meaning ten NiCD battery or 6 2V lead acid.The circuit is pretty small and can be built in a housing network adapter.The incorect usage is impossible :connecting the batteries with reverse polarity ,shortcircuit of the output terminals or power loss have no impact on the charger or battery .
We can use a transformer with 18V on the secondary and then using a doide bridge to rectify the 18V AC voltage we get 22V DC ON C1
The completley discharged batteries are charged at the begining with 6 mA current thru R2-D2 and R6-D1 .One the bat have reached 03-05V, the base-emitter voltage of T1 is high enough to bring the transistor in conduction.
Green LED D4 is used as an charging indicator and opens T1
There is an 60mA current flowing thru R5-R6 , this means that the charging of a 500 mA NiCD battery will take 12 hours
If the battery is connected with reversed polarity or ther is a shourtcircuit, the power transistor T1 remains blocked and charching current can exceed 6-12mA.the current draw at maximum load around 80mA
Circuit Diagram :
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