# Overvoltage Protection Schematic Circuit Diagram

#### Customizable Overvoltage Protection Design

This design, tested and proven, offers robust protection against overvoltage while allowing customization to suit your specific requirements. Moreover, this circuit holds the versatility to detect under voltages when necessary; in such instances, the inputs of IC1 need to be reconfigured. The operational principle is straightforward: when the input voltage exceeds a certain threshold, the voltage at pin 2 (inverting input) of IC1 surpasses the reference voltage at pin 3 (noninverting input). Consequently, the opamp’s output turns ‘low’. This behavior can be likened to a mathematical equation where a higher value multiplied by a negative yields a negative result.

#### Driving the Relay for Equipment Isolation

In this configuration, the circuit drives T1, which subsequently provides power to the relay. The relay’s contacts can then be utilized to disconnect the equipment from the power supply, ensuring isolation. Notably, the circuit lacks hysteresis, potentially leading to relay chatter. However, practical implementation suggests this issue won’t be severe due to a slight voltage rise when the protected equipment is turned off. The circuit’s chosen supply voltage is 12 V, but any voltage between 12 V and 24 V is suitable. For the relay, select one with a working voltage matching the chosen supply voltage, allowing for a range within plus or minus 10%.

#### Considerations for Current and Voltage Parameters

The BC558 (T1) can handle a maximum current of 50 mA. If the relay demands more current, substitute T1 with a BC516, capable of switching up to 0.5 A (typically 0.25 A). The voltage reference is derived from a zener diode (D1). While the zener voltage isn’t highly critical, opting for 5.1 V might be better than the utilized 3.3 V, as it minimizes zener voltage fluctuations due to temperature changes. R3 should enable a current flow of at least 3 mA through the zener. According to the 741 datasheet, the input voltage at which the 741 switches must be at least 1.5 V higher than the voltage at pin 4 (common-mode voltage). Consequently, the voltage set by P1 at pin 3 should not dip below 1.5 V.

In practical scenarios, a lower voltage, even down to about 1 V, is feasible. A suitable choice for R4 lies between 47 kΩ and 100 kΩ. Although this slightly narrows the adjustment range. It ensures the voltage can never be set too low. To extend the lower range to 0 V, opt for a different opamp, like half an LM358. R1, forming a potential divider with R2, should maintain a voltage at pin 2 close to the wiper voltage of P1 when it’s in the mid position, around 2.4 V. The formula to calculate R1’s value considers the required turn-off voltage minus 2.4 V, divided by 240 μA. For voltages surpassing 100 V, R1 should be around 407 kΩ, practically substituted with 390 kΩ. The circuit’s overall current consumption is a few mA, plus the relay current.

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