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12v Battery Load Cutout Circuit --
August 23, 2009
Lead-acid batteries should not be
discharged below a certain point if they are to last. Also, you don’t want certain
12v powered devices, which might be plugged into a car’s cigarette lighter outlet, to
drain the car battery down to a point where the car’s engine will not start. To
prevent battery damage or a dead battery, the circuit below disconnects a load from the
battery when the voltage dips below a fixed but adjustable voltage and will not reconnect
the load until a reset button is pressed. To keep the whole circuit efficient and
compact, a medium current rated p-channel FET forms the power switch. A 8.2v zener
diode forms the voltage reference while a second 15v zener diode acts as a transient
voltage protector.
The circuit uses one half of a popular
LM393 dual voltage comparator to monitor the battery voltage and the adjustable reference
voltage. If the battery voltage drops below the adjusted level, the comparator
changes state. One half of a popular CD4013 dual flip/flop latches the on/off
condition. A reset button must be pressed to reestablish power to the external load.
A rather long 5 second time constant
filter is used in the voltage comparator circuit. This allows the circuit to ride
out the dip in battery voltage, which always takes place when the car’s engine is started.
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