12 V battery charger with PUT

A short-circuit proof battery charger will provide an average charging current of about 8A to a 12V 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 220V at the input , the circuit comences to function when the battery is properly attached.

The battery provides the current to charge the timing capacitor C1 used in the PUT relaxation oscillator.When C1 charges to the peak point voltage the PUT , the PUT fires turning the SCR on , which in turn applies charging current to the battery. As the battery charges , the battery voltage increases slightly which increases the peak point voltage of the PUT. The voltage on C1 increases until the zener voltage of D1 is reached , which clamps the voltage on C1 , 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 pf the PUT . In the circuit shown , the charging voltage can be set from 10 V to 14 V - the Lower limit being set by D1 and the upper limit by T1.

Part List :

Resistor

R1 = 10K

R2 = 50K trim

R3 = 47K

R4 = 1K

Capacitor

C1 = 0.1uF

Diode , SCR , PUT

B1 = MDA990-1

D1 = 1N5240 10V

SCR = 2N5164

PUT = MPU131

Transformer , Inductor

T1 = Stepdown 220V to 14V

T2 = 11Z12 1:1