedback fraction.
Core rules
- A linear supply: transformer steps down AC, rectifier converts to pulsating DC, filter smooths it, regulator holds output constant.
- Three-terminal regulators: 78xx gives fixed positive output (7805=5 V, 7812=12 V); 79xx gives negative. Dropout voltage is typically 2 V, so a 7805 needs at least about 7 V input.
- Regulator dissipation P=(Vin-Vout)×Iload; heat-sinking is required when this exceeds the package rating.
- The 555 astable frequency is approximately f=1.44/((R1+2R2)C).
- Barkhausen criterion: loop gain ≥1 and phase shift 360° (0°) at the oscillation frequency. RC phase-shift uses three 60° networks; Wien bridge uses a lead-lag network.
Worked example
A 7805 is fed from an unregulated 12 V rail and drives a 100 Ω load. Iload=5 V/100 Ω=50 mA. The regulator dissipates (12-5)×0.05=0.35 W. A TO-220 package without a heatsink has a junction-to-ambient thermal resistance around 50 °C/W, so the temperature rise is about 17.5 °C-acceptable in a cool enclosure. If the load were 50 Ω, current would be 100 mA and dissipation 0.7 W, so a small heatsink is advisable.
Traps
Assuming a 7805 can regulate from a 6 V input; ignoring the dropout voltage; and forgetting that the filter capacitor must be large enough at the load current. In oscillator questions, a 180° phase-shift network alone does not guarantee oscillation-the amplifier must also supply the other 180° and enough gain.