Automotive Electronic Fuse

Here we investigate a circuit design designed to work like an electronic fuse circuit in automobiles and for safeguarding the auto electrical from overloads, over-current, short circuit and related fire hazards.

What is a Fuse

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 there's 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.

How this Automobile Fuse Circuit Works

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

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 placed 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 this low value resistor, this voltage becomes responsible for instantly triggering 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 not go into an oscillating mode.

The relay contacts must be rated to handle the maximum allowable current specified for the vehicle's 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:

simple automotive electronic fuse circuit

Need Help? Please send your queries through Comments for quick replies!


Edgar said…
Hello there, Swagatam,
As you say, during a short circuit or overload, the relay switches off hence, latching itself and goes into an oscillating mode. What if instead I connect it to another load after the relay is switched off? Do I have to remove also the push button connection or just retain it for resetting?
Edgar said…
Hi Swagatam,
What if instead of a reset button and oscillation, I direct, upon switch off of the relay, to another load? Do I have to remove the push button wiring? Thanks...
Swagatam said…
Hello Edgar,

I think there are many typos in the above article explanation, I'm sorry about it

Actually it should be: "However in the process it also latches itself so that it does NOT go into an oscillating mode...."
Please read it as corrected here, I'll make sure the same is done in the above article also.
Edgar said…
Hello there Swagatam,

The low value resistor you stated has 6 turns but length of the 1mm wire is not indicated. What's the exact value of the resistor, is it R=0.6v/trip current? If yes, I placed a 0.33R to trip at 1.8A, but in vain. Regards.
Swagatam said…
Hello Edgar, you did not explain about the exact results you are getting, did it trip a lot earlier or did not trip at all.

If it's not tripping at all there could be some serious fault in your wiring or the components, if it's tripping at lower than 1.8 amp in that case you can try reducing the 0.33 to some lower value.

The circuit has been thoroughly tested by me and will surely work
Edgar said…
Load current reached 3A and it didn't trip at all. When power source is switched-off, it's then that relay coil is energized for a second. Will test individual components.Thanks again & cheers to you Swagatam.
Swagatam said…
It means there's something not correct in your circuit, check the voltage across the 0.33 resistor.... this voltage would be responsible for triggering BC547 and the BD140.

Make sure the supply voltage source is on the left side as exactly shown in the diagram.
Edgar said…
Hi Swagatam bai,
First of all, a new year's greeting to you.

I have successfully tested your circuit and it really works!
When it senses an overload based on the sensing resistor or a short circuit, the relay energizes and latches the NO contacts. But I noticed a drop of 1 volt in this situation. I am using LEDs as load to visualize the switching and my power supply is around 0.6A/12Vdc.

I would like to know what is causing the voltage drop?

Swagatam said…
Hi Edgar,

The drop may be because of the relay coil consumption which could be due to its lower resistance value, try using a higher coil ohm relay and check the response
Swagatam said…
......wish you too a very Happy New Year!!
Kanta said…
Hi Swa,

I am planning to use this protection circuit for a higher voltage DC which is about 40V. How do i modify the latching? Please kindly give me advice.
Swagatam said…
Hi Kanta, since the relay is 12V the emitter of BD140 will need to be connected with a 12V source derived from the 40V...and similarly the feed coming from the push switch will need to be passed through a 7812 IC before it connects with the relay coil

rest everything can be as is
Kanta said…
Hi Swa,
Can you please explain the function of the doode at the emitter of the BD 140.
Swagatam said…
Hi Kanta, it's not critical, however it guarantees a more reliable triggering of the BD140 and also a better charge holding capability for the 220uF capacitor

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