This is a tried and trusted design for the protection against overvoltages that can be configured for your own needs. The circuit can also be used to detect under voltages; in that case, the inputs of IC1 have to be changed over. The operation is straightforward. When the input voltage becomes too large, the voltage at pin 2 (inverting input) of IC1 becomes higher than the reference voltage at pin 3 (noninverting input). The output of the opamp will, in that case, become ‘low’. (It’s just like a maths lesson: higher times inverting can be thought of as + times – and that results in–.)
This then drives T1 on, which in turn supplies power to the relay. The contacts of the relay can then be used to isolate the equipment from the supply. The circuit doesn’t have any hysteresis, so in principle, there is a possibility that the relay could chatter. In practice, it won’t be that bad, because the voltage will rise a bit further when the protected equipment is turned off. We’ve chosen a supply voltage of 12 V for the circuit, but in practice any voltage between 12 V and 24 V is suitable. The coil of the relay should work at the chosen supply voltage; so select a relay with a working voltage the same as that of the supply, or if you already have a relay, make the supply voltage equal to the operating voltage of the relay. It doesn’t have to be exact, as long as you stay within plus or minus 10%.
Keep in mind that a BC558 (T1) can switch at most 50 mA. Should the relay require more current, replace T1 with a BC516; this can switch a maximum of 0.5 A (in practice 0.25 A). The voltage reference is simply derived from a zener diode (D1). The zener voltage isn’t critical and a value of 5.1 V may even be better than the 3.3 V used here because the change in zener voltage due to a temperature change is then at a minimum. R3 should have a value that lets a current of at least 3 mA flow through the zener. Keep in mind that according to the datasheet for the 741 the input voltage at which the 741 switches should be at least 1.5 V higher than the voltage at pin 4 (this is the so-called common-mode voltage). So the voltage set by P1 at pin 3 should not be less than 1.5 V.
In practice a lower voltage is possible, down to about 1 V. It would be better to use a value between 47 kΩ and 100 kΩ for R4; this reduces the adjustment range somewhat, but at least the voltage can never be set too low. If you really want the lower range to extend down to 0 V, then you should choose a different opamp, such as half an LM358. Resistor R1 forms a potential divider in combination with R2. R1 should have a value such that the voltage at pin 2 is about equal to the voltage at the wiper of P1 when that is in its mid position; that is about 2.4 V. The following formula is used to calculate the value for R1: it is equal to the required turn-off voltage minus 2.4 V, divided by 240 μA. So for protection against voltages greater than 100 V, R1 should be 407 kΩ; in practice, you would use 390 kΩ. The current consumption of the circuit is only a few mA plus the relay current.