OVER VOLTAGE UNDER VOLTAGE - Mains Over Voltage and Under Voltage Protection Circuit Explained

Mains Over Voltage and Under Voltage Protection Circuit Explained

The post discusses a mains 220V or 120V operated over voltage and under voltage protection circuit using relay cut-off, and using just a couple of transistors.

Our Mains AC domestic power may be full of ups and downs or fluctuations, which may be quite fatal to our sophisticated appliances like TV sets, DVD players, compotes etc.

A very simple circuit presented when installed in the house electrical may help in restricting the issue to a great extent.

Here we will learn two designs of over and under voltage circuits, the first based on transistors and the other one using an op amp.

Over/Under Voltage Cut Off Circuit Using 2 BJTs

You will be surprised to know that a nice little circuit for the said protections can be built using just a couple of transistors and a few other passive components.

Looking at the figure we can see a very simple arrangement where T1 and T2 are fixed as an inverter configuration, meaning T2 responds oppositely to T1. Please refer the circuit diagram.

In simple words when T1 conducts, T2 switches OFF and vice versa. The sensing voltage which is derived from the DC supply voltage itself is fed to the base of T1 via preset P1.

The preset is used so that the tripping thresholds can be determined precisely and the circuit understands when to execute the control actions.

How to Set the Preset for Automatic Cut off

P1 is set for detecting high voltage limits. Initially when the voltage is within the safe window, T1 remains switched OFF and this allows the required biasing voltage to pass through P2 and reach T2, keeping it switched ON.

Therefore the relay is also kept activated and the connected load receives the required AC voltage.

However in case suppose, the mains voltage exceeds the safe limit, the sensing sample voltage at the base of T1 also rises above the set threshold, T1 immediately conducts and grounds the base of T2. This results in switching OFF of T2 and also the relay and the corresponding load.

The system thus restricts the dangerous voltage from reaching the load and safeguards it as expected from it.

Now suppose the mains voltage goes too low, T1 is already switched OFF and at this situation T2 also stops conducting due to the settings of P2, which is set so that T2 stops conducting when the Mains input goes below a certain unsafe level.

Thus the relay is once again tripped OFF, cutting of power to the load and prompting the required safety measures.

Though the circuit is reasonably accurate, the window threshold is too wide, meaning the circuit triggers only for voltage levels above 260V and below 200V, or above 130V and below 100 V for 120 V normal supply inputs.

Mains Over Voltage and Under Voltage Cut of Circuit Explained


Therefore, the circuit may not be very useful for folks who might be looking for absolutely accurate tripping points and controls which can be optimized as per ones personal preference.

To make this possible a couple of op amps may be required to be included instead of transistors.

Parts List for the above AC mains over voltage, under voltage protection circuit.

R1, R2 = 1K,
P1, P2 = 10K,
T1, T2 = BC547B,
C1 = 220uF/25V
RELAY = 12V, 400 OHMS, SPDT,
D1 = 1N4007
TR1 = 0-12V, 500mA
Using an Op Amp:
 The above discussed
operations can also be done using an op amp, with precise results.

97 thoughts on “Mains Over Voltage and Under Voltage Protection Circuit Explained

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. the project is basically DC motor Protection module(protecting the DC motor from over voltage and under voltage and also excess temperature)…would love to get your response soon..

  3. Hey swagatam, u've reduced the circuit 4rm last time, but I'm thinking of adding a low pass filter to cut off signals with high frequencies, what do you think?

  4. Hey swagatam, i have a few questions:
    i.How come this circuit uses half bridge rectification & does full bridge rectification offer any relative advantages.

    ii. The circuit has some delay in cutting off supply to my load(a 60W bulb), is this normal or am i doing something wrong.

    iii.Isn't the push button switch from the previous circuit necessary or does this circuit automatically resume supply to the load?

    • Hi, bourgeoisie

      A full bridge is recommended and is good, but not absolutely necessary.

      Delay in operation should not occur, so there's some problem with your circuit. By the way how are you testing it?

      The push button was used for a different configuration where the circuit was wired to get switched off completely, but here's it automatic and does not required a push button.


  5. hey Swagatam,

    The transformer i use has 2 tap outs(12v & 24v), the circuit like you said is supposed to run smoothly for voltages between 10-13v.Hence, i used the 24v tap out to test for high voltage & it delayed before shutting off and now the circuit dosen't even go off again, PLEASE what can be wrong?!

    I still prefer using your old configuration, but would it resume automatically if i remove the switch and connect the transformer directly to the phase(live)?


