Fast Electronic Fuse

Figure 1 

The function of this circuit is as an electronic fuse which cut the current off when it is exceeds certain limit. Time response is fast and the current limit is adjustable

The current level of the wiper on 1k wire-wound pot should be determined first, this can be adjusted. It works whenever the load reaches current level as determined before, then it will shut the load off immediately. The current range when switch S1 in “open” condition is 300 ~ 650 mA while it becomes higher 0.8 ~ 2A when it is closed.

Voltage drop across the power resistor(s) is the main key of the entire circuit. These power resistors are set series with the load. Triac T1 could not be triggered to active when the current and voltage drop across resistors are low. T2 will be conducting since it has voltage supplied from a small power system located around a negative voltage regulator. It will lead “ON” the load. Please use TIC225M triac for T1, this triac has a very low trigger current characteristic. While triac T2 would be BTA12-600CW or BTA12-600BW (both can be used without snubber circuit).

If load gets higher current across, it will make voltage of resistors trigger the T1’s gate through resistor 330R and the pot. This causes T1 starts to conduct while it cut off all current from the gate of T2. The voltage across T1 now will be only 0.7 while T2 is off. This condition will stay still until push-button Reset is pressed which will drop current of T1 below the hold level and makes it turn off.

In addition, depending on the load, AC output will be 1-5 volts below the input level. If the gates of triacs are DC-controlled to switch AC, the DC to control the gates MUST be negative – flowing from the gate, which is the negative voltage regulator. Varistor’s value are 250V AC. Polarities of the electrolytic capacitor and diodes before IC 7906 should not be reversed.

Source:www.zen22142.zen.co.uk/Circuits/Power/fuse.htm