Solar Battery Protector Prevents excessive Discharge

This circuit prevents
the battery in a solar lighting system from being excessively
discharged. It’s for small systems with less than 100W of lighting, such
as several fluorescent lights, although with a higher rated Mosfet at
the output, it could switch larger loads. The circuit has two
comparators based on an LM393 dual op amp. One monitors the ambient
light so that lamps cannot be turned on during the day. The second
monitors the battery voltage, to prevent it from being excessively
discharged. IC1b monitors the ambient light by virtue of the light
dependent resistor connected to its non-inverting input. When exposed to
light, the resistance of the LDR is low and so the output at pin 7 is low.

Circuit diagram:

Solar Battery Protector Circuit

Solar Battery Protector Circuit Diagram

IC1a monitors the battery voltage via a voltage divider connected to
its non-inverting input. Its inverting input is connected to a
reference voltage provided by ZD1. Trimpot VR1 is set so that when the
battery is charged, the output at pin 1 is high and so Mosfet Q1 turns
on to operate the lights. The two comparator outputs are connected
together in OR gate fashion, which is permissible because they are
open-collector outputs. Therefore, if either comparator output is low
(ie, the internal output transistor is on) then the Mosfet (Q1) is
prevented from turning on. In practice, VR1 would be set to turn off the
Mosfet if the battery voltage falls below 12V. The suggested LDR is a NORP12, a weather resistant type available from Farnell Electronic Components Pty Ltd.
Author: Michael Moore – Copyright: Silicon Chip Electronic

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