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Designing a relay based over current protection circuit involves designing an automatic cutoff that interrupts power when current exceeds safe limits. The core of this circuit is a current sensor, a comparison mechanism, and a relay that acts as the switching element.

First, pick an appropriate current detection technique. A common approach is to place a low-ohm current-sensing resistor directly in the load circuit. The resistor should be rated to handle the maximum expected current without overheating. When current flows through the shunt, a millivolt-level signal is generated that scales with the current.

Then, feed the sensed voltage into a differential amplifier.

The comparator compares the sensed voltage against a reference voltage that corresponds to the desired current threshold. You can establish the threshold via an adjustable resistor network and a stable reference IC. The digital output will be a logic high when overcurrent is detected and low otherwise.

This output must activate the relay via an isolation stage such as an N-channel MOSFET or a general-purpose NPN transistor, to protect the comparator from the relay’s inductive load.

Choose a relay with a coil voltage compatible with your control circuit and contact ratings that exceed the maximum load current and voltage. The relay contacts should be inserted directly into the power path to the load so that the connection is physically severed during a fault condition.

To eliminate unwanted relay flutter at borderline levels, add hysteresis to the comparator using positive feedback. This ensures the circuit switches off at a higher point and resets only after the load current falls well below the setpoint.

Supplement the system with an upstream overcurrent device for fail-safe redundancy. Also, add a physical reset control so the system can operate only after intentional human intervention following a trip.

Calibrate using an adjustable supply and resistive load to confirm accuracy and delay. Always ensure all components are rated for the operating voltage and current, and follow proper electrical safety practices when working with mains powered systems.

A well designed relay based over current protection circuit provides a robust, mechanical solution that is simple to understand, maintain, and repair.