Minnie Low-Voltage Cutoff Circuit Test

Developing and testing the Minnie LiPo Low Voltage Cutoff circuit. Schematic at the end 5:14. NOTE: The S Weir mods make the circuit work better. Use a 2n7000 mini-mosfet instead of the BC337 and use a 68k (or 47k) instead of the 10k from the reset switch. 1 uF across the switch. Thanks Steve!

This is a basic comparator circuit. The Zener diode provides a reference voltage that is compared to the voltage at the setpoint trimpot. When the supply voltage goes below the setpoint the comparator’s output goes low and this turns off the NPN transistor, so the reference voltage goes high and this keeps the comparator off. When the supply voltage is restored the comparator stays off because the transistor is off so the reference voltage (on the op-amp’s inverting input) stays high so the output of the op-amp stays low and the mosfet stays off. When the reset button is pressed it turns the NPN transistor on, which drops the reference voltage to below the setpoint pot voltage (on the non-inverting input), which turns the op-amp output high, turning on the mosfet and also keeping the NPN transistor on. The circuit draws 11-12 mA when in the “off” state, so isn’t really suited for completely unattended operation for days at a time. It’s just designed to help you avoid overdischarging your LiPos when using them for regular tasks where you can keep an eye on things.

The specified mosfet is a high-current unit and should handle 20 amps without problems. It should probably be on a heatsink just in case, and the unit should be fused to avoid battery damage if there should be a short in the circuit somewhere. And of course the battery should have a “C” rating that can handle the load’s current demand.

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