This simple Circuit Breaker can be added to the Work Bench power supply, to break power to the load when the current flow through the load increases more than 470 mA due to short circuit or power surge. It immediately breaks the power to the load and the load again gets power only if the current flow returns to normal.
Circuit working is simple. Power to the load is connected through the Common and NO contacts of the relay with a bypassing push switch for relay activation. Current from the negative of the load passes through the Current Sensor resistor R2 (1.5 Ohms 2 Watt). A Medium power NPN transistor (T1) is used as a switch to break the power. When the Push switch is pressed and released, Relay turns on and makes the contacts closed. Load gets power. When the current passing through R2 increases more than 470 mA, T1 turns on. As a result, current reduces through the relay and the contacts break. If the over load is ceased, T1 turns off. The Relay can be turned by pressing SW1.
Selection of Resistors R1 and R2 needs some calculations
Tripping current value through R2
R2 = 0.7 / It
Where 0.7 is the voltage across R2 due to T1 and It is the tripping current value.
Selection of R1 for different supply voltages
R1 = (V supply – VR) / IR
Where VR is the Relay voltage and IR is the Relay current
For example, in the circuit, a 6 Volt 500 Ohms relay is used , so 240 Ohms resistor is selected as R1 and for 470 mA Tripping current, R2 is selected as 1.5 Ohms.