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Mains AC Short Circuit Breaker/Protector – Electronic MCB

Mains AC Short Circuit Breaker/Protector – Electronic MCB

In this post we will try to understand the making of a simple 220 V, 120 V AC mains short circuit breaker using an SCR and a triac combination, (researched and designed by me).

The circuit is an electronic version of the normal main circuit breaker MCB units that we use in our homes.

Note: I did not use a relay for the cut-off, because relay contacts will simply fuse with each other due to heavy current arcing across the contacts during a short circuit condition, and therefore it is highly unreliable.

Why Short Circuit in Homes Can be Hazardous

A short circuit in a house wiring may appear to be something which happens very seldom and folks aren’t too interested to get any relevant precautionary measure installed in their houses and take the hazard very casually.

However once in a while due to some accidental fault, a short circuit in the mains wiring becomes inevitable and it the happening causes a disaster and huge lose.

At times the consequence leads to fire hazards and even lose of life and property.


Though many types of short circuit breaker units are available ready made in the market, these are generally very costly.

Moreover an electronic hobbyist will always want to make such an equipment all by him and enjoy its display in the house.

Making a Cheap yet Promising Electronic Circuit Breaker Unit

A short circuit breaker circuit described in this article is indeed a piece cake as far making it is concerned and once installed will provide a life long protection against all short circuit like conditions that might accidentally take place.

The circuit will also safeguard you house wiring against a possible overload conditions.

Electronic Mains AC Short Circuit Breaker/Protector


How it Works

The circuit shown in the schematic looks pretty straightforward and may be verbally simulated as follows:

The sensing stage of the circuit in fact becomes the heart of the whole system and consists of an opto-coupler OP1.

As we all know, an opto-coupler internally consists of an LED and a switching transistor arrangement, the transistor is switched ON in response to the illumination of the built-in LED.

Thus the triggering of the transistor which forms the output of the device takes place without any physical or electrical contact rather through the passage of light rays from the LED.

The LED which becomes the input of the device may be switched through some external agent or a voltage source which required to be kept aloof from the output stage of the opto-coupler.

Why an Optocoupler is Used

In our circuit, the opto coupler LED is powered through a bridge network which obtains it voltage source from the potential generated across resistor R1.

This resistor R1 is connected in such a way that the AC mains current to the house wiring passes through it and therefore any over-load or over-current is subjected over this resistor.

During an over load or short circuit conditions, the resistor instantly develops a potential across it, which is rectified and sent to the opto coupler LED.

The opto LED immediately illuminates, switching ON the corresponding transistor.

Using an SCR for triggering the main Triac Cut out Stage

Referring the circuit we see that the opto transistor’s emitter is connected to the gate of an external SCR, whose anode is further connected to a Triac's gate.

During normal conditions, the triac remains switched ON, allowing the load connected across it to remain operational.

This happens because the SCR remains switched OFF and allows the triac to acquire its gate current through R3.

However in case of an over load or a short circuit, as discussed earlier, the opto-coupler transistor conducts and triggers the SCR.

This instantly pulls the gate potential of the triac to ground, inhibiting it from conducting.

The triac immediately switches OFF, safeguarding the load and the house wiring to which it is configured.

The SCR remains latched, until the problem is rectified and the circuit is restarted.The section comprising C1, Z1, C2 is a simple transformerless power supply circuit, used for powering the SCR and Triac circuit.

Parts List

  • R1 = iron coiled wire; its resistance is calculated to produce 2 volts across it at the determined critical load conditions.
  • R2, R3, R4 =100 Ohms
  • R5 = 1K,
  • R6 = 1M,
  • C1, C2 = 474/400V
  • SCR = C106,
  • Triac = BTA41/600B
  • Opto-Coupler = MCT2E,
  • ZENER = 12V 1W
  • Diodes = 1N4007

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

56 thoughts on “Mains AC Short Circuit Breaker/Protector – Electronic MCB”

    • wind copper wire on an iron core until its resistance value becomes equal to the value obtained by the Ohm’s law result:

      R = V/I
      here V is equal to the LED forward voltage rating, and I is equal to the maximum current at which the cut-off is required.

        • Core size is not important, only the resistance value is important and the wire gauge is important, use a 0.5mm thick wire.

          I think an iron wire will work much better, and will satisfy the resistance value with minimum turns.

  1. Three solutions to turning off quicker than a half cycle. First, use an inductor to inhibit rapid current changes to the point that your circuit can switch off. Second the old school way is forced commutation…add a reversed SCR to short out the “main” SCR and force it open. There is a reason these aren’t used anymore. Third option is use something that can both close and open like power MOSFET’s, SiC JFET’s, or IGBT’s.

