Batteries, Power Supplies and Semiconductor Devices
What this domain tests
This domain links electrochemical cells, DC power supplies and basic semiconductor devices. Expect capacity and back-up time calculations, rectifier descriptions and diode behaviour.
Core rules
- A primary cell is not rechargeable; a secondary cell such as lead-acid or lithium-ion is. Capacity in ampere-hours = current × time, but actual back-up time depends on discharge rate, temperature and depth of discharge.
- Cells in series add voltage while capacity stays the same; cells in parallel add capacity while voltage stays the same.
- A diode conducts when forward biased above about 0.7 V for silicon and blocks reverse current; a Zener diode operates reverse-biased in breakdown to regulate voltage.
- A half-wave rectifier uses one diode and gives 50 Hz ripple from 50 Hz mains; a full-wave bridge uses four diodes and gives 100 Hz ripple with better transformer utilisation.
- A transistor can amplify or switch; an LED emits light when forward biased and needs a series resistor to limit current.
Worked example
A 150 Ah battery supplies a 10 A load. Ideal time = 150/10 = 15 h. If only 80% of capacity is usable to protect battery life, usable capacity is 120 Ah and the realistic time is 12 h. At a higher 20 A load the time is not simply 7.5 h, because Peukert-type losses reduce available capacity further.
Traps that cost marks
Treating Ah as a unit of energy; adding voltages of parallel cells; assuming a bridge rectifier gives 50 Hz ripple; and forgetting the diode drop when estimating DC output voltage.