Electronic battery charger

Claim 1

A battery charger comprising: A power transformer having a primary and a secondary winding; an input circuit for said winding comprising a pair of input terminals for connection to a source of alternating current and a normally open gated junction reversible AC switch connected in series between one of said input terminals and said primary winding, said switch adapted to close and conduct said AC current for the duration of any half cycle thereof only if subjected to a gating signal during said half cycle and to reopen at the end of said any half cycle; an output circuit for said secondary winding comprising a pair of output terminals for connection to a battery and rectifier means connected between said secondary winding and said output terminals for producing in response to excitation of said secondary winding by current flowing in said primary winding charging current pulses in cadence with conduction of said AC current by said switch; and means for gating said switch at a phase angle that increases with increasing state of charge of said battery so that the average charging current supplied to said battery decreases as said battery is charged up; said means for gating switch comprising a unijunction transistor circuit, means for triggering said unijunction transistor to conduction according to the open circuit potential of said battery, and means for generating a gating signal each time said unijunction transistor is triggered to conduction; said means for triggering said unijunction transistor to conduction comprising means for establishing a pedestal voltage on the emitter of said unijunction transistor that varies inversely with the state of charge of said battery and means for exponentially raising the voltage of said emitter from said pedestal to the level at which said unijunction transistor is triggered during each phase reversal of the current from said AC source; said means for establishing said pedestal voltage comprising a resistor network connected across said output terminals, a transistor connected in circuit with the emitter of said unijunction transistor so that said pedestal voltage decreases with increasing conduction of said transistor, and a zener diode connected for reverse conduction between said resistor network and said transistor so that the degree of conduction of said zener diode is determined by the open circuit potential of said battery and the degree of conduction of said transistor increases with increasing reverse conduction of said zener diode.