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Mains AC Overload protection Circuit for Voltage Stabilizers

Mains AC Overload protection Circuit for Voltage Stabilizers

In this article we discuss how to make a cheap yet effective mains operated AC overload and over-current protector circuit using very ordinary discrete components.


I have published a few mains voltage stabilizer circuits in this blog, these units are designed and intended for safeguarding the connected appliances at their outputs.

However these equipment lack one protection which is the overload protection.

The Importance of an Overload Protection Circuit

A particular stabilizer unit may be rated for handling a maximum specified limit of power, beyond which it's effects may start diluting or might become inefficient.

Overloading a voltage stabilizer might also result in heating of the transformer and fire hazards.

A simple circuit shown below may be incorporated with a stabilizer circuit or any such protection circuit for reinforcing the safeguarding capabilities of the units.

How it Works

The diagram shows a very simple and straightforward configuration where only a couple of transistors and few other passive parts are used for forming the intending design.

The mains stabilized AC is derived from the stabilizer outputs and allowed to switch through another RL1, via its N/C contacts.
One of the wires of the AC mains connections is added with a series resistor of a calculated value.

As the load across the mains output increases, a proportionate magnitude of voltage starts developing across this resistor.

The value of the resistor is so selected that the voltage across it becomes just enough to light up a connected LED in response to a load that might be considered as dangerous and over the maximum tolerable limit.

When this happens, the LED just lights up, an LDR positioned and enclosed in front of the LED instantly drops its resistance in response to the illumination generated by the LDR.

The sudden reduction in the resistance of the LDR, switches ON T1 which in turn switches ON T2 and the relay, initiating the latching effect of the circuit and the relay.

The load or the appliance at the output is thus immediately switched off when an overload situation is detected.

The whole action takes place within a fraction of a second, giving no chance for any untoward consequence and the whole system is safeguarded by the inclusion of this simple AC mains overload protection circuit.

Formula for Calculating Current limiting Resistor

R1 = 1.5/I(specified current limit), example if I=15amps, then  (R1 = 1.5/15 = 0.1 Ohms)

Mains AC Overload protection Circuit for Voltage Stabilizers




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!

68 thoughts on “Mains AC Overload protection Circuit for Voltage Stabilizers”

  1. sir is can you explain what led/LDR means because i dont have enough knowledge about electronic parts or is it found in chargers.
    can i get the part number for the LDR thanks

    • To make this, you have to pack an LED and LDR face to face inside a light proof box….or you can simply use a ready made opto coupler.

      LDRs do not have any number.

  2. Hello sir, is it possible to modify this circuit by adding a delay timer for automatic restarting? If yes, pls throw more light to that. Thanks.

  3. Hi Swag,

    Will this also help in short circuit?

    also I have both MCT2E (6pins) and PC817 (4 pins). which one should I use for better detection?

    I know PC817 is most used in many power supply circuits and MCT2E is absolute i think.

  4. Hello sir Swagatam,
    Please I have one more question on this schematic. Can I connect the positive rail of the schematic to the same 12V regulator that is supplying the inverter driver without it affecting anything?

  5. Replying to your response…
    So for example, supposing I build a 2KVA inverter which I want to use for a maximum load of 1,600W.
    Shutdown current = 1,600/220 = 7.27A
    R1 = 2/7.27 = 0.28Ohm
    R1 wattage = (1.2 + 2) x 7.27 = 23W (approx)
    Is this correct sir?

    • Hello Maruti, it will be difficult for me to guess what mistake you might have done in your circuit, please read the article and check what might be missing in your circuit

  6. Sir, in this circuit u used stabilizer o/p as circuit i/p.
    Can i apply direct 230v ac in I/p?
    If I can then how I select all components?

  7. Hello sir I m anmol from Lucknow
    I started STYBLIZAR menufecturing as a business will u help me for my business technical support I need your help my watsup no. 08115720798

    • Hello Anmol, phone or wattsapp communication may not be possible, but you can consult the issues through email, I'll try to help.

