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.
WARNING - THE PROPOSED CIRCUIT IS NOT ISOLATED FROM MAINS AC, THEREFORE IS EXTREMELY DANGEROUS TO TOUCH IN UNCOVERED POSITION AND WHEN POWERED.
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.
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 5W
- Diodes = 1N4007
Hi Swagatam;
As the sample there is a circuit in which AC 220 and DC 5 V are together at the PCB as in the circuit AC load with BT136 triggered by DC volt. It is possible to say about that for example the DC electronic parts would be negatively affected by AC voltage(i.e. magnetic field interaction). So we should add some parts to circuits like coils for the isolation of AC and DC. Kind Regards.
Hi Suat,
for the components there will be no effect at all, but for a human it can lethal to touch any point in the circuit.
When you use a triac with a DC circuit without an optocoupler then you will have to involve one AC line as the common line in the circuit. It is a standard practice.
There’s no need to add any extra parts in the circuit.
Sir, how can i achieve this the same purpose ( short-circuit, & over current protection) using arduino ?
Godstime, My Arduino knowledge is not good, so not very sure how that can be done!
Okay.
Thank you sir.
Goodnight
In the circuit, you can replace the scr and transistor optocoupler with an scr optocoupler, such as 4n40 that already has scr internally
Thanks in advance for your attention
Yes, I think it can be worth trying an optocouper with an internal SCR, so that the external SCR can be avoided.
Goodnight
thanks for the answer
The circuit is not tripping..
What should be done now????
Is the SCR switching on when the maximum current limit is reached? You can connect an LED in series with R2 to check this….If this LED does not light up means your opto is not working or there’s some other fault.
Hey,
We practically made this practically but a problem is there The MCB is tripping before this ckt.
So what should we do now???
Hi, to verify the working of the circuit, you can increase the value of the resistor R1 to some higher value. MCB is highly sensitive and fast so it may trip quickly…increasing R1 might help to complete the preliminary tests.
Can’t we replace R1 coil with a simple resistor??
If yes then how to calculate the resistance of that resistor or which resistor to use as R1
R1 handles the full output load therefore needs to be rated high, depending on the load current. You can try a wire wound resistor, using the following formula:
R1 = 2.2 / tripping current.
Assuming tripping current is for 2000 watt / 220V load, then 2000/220 = 9 amps or 10 amps.
R1 = 2.2 / 10 = 0.22 ohms
wattage will be 2.2 x 10 = 22 watt.
so yes using a wire wound resistor is feasible for R1.
Can you please tell me which wires to use to build this ckt. And how to safely test it…
I want to do this as my Minor Project in my Diploma Course..
Sorry I did not understand what you meant by “wire”….by the way this is not a minor project, since it involves AC mains which is not isolated and there may be a possibility of a fire hazard if not done correctly.
This circuit is recommended for the experts who know how to handle mains AC circuit safely.
Ok, advice taken..
So can you please suggest me a good idea for the project.
You can go with any project which is not dangerous in terms of shock or fire hazard. There are plenty of projects already explained in this site….you can select any one of them…
Hello,
I didn’t understand how the voltage cross the diode bridge to the led of the opto coupler since the diodes are opposite to the voltage direction.
Can you explain it please?
Thank you
Hello, yes the opto LEd polarity is wrongly shown in the diagram, it should be the opposite way….LED anode towards to the bridge cathodes, and LED cathode towards the bridge anodes
The LED voltage is 6V, the AC current I want to draw is 2A, so the R will 3ohms. Sir, is this correct.
Also, how can I make the iron coil and measure it.
Please Swag, Is there any easier way.
Thanks Sir
The LED voltage looks too high, the LED voltage should not eb more than 2V. So please change the LED accordingly.
for 2V, the value of the resistor will be 2 / 2 = 1 ohm / 4 watts
please Sir, does diode in series with circuit breaker affects sensitivity, because it didn’t break as before with overload, all MOSFETs blew. And I need the diode to be there.
Seun, which schematic are you referring to?
No schematic, I just connected diode to circuit breaker to give unidirectional path. Please does that affect as requested above.
without seeing the schematic and the position of the diode it can be difficult to tell how the MOSFETs blew.
Just asking does diode affect sensitivity of breaker, because there was a short circuit from a load that affected the MOSFET and the breaker failed to trip off,which used to trip before the diode was placed.
If you connected it in series with the DC supply of the circuit then it is not supposed to affect the working of the circuit
I’m trying to simulate the circuit on Proteus I can’t find the right Optocoupler components that you used. Kindly help me out with the right value. Thanks
Hi ,sir
Can you attached the simulation of this circuit here . I’m trying to simulate it in NI multisim but there is a lot of missing details of the components .I really want this simulation please.
Hi Ahmed, sorry I cannot simulate it for you, however, if you want to know about any part specifications, I can help you in that regard.
Hi Swagatam,
The trick the user in the circuit how many time take power OFF and power ON.
sorry sabah, I did not understand your question correctly!
Please how can I make R1
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.
Please Sir What is the size of the iron core . thanks
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.
Good day Sir, please sir what is the value of LED Voltage rating to calculate R1.Thanks
Hi Seun, the LED is inside the opto-coupler, the specifications can be checked from the datasheet of the selected device….
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.
Thanks for updating the information, appreciate it!
The second option looks more feasible and technically viable.
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.
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).
Please check the datasheet of the triac and check for how long the triac can withstand as high as 200 amps