    • Hi Bourgeoisie,

      If the input voltage to the circuit is 12 at 220 mains voltage then you can use the following formulas for determining and setting the equivalent DC voltages for the above circuit.

      12/x = 220/250 for calculating the equivalent high voltage DC threshold and,

      12/x = 220/190 for calculating the equivalent low voltage DC cut-off threshold

      The transformer volt should match the relay volts.


      The basic design given in the Brighthub is same as the above circuit.

    • Hey Swagatam,
      Thanks so much for all your replies, awesome stuff.

      With this formula, you'll still get the working range for the circuit to be 10-13V. Is there anything wrong with my method of testing the circuit, that is using 24v input?
      If i want the total load to be able to draw maximum of 15A, what part of the circuit would i need to modify, the relay, the transformer,or both?


    • Hey Bourgeoisie,

      If 10 corresponds to the lower threshold and 13 to the higher threshold obviously the normal voltage would lie between these thresholds, possibly 12 volts, therefore a 12 volts input would be good, moreover this voltage should also match with your relay operating voltage.
      If you are using 24 voltage as the input then set the high and the low threshold around 24 and also make sure the relay operating voltage is 24.
      For 15 Amp loads make sure the relay contacts are rated at 15 Amps @ the specified volts (220 or 120)


    • Hey SWAGATAM
      Thanks again.

      So how do you suggest i test the circuit for under and over voltage?
      If i use a relay rated 15A, would it affect the overall performance of the circuit since the rating of the one you used in your circuit is 10A.

    • Hi Bergeoisie,

      You will need to test it using a variable power supply and provide the circuit with the relevant voltage thresholds and adjust the relay so that it just trip at those voltage.
      After the settings are done, you can connect the circuit with a regular transformer having a voltage in between the above set thresholds.
      Relay contact does not have connection with the circuit, it's an external issue.

    • Hey swagatam

      I can't seem to get the settings of my potentiometers correctly, i don't really know what and how to set them.

      Please & Please, could you please explain to me how the circuit energizes only T2 during operation without energizing T1 i would really appreciate it.


    • Hi Bourgeoisie,

      Suppose you set P1 such that T1 conducts at 13 volts which corresponds to say 260V AC, so when T1 conducts it pull the base of T2 to ground and therefore T2 switches OFF and also the relay.
      Also Suppose you set P2 such that T2 just stops conducting at 10V corresponding to say 190V because of low biasing voltage (T1 is already off) so again the relay switches off.
      In this way at both the levels the relay is switched OFF.

    • Hey swagatam,

      I'm eternally grateful for your help, and i'll keep you posted on the progress i make as well as any developments.

      Thanks again.

    • Hey Swagatam

      I'm back with some more questions, mostly conceptual and design related; because i'm trying to understand the circuit to its details.

      i.Y 100 microFarad? What happens if the size of the the capacitor is decreased or increased.

      ii.Y the connection of 1k resistors to the pots? are they voltage dividers? if yes y are the voltage divisions necessary?

      iii.Y r d zener diodes rated at 3V? they seem to be in series with the base of the transistors, so they're not really regulating the voltage at the base of the transistors to 3V, so whats their purpose?(Zener diodes from ur initial configuration).

      I just found that i couldn't answer these questions satisfactorily, i'd appreciate it if you could help out.


    • Don't forget to give a plus-one-like to my blog if you find my answers satisfactory…OK?

      Nothing is critically set here, 100uF is selected because the circuit works fine with this value, you can even put a 1000uF, it would be much better but would increase the cost.
      1K resistor is kept for safety concerns, if they are not placed then while adjusting the preset if accidentally its shifted at the positive end, the transistor would get damaged.
      Zener diodes might be actually not required, in my earlier circuit (in BrightHub) I had included them in an attempt to make the tripping points sharper, however later on I realized that it was absolutely not required.

      If you are happy with the answers give me a "like"


    • Thanks so much, i 've hit the like button twice with joy! 🙂

      If i were to add a filter circuit for cases where there are unwanted frequency changes, what values of capacitance would you advise i'd use?.

    • Thanks!

      What kind of frequency are you referring to?

      This circuit cannot get affected with any frequency because it is just not sensitive enough to get disturbed by frequencies or RFs of any kind….

    • Hey Swagatam,

      The thing is, i want to build a fairly more robust circuit for abnormal voltages, and i want this to also protect against frequency fluctuations from the RF interferences like you mentioned as well. I want to add a simple passive low pass filter to attenuate high voltage signals from getting to the load(the control circuit is not my concern for these frequency variations). So what value(s) for capacitors would you advise & y?

