Electronic Fuse

Circuit of fuse (sekring) electronic designed to operate on 230V AC with an adjustable trip current. When the current through the load exceeds a level determined by the position of the wiper on the 1k wire-wound pot, this circuit cuts off the load immediately. If S1 is open, the range is approximately 300-650 mA, and 0.8-2A when it is closed.

This circuit connects directly to 220-230V AC which can be lethal! Please do not attempt to build any of the circuits/projects unless you have the expertise, skill and concentration that will help you avoid an injury.
  • D1: 1N4001
  • T1: TIC225M
  • T2: BTA12-600CW
The key variable in the operation of the fuse is the voltage drop across the power resistor(s) which are connected in series with the load. This voltage drop is directly proportional to the current the load draws. When this current is low, the voltage across the resistors is also small and cannot trigger T1. At the same time the gate of T2 is fed from a little power supply built around a negative voltage regulator. T2 is conducting and the load is on.

If the current through the load then gets too high, so that the voltage created across the resistor(s) can trigger the gate of T1 through the 330R resistor and the pot: T1 starts to conduct, swiftly taking away all the current from the gate of T2. The voltage drop across T1 (MT1-MT2) will then be only 0.7 V and T2 will be firmly off. T1 stays this way all until the momentary (normally closed, "push-to-break") Reset push-button is pressed: this causes the current through T1 to drop below the hold level and forces this triac to turn off. Releasing the Reset button re-enables the current flow to and through the gate of T2, switching it on.

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