Most of the battery chargers do not have current and voltage regulation provisions. The step down voltage is simply used for charging. These chargers develop internal resistance so the output voltage drops when the battery is connected to the charger. This is a design circuit for the charger circuit. This is the figure of the circuit;
0-15 1 Ampere step down transformer drops 230 volt AC into 15 volt AC which is rectified through the bridge rectifier comprising D1 through D4. The rectified DC is then made ripple free by C1 and send to the collector of the medium power NPN transistor T1 to give regulated output. Resistor R1 and Zener diode ZD are used for both voltage and current regulation. Output current from the emitter of T1 depends on the value of R1 which can be changed according to the requirement using the ohms law. 12 volt Zener diode gives constant 12 volts to the base of T1 so that output voltage remains 12 volt irrespective of the input fluctuations. Diode D5 is polarity protector that prevents short circuiting if the polarity of the battery is reversed. LED indicates the charging process.
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