Power Supplies and Batteries
What this domain tests
Power Supplies and Batteries tests the transformer–rectifier–filter–regulator chain, fixed and adjustable regulators, SMPS basics, and battery ratings, connection and charging. Numerical questions on regulator dissipation and battery back-up are common.
Core rules
- A linear supply: transformer steps AC down, rectifier converts to pulsating DC, filter capacitor smooths, regulator holds output constant.
- 78xx is a fixed positive regulator (7805 = +5 V); 79xx is fixed negative. The input must exceed the output by the dropout voltage, typically about 2 V.
- A bridge rectifier gives full-wave output; ripple frequency is twice the AC input frequency. Larger filter capacitance reduces ripple but increases surge current.
- Batteries in series: voltages add, Ah capacity stays the same. In parallel: capacity adds, voltage stays the same.
- Lithium cells need a proper charge controller; lead-acid cells must not be overcharged or deeply discharged.
Worked example
A 7805 has 12 V input and supplies 500 mA. The regulator drops 12 − 5 = 7 V, so it dissipates P = V × I = 7 × 0.5 = 3.5 W. A small TO-220 regulator without a heatsink will overheat. The load gets 5 × 0.5 = 2.5 W, so efficiency is 2.5/6 = about 42%. Fit a heatsink and keep the input as low as the dropout allows, such as 8–9 V, to reduce wasted heat.
Traps
Reversing 7805 pinout; feeding a regulator below dropout; omitting the filter capacitor; adding battery voltages but also adding Ah in series; and assuming a 5 Ah battery runs a 1 A load for 5 hours at full efficiency. Real runtime is lower.