Low-Voltage Cutout For 12V SLA Batteries

This simple circuit protects an SLA
battery from over-discharge by disconnecting the load when the terminal
voltage drops below a preset level. In operation, a sample of the
battery voltage is derived from the 22kΩ resistor and 20kΩ trimpot
divider. This is applied to the non-inverting input (pin 3) of IC1,
where it is compared with a reference voltage on the inverting input
(pin 2). When the sampled battery voltage falls below the reference
voltage, IC1’s output (pin 1) swings towards ground, switching Mosfet Q2
off and disconnecting the load from the battery. The reference voltage
is derived from a 4.7V zener diode (ZD1), which is connected to ground
via the collector-emitter circuit of Q1 (ie, when Q1 is on).

Low Voltage Cutout For 12V SLA Batteries

Low-Voltage Cutout For 12V SLA Batteries

However, when the op amp’s output is driven low, Q1 is switched off,
causing the non-inverting input to rise towards the full battery
voltage. This greatly reinforces the switching action, latching the
circuit in the “off” state until the battery is recharged and the reset
switch (S1) pressed. The Mosfet used for Q2 should be selected to suit
the intended application. The circuit could also drive a relay simply by
connecting the coil across the “load” terminals. As is usual practice, a
diode should be connected across the relay coil to limit back-EMF spikes.

Author: Tim Nuske – Copyright: Silicon Chip Electronics

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