Posted on Jun 18, 2012

A short-circuit-proof battery charger will provide an average charging current of about 8 A to a 12-V lead/acid storage battery. The charger circuit has an additional advantage; it will not function nor will it be damaged by improperly connecting the battery to the circuit. With 115 V at the input, the circuit commences to function when the battery is properly attached. The battery provides the current to charge the timing capacitor Cl used in the PUT relaxation oscillator. When Clcharges to the peak point voltage of the PUT, the PUT fires turning the SCR on, which in tum applies charging current to the battery.

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As the battery charges, the battery voltage increases slightly which increases the peak point voltage of the PUT. This means that Cl has to charge to a slightly higher voltage to fire the PUT. The voltage on Cl increases until the zener voltage of Dl is reached, which clamps the voltage on Cl, and thus prevents the PUT oscillator from oscillating and charging ceases. The maximum battery voltage is set by potentiometer R2 which sets the peak point firing voltage of the PUT. In the circuit shown, the charging voltage can be set from 10 V to 14 V-the lower limit being set by Dl and the upper limit by TL

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