Constructing a relay-controlled overcurrent safeguard involves creating a system that can detect when excessive current flows through a load and automatically disconnect the power to prevent damage. The centerpiece of this circuit is a current-sensing unit, a threshold comparator, and a relay as the final disconnect switch.

Begin with choosing how to monitor current. A widely used approach is to use a low value shunt resistor placed in series with the load. The resistor should be chosen with sufficient power dissipation rating to withstand surges. When current flows through the shunt, a measurable voltage differential appears, directly correlating to the magnitude of current flow.

Next, connect this voltage signal to a comparator circuit.

The circuit compares the feedback voltage with a calibrated setpoint indicating the maximum allowable current. You can generate the reference voltage using a simple voltage divider and a zener diode. The output of the comparator will be a binary state indicating trip or normal conditions.

This signal should drive the coil of a relay through a transistor buffer such as a BJT like the 2N2222 or an IRF540 MOSFET, انواع رله to isolate the low power control circuit from the higher current needed to activate the relay.

Choose a relay with a coil voltage compatible with your control circuit and contact ratings that exceed the maximum load current and voltage. The power terminals should be wired in series with the load and the power supply so that when the relay is de-energized, the circuit is broken.

To eliminate unwanted relay flutter at borderline levels, implement a feedback loop to create a turn-on. 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, incorporate a manual reset button or switch so the system can be manually reactivated once the overload is resolved.

Validate performance with programmable current source and simulated load conditions. Always ensure all components are rated for the operating voltage and current, and adhere to NEC and IEC safety standards for line-voltage applications.

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