This is one of the simplest electronic fuse circuit one can make. The circuit uses only one transistor, one SCR, one push button switch and two resistors.
The value of R1 can be obtained from the equation; [Imax] X [R1] = 0.7V.
R2 can be obtained from the equation; R2 = [Vs] X [1K Ohms].
Wattage rating of R1 can be obtained from the equation; W = [Imax] X [Imax] X [R1].
For this circuit to work the current consumption of the load must be greater than the holding current of the SCR.
The working of the circuit is very simple. Initially the load current flows through SCR and resistor R1.The value of R1 is so selected that, the maximum load current multiplied by the resistance of R1 is equal to 0.7 volts. When the load current exceeds the maximum value the voltage drop across R1 becomes more than 0.7V and switches transistor Q1 ON. Now the transistor completely bye passes the load current and the current through triac falls below the holding current. This makes the triac OFF. When SCR is OFF there will not be any current flow through R1 and so the voltage across it falls to 0.This makes the transistor OFF, completely isolating the load circuit.The fuse can be resetted by pressing S1.When S1 is pressed the SCR is again triggered and remains latched to conduct the load current.