3 Simple Low Voltage Protection Circuit

These circuits are very easy to construct and has very minimal parts. The circuit diagram below is a simple low voltage protection. The relay is a normally open (N.O.) and must be very sensitive type( operates even low current input). When the input voltage (Vin) is less than the forward voltage of the zener diode (VDz), no current will flow to the relay coil thus the load is isolated from the supply. 
simple low voltage protection
Circuit below is an improvised low voltage protection relay. When Vin is higher than VDz plus 0.7 (diode base emitter drop), the relay is close. On the other hand when Vin is lower, relay is open and load is isolated to the source. Main advantage compared to the circuit above is higher sensitivity in voltage change and requires only an ordinary type of relay.
modified version
The load is any device or equipment that works on DC. 


  1. This is a great simple circuit. Could a transistor,FET, or SCR be substituted for the relay, or is there a way to make a solid state circuit such as this that would cut out around 3.0 volts? I need something to disconnect a 1watt LED light circuit from a battery pack when the voltage drops to 3 volts or so. Any help would be appreciated.

  2. You can remove the relay and replace it with FET or BJT and just series your LED.

  3. I want to use this to build a voltage sensitive relay for my boat. Will this circuit drain my battery if left for a long time.