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Myo Min, Yangon, Myanmar



THERE are two main types of battery charger constant voltage and constant current. Both have their advantages and disadvantages. For constant voltage, the battery cannot be overcharged but the charging rate is slow. Constant current mode can charge batteries more swiftly, but there is the danger of overcharging them. The circuit in Fig.3 was designed to combine both modes, but without their disadvantages, for use with a 6V sealed lead-acid battery. The main players of the circuit are voltage regulator IC1, which is used for constant current mode, and precision adjustable shunt regulator IC2, which is used for constant voltage mode. In constant current mode, resistor R4 sets the current at 370mA, according to the equation:


R4 = (125/I) 1000


I = the constant current required, in milliamps.


Diode D3 prevents the battery from discharging back into IC1 if the input supply is disconnected. Resistor R3 provides current to switch on transistor TR1 when the input supply is present. Shunt regulator IC2, resistors R6, R7 and preset potentiometer VR1 form the network which determines whether or not the battery has reached its required voltage.

Figure 3.  Circit diagram from dual-mode charger


When the voltage at IC2s reference input reaches 25V, IC2 switches on its internal transistor, connecting IC1s ADJ (adjust) pin to 0V. In this condition, IC2 holds IC1 in constant current supply mode. Capacitor C3 helps to stabilise the switching of IC2. Capacitors C1 and C2 decouple the d.c. input supply voltage. Light emitting diode D1 is a power-on indicator, and l.e.d. D2 is turned on when constant voltage mode is activated.

A heatsink may be needed with IC1. In use, adjust preset VR1 so that the voltage at the output suits the peak voltage required by the sealed lead-acid battery, which is usually printed on its body. Once adjusted correctly, it should not need further adjustment. The author used a 12V 600mA d.c. adapter for powering the circuit. The battery with which it is used has a peak voltage range of 6.9V to 7.12V.