Over and under voltage switch with relay

    In Fig.1 A precision DC under�voltage relay switch. The opamp is wind as a voltage comparator, with a reference voltage applied to pin 2 and the test voltage applied to pin 3: the relay turns on when the pin 3 voltage exceeds that of pin 2. The circuit can be made to trip at any voltage in excess of 5 volts by suitable choice of R1 value. In Fig. 2 An over-voltage switch that can be used to trip at any pre-set voltage in excess of about 10 mV. The input voltage can be connected directly to pin 2 if trip values in the range 10mV to 3Volts are required. For voltages in excess of 3 Volts, a suitable range resistor must be connected in the position shown, to keep the pin 2 Voltage drive to suitable level. The circuit can be converted to an under�voltage switch by transposing the pin 2 and pin 3 connections of the op-amp. The circuit can be used as an AC voltage switch by first rectifying the AC input signal.