      • There are commercial static breakers on the market. DC breakers meet some tough FAA requirements at better pricing since DC breakers don’t get zero crossings. There is one company in Charlotte in early commercialization stages with an AC one. I think they’re current limiting such as opening an IGBT bypass around an inductor just behind dc/dt snubbing so that the IGBT filter is small and the SCR/TRIAC valve survives. They said they can soft start for free. By itself it is not cost effective but as an all in one breaker/soft start it has very realistic possibilities.

  2. The triac will be ON until zero cross occurs even gate control is trying to turn it OFF.
    Therefore, a short circuit will break the triac within 10ms (50Hz).

  3. I want to know sir that is possible we use scr rectifiers to get the desired dc control output. I work on it since two weeks but i cannot get the control output if you have any knoldge about it kindly share with me. It is the part of my fyp.

  4. Thank u sir for the answers. On more question: after the short circuit occurs and the scr triggers shutting down the circuit, will the circuit automatically restarts it self? And and to which time duration? Or one need to restart the circuit manually, and how can one modify the circuit to circuit to restart within short period of time? Thanks

    • You are welcome Solomon,

      The circuit will not restart automatically because the gate SCR will be latched.

      You will have to switch OFF power and switch it ON again for restarting the circuit

      remember this circuit has not been tested practically by me, so proceed with caution.

      Initially use a 200 watt bulb in place of R1, and check by short circuiting the output terminals, if it works then you can replace the bulb with a resistor and proceed identically

  5. Hello sir, im so interested with this circuit, in fact, i bought almost all the components, but i have some problems and questions about the circuit:
    1. Does the triac need heat sink?
    2. Opto coupler of that number is not available in my is there any alternative for that? Or there any conponent to use insted? Or can any opto coupler work?
    3. Capacitor 474, 400v also is in my area, only 474, 680v and 473, 400v, which of the two can i use instead?

    • Hello Solomon, the replies are as follows…

      heatsink will be required if the load is bigger and tends to make the triac hot during normal use.

      you can try any 4 pin optocoupler, it is not critical.

      474/680V is OK, can be used.

      remember this project is very much dangerous so proceed only if you are sure of what you are exactly doing….

      • hello sir, i was opportuned to get one optocoupler 4N35, But it has 6 pins (anode, cathode, base, collector, emitter and NC), Can it serve same function?

        • yes it will work, check the datasheet of the opto and then you can select the relevant pinouts and ignore the other irrelevant ones..

    • Hi Solomon, a piezo buzzer can be added in series with R2 for the required alarm sound…and R2 reduced appropriate for generating a reasonably louder alarm sound.

  6. when the triac set on , it stay on to end of the cycle and for example if occurs short circuit first of the cycle triac stay on to end of the cycle and for 60Hz it will on for 8.3 ms and it is dangerous and we should find the way to cut off triac immediately after the short circuit. So this circuit can’t protection the load for short circuit

  7. Hai sir….
    Pls tell R1 value and watts sir….

    Instead of using TRIAC we can use relay….If u have relay circuit pls give me the link sir

    • Kesav, relay contacts can easily fuse and stick due to high current arcing across its contacts, whereas a triac can withstand high currents for short durations of time and operate more efficiently…..therefore triac is a better choice than a relay

      however remember this circuit is recommended only for the experts in the field.

  8. sir i want to intentionally short circuit an ac L wire with a Neutral wire taken from a wall socket.. my intention is to light a matchstick with the fusing action caused by the above case. The matchstick is followed by a cracker's fuse wire… i have tried connecting 2 MCBs of 6A rated in series to each L and N wires followed by the portion of wires to be shoted.. but what i am getting is Matchstick with the crackers wire is burned, MCBs tripped simultaneously but the main MCB of the room which is situated in the ELECTRIC DISTRIBUTION BOX nearer to the Electric Meter is also getting tripped which is actually not needed at all.. I have used a very thin copper wire to connect between L and N wires…

    I dont require any batteries in place of AC.. i need this to be operated with AC only..
    I dont need the main MCB to be tripped other than the externally connected MCBs..

    please suggest me a better way sir

    • RT, shorting mains wire in any manner is never recommended, you can do it by charging a high value capacitor through a bridge rectifier and then short this capacitor for getting the required igniting spark.

      You can use a 50uF/400V capacitor, charge it using a bridge rectifier made up of 4 x 1N5408, through mains AC.

      I hope you are aware of the dangers of playing with AC mains experiments, be extremely cautious while doing all these…..

