Building the Main Circuit
Building the main circuit of this short stop circuit breaker is much simpler as it includes just two transistors and few other associated passive components.
The whole circuit assembly may be easily completed by inserting the listed components neatly over a piece of general purpose PCB and soldering their interconnections following the circuit schematic.
R1 may be constructed at home using a piece of iron wire and calibrating it with the help of the following formulas and calculations:
According to Ohm's law we know that:
E = IR,
Where E is the potential difference developed across R1 = minimum forward voltage drop of the LED used and required to illuminate it.
The value of E may be selected as 2 volts.
I is the maximum load current passing through R1 at which the circuit needs to trip.
Therefore, the final formula becomes R = 2/I.
Its wattage also has to be properly calibrated using the formula:
W = VI,
Where V is the input AC voltage and I is same as above.
The thickness of the wire may be calculated using the formula:
R = ρL/a,
Where ρ = resistivity of iron (ΩMeter), L = length of the wire in Meters and a = area of the conductor (mm2).
Now, if we substitute the above value of R in Ohms law we get:
E = IR = IρL/a,
a = ρIL/E,
The above expression when solved, will give you the exact thickness of R1 (iron wire).
The thickness of R1 may also be selected through some trial and error; the rule of thumb is to make sure that it does not heat up around tripping points (use a sufficiently thick gauge wire).
If R1 is calibrated perfectly through the above methods, it will produce the required 2 volts of potential across itself during the set tripping point. The remaining safety operation of the circuit is also pretty assured after this.