      Please make sure to write the questions in proper English and elaborately because I would be publishing it in my site also.

  8. Brother actually p1 is for sensitivity only know, i need to adjust current limit resistor? And the above circuit is only for overload current limit, also i need a under load current limit circuit?

    • Brother, at lower P1 setting the circuit will activate at higher currents, and at higher P1 value it will operate at lower current….so it can be used effectively for setting up current.

      for under current cut off we might require two opamp circuits, one for controlling the upper limit and other for the lower limit….I'll think about it.

  9. Dear Brother i need under and overload protection circuit for my openwell submersible water pumb 5hp 4kw 415v 10amp , for that i have to use variable protection should be 5amps to 20amps? if you have any circuit like this? Or pls design a circuit for me? If you need some more details pls ask me…

    • Dear Brother, the above design already has an adjustable feature in it through P1, you can set P1 to different positions for achieving any desired cut off point

  10. Please there is a small confusion about the actual constant need for calculating various values of R1.. The post said 1.5 you said it is 2 and later said 3 so please explain the changes. Thanks

  11. Hello sir
    I don't quite get how vijay set up the 1ohm 20w and the 10k preset.
    Then what value can we use for P1.
    Also there seem to be some confusion as to how to calculate R1 value as I read through the comments.

    • Hello Michael, the delay effect shouldn't be so critical actually….and yes all the above parameters will interact some and influence the delay effect, so it may require a little experimentation until the right delay is determined…

      • Sir assuming i want to use this circuit for 3kv inverter overload protection how could i calculate R1? sir the relay connection it seem not clear well to me, first, connection to coil terminal is not show in the circuit,secondly, N/O terminal and N/C seem some how confusion. Thanks, hope to hear from you soon.

        • Adelusi,

          the formula is given in the article, please use it to calculate. the current limit in the formula could be calculated by dividing 3kv with the voltage.

          please click the diagram to enlarge, you will be able to see both the contacts although slightly faded, but anyway the N/O is unconnected so its not important, the N/C is the one which is associated with the load and the current.

        • you can try any IC 555 based boost converter circuit or 555 IC based flyback converter circuit, please Google this you will find many

  12. Hello Sir
    Just stumbled upon this post.
    Since vijay made it work then I think I can use it as an overload protector with some changes
    1) I get rid of the relay and instead pass the positive signal from collector of PNP transistor to pin10 of my sg3524.
    2) while I am studying the other post you referred me to this one looks more sophisticated.
    3) where and how will I include a time delay before passing to in 10 of IC so the when the current goes above the limit it first gives a de lay of say 5secs and if load does not reducez current then it shuts off inverter
    4) if per adventure the load settles to normal like in the case of surge loads capacitor should discharge rapidly so that it does not trigger shut down since starting current has reduced to normal

    • Hello Michael,

      for including a delay effect, you can simply add a high value capacitor across the base and ground of T1…make sure the signal from the opto comes through a resistor if you are using an opto coupler…this resistor value can also be tweaked for setting the delay response

  13. Hi Swagatam,

    I redesigned this circuit and assembled. As my stabilizer is 1KVA and it can handle 4A, I have used a 1ohm 20W resistor as the voltage sensor. I know this is a little high value but I have created a voltage divider using a 10K preset and adjusted the voltage so that only at 4A the optocoupler is triggered. Even that's not perfect because even a tiny current through the optocoupler triggers T1. To mitigate this I played around and connected a 100K resistor between the optocoupler and T1 base. I tried the 4A current using an induction stove, set the power at 1000W, and I measured exactly 4A using a multimeter. This seems to work fine. When I increase the power to 1200W, it triggers.I haven't yet tried 1050W, 1100W etc yet, which I'll do a little later. While doing this, I found another issue; when I connect the output to my voltage stabilizer (already assembled using one of your circuits but this is 500VA), because of the inrush current to the autotransformer, the circuit triggers. I mitigated this issue by connecting a 47mf capacitor (anything less doesn't do the trick) between P1. Now when I increase the power to 1200, the circuit triggers but takes about 1 second to do so.

