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Abstract

A battery protection circuit particularly for portable lamps employing rechargeable batteries is provided to prevent overdischarge of the batteries and includes a bulb drive transistor connecting a bulb across the battery with a switch interposed between one side of the battery and the bulb, a voltage sensing resistor connected to one side of the switch, a regenerative switch connected between the sensing resistor and the other side of the battery, and the regenerative switch including an output terminal connected by a base resistor to the base of the drive transistor. The regenerative switch includes a pair of transistors connected and cooperating such that when one of the transistors is turned on, the other is caused to be turned on, the transistors, further, maintaining each other on. As the dropout voltage of the battery is approached, the base drive of the drive transistor decreases which causes the bulb to become dim to thereby warn of the discharged state of the battery. At the dropout voltage, the voltage across the sensing resistor, which is substantially in parallel with the base-emitter junction of one of the switch transistors, is reduced to a level which is not sufficient to maintain the sensing transistor on whereby the switch transistors are turned off, resulting in the turn-off of the drive transistor so that the battery is protected from overdischarge.

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First Claim