The battery charger shown in the diagram may be used to charge a battery, or batteries with a total nominal voltage of 12 V (that is, ten NiCd batteries or six 2-V lead-acid batteries. It is small enough to be built into a mains adapter case. Misuse is virtually impossible: batteries connected with incorrect polarity; ‘a short-circuit of the output terminals, or a mains failure has no effect on either the charger or the batteries. Power is derived from the mains via a transformer with an 18-V secondary. The output of the transformer is rectified by diodes D1-D4 and smoothed by C1, whereupon a direct voltage of 22 V is available across C1. Completely discharged batteries are first charged by a current of some 6 mA. via R2-D5 and R4-R6-D8. Once the battery to be charged has an e.m.f. of 0.3-0.5 V, the base-emitter voltage of T1 becomes high enough to switch the transistor on. Charging indicator D6 then lights and T2 is also switched on. A charging current of some 60 mA then flows via R5-R6. This means that 500 mAh ‘NiCd batteries will be charged in about twelve hours. If the battery is connected to incorrect polarity or the charging terminals are short-circuited, power transistor T2 remains off and the charging current cannot become higher than 6-12 mA.
The current drawn by the circuit in full operation is about 80 mA.