Designing a relay based over current protection circuit involves creating a system that can detect when excessive current flows through a load and automatically disconnect the power to prevent damage. The core of this circuit is a current-sensing unit, a threshold comparator, and a relay as the final disconnect switch.
First, pick an appropriate current detection technique. A common approach is to install a precision shunt resistor in series with the current path. 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 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 output of the comparator will be a digital signal high or low depending on whether the current exceeds the threshold.
This output must activate the relay via an isolation stage such as a BJT like the 2N2222 or an IRF540 MOSFET, to protect the comparator from the relay’s inductive load.
Select a relay whose coil matches your supply voltage and whose contacts handle peak load specs. The switch contacts should be wired in series with the load and the power supply so that the power is fully interrupted upon activation.
To avoid contact oscillation near the trip point, implement a feedback loop to create a turn-on. This ensures the circuit deactivates at the upper limit and reactivates only after a substantial current reduction.
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.
Validate performance with programmable current source and simulated load conditions. Verify component specifications against worst-case operating parameters, and follow proper electrical safety practices when working with mains powered systems.
This type of protection offers a dependable, electromechanical safeguard that’s easy to troubleshoot and service.
