Power Supplies and Battery Maintenance
This domain tests linear and switch-mode supply blocks, voltage regulation and safe battery charging and maintenance. Expect questions on ripple, dropout and battery chemistry.
Supply blocks
A basic linear supply has a transformer, rectifier, filter capacitor and regulator. A 7805 gives a fixed 5 V output but needs an input several volts higher; the difference is dissipated as heat. An SMPS switches at high frequency for efficiency and is common in TVs, chargers and computers.
Batteries
- Lead-acid: check electrolyte level and specific gravity on serviceable types; charge in a ventilated area; clean terminals; avoid deep discharge.
- Li-ion: charge with a proper CC/CV charger and protection circuit; never overcharge or puncture; capacity loss is normal with cycling.
- NiMH/NiCd: avoid memory effect in NiCd by full discharge cycles.
Worked example
A 7805 regulator is supplied with 12 V and delivers 0.5 A. The regulator dissipates (12 − 5) × 0.5 = 3.5 W. A small TO-220 package without a heatsink will overheat, so a heatsink is required. The output remains about 5 V only while the input stays above the dropout voltage.
Traps that cost marks
Reversing battery polarity; charging a Li-ion cell without a protection circuit; ignoring ripple on a supply output; expecting a regulator to deliver more current than its rating; assuming a fuse blowing is the root fault rather than a symptom.
Exam focus
Trace the transformer–rectifier–filter–regulator chain and explain each block's function, and state one safety rule for each battery type.