Electronic fuse circuit

  
Full short-circuit and overcurrent protection is given by this circuit suitable for workbench applications in technical schools and laboratories where there is a need to work directly with the mains. Additional features are a clearly visible red lamp indicating that the voltage is present, good isolation of the output circuit when the unit is off, only a few millivolts were measured with no load, current threshold adjustable over a limited range and the possibility of remote cutout: the 6V from the secondary can be taken anywhere, normally where you are working, even far away from the protection circuit.
Electronic fuse circuit - schematic

Pressing the push button will short-circuit the winding and the circuit will switch off thus removing the mains voltage. A suitable led is placed together with the push button to show whether the circuit is in operation or not. Additional remote cutout circuits can be wired in parallel if so required. The circuit will switch off if a short is applied at the output without blowing the fuse but it will blow if you try to activate the circuit if a short is already present. If in doubts, first activate the circuit and then apply the load. The BTB12-600SW is a snubberless triac while the T0805 is a standard triac: you may use other equivalent types but because of the way triacs are driven you cannot use, in this circuit, a snubberless triac instead of a standard triac and viceversa. The 250 ?H inductor is a coreless inductor made with 100 turns of 1mm enamelled wire over a form 27mm diameter and 12mm wide. The mains transformer is a standard transformer with split primary wired in such a way that the circuit will self-sustain once it is activated. The same circuit was implemented with a current limit between 0.1 and 0.3A. In this case you have to change the fuse from 6.3A to 1.5A and the sensing resistor from 1? to 10?.



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