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:
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Kanta says
Hi Swa,
Can you please explain the function of the doode at the emitter of the BD 140.
Thanks
Kanta
Swagatam says
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
Kanta says
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.
Thanks
Kanta
Swagatam says
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
Edgar says
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?
Swagatam says
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 says
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 says
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 says
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 says
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 says
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?
Cheers!
Swagatam says
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 says
……wish you too a very Happy New Year!!
Anonymous says
Thanks Buddy, keep up the good work you do and have a nice day
Anonymous says
Hi Buddy, what can I add to this circuit to replace the push button switch so that after a short interval it resets on its own.
Thanks,
Swagatam says
Hi Buddy,
Remove the push button and the connections entirely.
Make the base resistor of BD140 to 10K.
Connect a 1000uF/25V cap across the collector of BC547 and positive.
The above mods will keep the relay triggered for a few seconds and then restore it back to its original position.
Anonymous says
Thanks for your quick response, just to be on the same page, are you saying after removing the bush buton switch and all its leads entirely to replace the 1K at base of BD140 with a 10K and the 100mF with a 1000mF, is that correct
Swagatam says
Replace 1K with 10K, that's correct,
Add an additional 1000uF/25V across the collector of BC547 and positive.
The 100uF capacitor can be eliminated, it won't be required now.
Edgar says
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…