    I have one query: Since this circuit allows a second of over current, will it damage the autotransformer? I feel it can take more current than it is rated at for some time but I just wanted to be sure.


    • Hi Vijay, congrats to you!

      You have done all the modifications correctly, and everything seems perfect to me, a 1 second delay will not affect your autotransformer in any manner, because before getting damaged the transformer will first have to heat up, and when it reaches above 150 degrees only then it might start smoking and this might take more than 20 to 30 seconds.

      By the way what did you use for the opto coupler? is it a readymade LED/phototransistor opto or the recommended LED/LDR type homemade opto?….the LED/LDR would provide better and more reliable results compared to the conventional one.

    • Hi Swagatam, Thanks. I used MCT2E. I was thinking of trying an LED/LDR combo if 100K between MCT2E and T1 base didn't do the trick but it did.


  14. Yes, I used two .47E resistors in series as I couldn't get 1 ohm for a 3amp load. Anyway, I'll try again and update.


  15. Hi Swagatam,

    Just like your voltage stabilizer circuit using opamps, I have also tried this circuit (with an optocoupler) and it works. But it triggers quite often even when there is not so much load. Adjusting the preset does not help. Any ideas?

    • Hi Vijay,

      did you calculate R1 correctly? please calculate such that the opto led lights up only when there is a true overload as per the required specifications.

      alternatively you may also try adding a 1uF capacitor across base and ground of the T1

  16. Hello Swagatam,

    Thanks for the quick response, to be very precise I have recently bought a Hitachi AC model RAW318KTD, my previous AC compressor was blown due to the stabilizer was generating 300VAC. I have got the stabilizer repaired how ever want to be very cautious this time.

    My AC specs are Current drawn-4.7 amps, Rated power supply -(230/50/1 Volts/Hz/Phase), Total Power input-1055 Watts.

    Sorry for bothering you again.



  17. Hello Swagatam,

    I want to apply this CKT to my Air Condition stabilizer which is 3Kva can you help me decide on the relay rating and R1 my AC is 1400Watts.



    • Hello Ajay,

      Here are the details

      3000/220 = 13 amps

      Therefore R1 = 2v/13 = 0.15ohms 3watts

      Relay contacts must be rated at 20 amps

      • Hello sir Swagatam. I really appreciate your effort. I’ve been looking for an overload protection for inverter circuit. You have so far referred me to two post but this one is just the perfect one for the intended purpose. I’ve read through the comments and have gotten more info from your responses. Thank you sir. I have a few questions:
        1. In your response above as regards how to calculate the sensing resistor wattage, you said that: P = VI, V = voltage drop across resistor. How do we determine this voltage?
        2. I want to design the circuit in such a way as to use it to power off the inverter when overload is sensed at the transfo output. So i want to use a smaller relay such that the contacts will be used to disconnect power to the inverter oscillator during overload condition. So what i want to do is to allow the output of the transfo to pass through the sensing resistor directly to the load. Can i go ahead with that arrangement?
        3. From the schematic, which side/pin of C2 is positive?

        • Hello Godson, you can use the relay contact for the mentioned purpose, so in that case connect the indicated relay contact wire directly with the load.

          watt of the resistor will be = bridge drop + LED drop / shut down current
          = 1.2 + 1.5 / D current.
          the white side of c2 is the positive.

          • Please pardon me sir, I still don’t fully understand the explanation you gave in your response to how to calculate the wattage of the sensing resistor. Please could use the example you gave above for a 3KVA inverter to explain, I’ll understand it better that way. Thank you sir.

            • Godson, the wattage will equal to the voltage drop across the resistor multiplied by the shut down current limit selected by you.

              the voltage drop across the resistor can be achieved by adding the bridge rectifier drop (approximately 1.2V) and the LED FWD voltage drop (approximately 2V)

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