Sunday, March 22, 2015

How to Make an Automotive Electronic Fuse Circuit

A fuse is a device used in electrical wiring for preventing accidental fire hazards due to to a short circuit or overloads. In ordinary mechanical type of fuses, a special fusible wire is used which melts when theres short circuit at some point in the wiring.

Though such fuses are fairly reliable, are surely not so efficient or elegant with their performance.
A mechanical fusible type of fuse requires careful selection as far as the rating is concerned and once blown, again requires careful replacement of the device correctly.
Even automobiles incorporate largely the above fusible types of fuses for the discussed precautions concerns.
However the above inefficient fuse can be very effectively replaced with more versatile types of electronic fuse circuit with little consideration.

The idea has been exclusively developed by me and the test results were pretty encouraging.

The CIRCUIT DIAGRAM is very simple, a relay is used to switch the battery power to the rest of the electrical of the vehicle via its contacts.

A low value resistor is place across the base emitter of a transistor for sensing the rise in the current levels.

When a possible short circuit is sensed, an equivalent amount of voltage is developed across the low value resistor, this voltage instantly instantly triggers the transistor which in turn triggers the relay driver stage.
The relay quickly reverts and switches OFF the supply to the vehicle electrical.
However in the process it also latches itself so that it does go into an oscillating mode.
The relay contacts must be rated to handle the maximum allowable current for the vehicle normal needs.

The value of the sensing resistor should be carefully selected for the intended tripping operations at the correct over load levels.

I used an iron wire (1mm dia. 6 turns) in place of the sensing resistor and it could handle well up to 4 amps after which it forced the relay to trip.

For higher currents lower number of turns may be tried.

The "push to OFF" switch is used to reset the circuit, but only after the short circuit condition is properly rectified.

A simple electronic fuse circuit developed by me is shown below:

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