Battery charger shown in this circuit diagram can be used to charge one or more batteries with a total nominal voltage of 12 V (ie ten 1.2V or six 2V NiCd batteries or lead acid ) Misuse connection is impossible, because the batteries connected with incorrect polarity, output short circuit terminal or network loss have no effect on the charger or batteries.
Circuit diagram :
Power is taken from the network through a transformer secondary voltage of 18 V. The output voltage from transformer is rectified by diodes D1 - D4 and filtered by C1, resulting in a voltage of 22 V across its C1. Exhausted batteries are charged in advance with a current of about 6 mA through R2-R4-R6, D5 and D8. Once the batteries have reached a voltage around 0.3 + / - 0.5 V, base-emitter voltage of T1 is large enough to bring the transistor into conduction. Charge indicator, D6, lights and also opens T2. Through R5-R6 pass a charge current of 60 mA.
If the battery is connected with reverse polarity or shorts power transistor T2 remains blocked and the current can not exceed 6-12 mA.