  9. Good day sir! Can i ask if there would be any alternatives for the triac BTA141a? i live in philippines and this kind of components are really rare. Looking forward to your reply!

  10. Hi Swagatam
    thanks for your quick reply,
    yes as per datasheet the bta should not burn.
    i guess i ll have one more try on this.
    i ll update here after that try,
    if that try fails i am thinking to upload the circuit i have made.
    soon u hear from me.
    thanks and warm regards

    • Hi Swagatam,
      i tried and triac fried… 😀
      btw i m looking to upload the circuit diagram so you can find where the fault is

    • Hi prashant, that's strange, because even if we ignore the 400amp factor still the resistor in series would absolutely prevent the triac from burning…

      anyway you can upload it to any suitable picture upload site or in Google drive and post it here for an assessment.

    • Prashant, I can't see how the trigger circuit in your diagram would latch when a short is detected?…In my circuit the SCR is selected precisely for executing the latching action as soon as a short is detected……moreover the MOC configuration is not exactly as per its datasheet diagram

    • great thanks Swagatam…
      actuallly i was looking for that latching factor…
      ok i ll try with that SCR …
      its me only 😐 .. i ll try to improve

    • Dear Swagatam in my circuit tha 1000 uF cap accross the bridge output itself latches the triac in off state for some time..
      the 1kohm resistor across it was used to discharge cap. later on i changed that value to a higher one so it will take more time to discharge and ultimately the opto817 will be in on state which will keep moc3021 in off state. in this way we can change the latch time by altering the resistor value across 1000uFcap.

      this circuit works well in overload condition. triac gets short in actual short circuit condition

    • Hi Prashant,

      it's dangerous to put capacitor across the bridge because it may cause a delay in switching ON the opto which we don't want and the prevent the cut off action from happening instantly…..

      the latching effect should be permanent and not momentary, otherwise the traic will be forced to switch ON/OFF and subjected to huge short circuit currents….

      the circuit concept should be similar to what I have created in the above article, otherwise it will not work.

  11. Hi Swagatam,
    wanted to update you, i already was working on the same purpose but with little different circuit(triac and opto is common factor), i have implemented the circuit and found that it is properly working on overload condition and not for short circuit. what i figured out is the rate of of rise of current in short circuit condition which is very fast. on the other hand triac needs max 10 ms of time to get turn off if considered that ac cycle just started on every zero cross of ac cycle. when short circuit happens the current in triac will increase in less than few microseconds as load resistance is now ZERO ohms i.e. current = voltage/0ohm. and so before getting next zero cross the bta has laready burnt…
    so finally after many tries i am thinking to have a try with relay based circuit… need you suggetion

    • Hi PK,

      If you check the datasheet of the triac BTA41/800 it shows the maximum non-repetitive current handling capacity of this triac to be 400 amps for 20ms.

      20ms corresponds to one full AC cycle (50Hz), and here we need the triac to hold ON only for 10ms (the first zero crossing).

      Moreover 400amps is massive, and I don't think our domestic AC is specified to deliver this much current.

      So I think this triac would work fine, furthermore we have a resistance in series so it would be the resistor that would burn first in case the triac fails to fire.

      A relay can be worst in such a scenario because a relay will be too sluggish and by the time its contacts are about to changeover, the current might already reach huge values causing a spark across the changeover contacts which in turn would fuse the contacts together making it impossible to move and ultimately burn the contacts

  12. Hi Prashant, no problem, it won't be an issue, my reply under it will probably be enough to make the folks understand regarding the question.

  13. hi Swagatam,
    by mistake i deleted my post while doing some other things.
    should i put the same post again or u ll do something???

  14. Thanks Prashant, yes it seems the opto LED is connected with wrong polarity by mistake…it needs to be connected the other way round.

    a current limiter resistor can be included for greater safety to LED, a 50 ohm resistor will be just enough.

    • which CDI circuit are you referring to?

      use a good quality capacitor and SCR…it will surely work….use only BT151 SCR

    • Hi, it's not designed to switch ON automatically, you'll have to switch OFF and restart the circuit for initiating the circuit and the load.

  15. Hello. I don't know if I'm wrong, but the optocupler's LED is not connected in the wrong way? How can it light if the bridges diodes don't allow current to pass through it?

  16. Hello,
    Please, check Triac will be on until zero cross even gate control try to OFF.
    So, Short Circuit will be broke Triac within 10ms (50hz).

    • Thanks, yes you are right!
      immediately i am unable to find a solution for this, I'll think and try to figure some appropriate remedy

  17. If you have used a 7805 IC then any external protection would not be required as the IC has built-in protections from short circuit, overload and also high voltage.

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