protect elect devices from over and under current using LM339

hi
need help to under stand how LM339 and relay circuit protect my devices for current desturbance

LM339 is a comparator, it cannot itself protect anything but in a suitable circuit it may be able to detect a voltage (which may be an indicator of current) and see if it has exceeded a pre-determined amount. If excess voltage has been detected the LM339 output can be used to drive a relay or other circuit to limit or disconnect the offending current.

Brian.

A high current through a resistor will generate a voltage. You can compare the resistor voltage (or just between that resistor and the charge circuit), with another set by you with a resistive voltage divider or a reference generator. When the output runs below the reference voltage it means that current is so high that caused a high voltage loss through the resistor between +V and the circuit or charge.

When that happens, it should activate the relay and disconnect the charge. Don't forget to put a delay, otherwise your relay will oscillate since no voltage loss through the resistor(caused by relay activation) will deactivate it again, and the loss over the resistor will appear again and will make the relay to activate, and that's how you can build your own bell.

If I were you I would look for another comparator, because you will compare two similar voltages.