    • Hi Bourgeoisie,

      I still did not get your point, especially what you meant by this:

      "I want to add a simple passive low pass filter to attenuate high voltage signals"

      because high voltage signals have no connection with the generation of RFs.

      Is the circuit that you are referring related to the above circuit? or is it a separate configuration?
      Also provide me with the range of frequencies that you need to filter out and also inform me regarding the units which would need these protections.

      Thanks and Regards.

    • Hey Swagatam,

      In this part of the world, the supply is given at 50Hz 230V, so what i'm saying in essence is that i want to design a low pass filter to filter out voltage signals with frequencies greater than 50Hz that can b caused by harmonics from RF interference and add it to YOUR CIRCUIT, so that these signals don't get to the loads and damage them.

  6. Hey swagatam i have some questions
    1.Is it possible to build a protection module for just any DC motor??
    2.When the field winding of a shunt DC motor is opened, how can that fault be detected?
    3.Thanks for the response on the use of the 78xx and a DC power supply..but from your over and under voltage protection from brighthub, you made use of AC supply which u rectified and filtered to energize the relay whenever there is over or under voltage…Now the output was an AC load and was powered by AC…but in this case its a DC load..so the DC load will be powered by a DC supply but how will the output be connected for the relay to switch off the load..

    • Hi Gbengoose,
      For regulating DC loads voltages we can use ICs.
      Using the above circuit would become a very crude idea for the application.
      Detecting open field winding will require a different sensing circuit, not the above one.
      For connecting a DC load to the above circuit, simply connect the NC of the relay to positive supply of the circuit, connect the positive of the motor to the central pole of the relay and connect the negative of the motor to ground rail of the circuit.


  7. can the 12 volt part be used to protect 12volt lithium battery's with cutoff ad 10,5volt minimum and 14,5 volt maximum ?

  8. I'm sorry, if I give 220V to input ,How much Volt at out put

    and if more 220V at input, Volt at out put become 220V??

    Thank a lot..

    Ps. I,m a Thai people ,Eng. language is bad

    hope your understand

    • The output will be the same as the input, this circuit is intended for securing the load from high and low voltages by cutting the supply OFF when such voltages appear at the input.


  9. Hai Swagatam
    i would like to get a simple 12v 7ah battery charging circuit with low discharge and high charge protection with a transformer of 12 0 12 1A. i am not an expert in electronic but i have some very basic idea. so plz provided me a very simple circuit

    thanks for these posts

  10. Swagatam
    Could you plz provided similar type of circuit for 12v 7ah sealed battery charger with low and heigh voltage cut off circuit with 12v 1A transformer. plz provided the components which should be commonly available in the market too. plz provided a replay soon its urgent. plz make sure that the circuit is simple where a even a normal persons also can make it like me.

    Thanks for your blogs.

  11. Swagatam
    Thanks for your support super fast replay, as per your instruction the below link
    "https://www.homemade-circuits.com/2011/12/simplest-smf-automotive-battery-charger.html" i made this circuit, which is working fine. i have a doubt. what will happened when the diodes parallel to the relay, what does this do, without giving that diode the relay was continuously toggling.

    i have some more suggestion. as in the circuit the transformer is working continuously. is any way to stop that ?. also there was some battery discharge due to the relay function any way to avoid ?

    Any why the circuit is very simple and easy to create
    Thanks for your support

  12. Dear sir, kindly design a circuit with High / Low Voltage cut off with (time & voltage adjustable ) 3 min time delay having detection time in the range2 milli Sec & response time of less than 10m Sec to avoid in damages to electronic equipment like HiFi, washing m/c etc, as no such protection systems are available in India

    • Hello swagatam,
      I am a female who is taking my last semester of electrical engineering technology control. In order to graduate, we must build a project. I have chosen to build a voltage protector for home appliances. Could you please help me build a circuit that would stop high or low voltages from going through to an appliance. I would also like tp include some kind of display to the circuit that would display the voltage. Any ideas.
      Thanks in advance

    • Hello Ayat,
      you can try the above explained circuit, it's been tested by me and is quite reliable. You can connect an LED across D1 for getting indications regarding abnormal voltage conditions.

  13. sir i want to be realistic you mean if the circuit is power by variable power supply the p1 should be calibrate so that the relay move from n/c to n/o and the p2 should also move it back to n/c isn't?

    • Hi, The DP outputs will need to be removed from its existing position and connected with the points indicated "AC mains input" next, the terminals indicated "to load" will need to be connected with the points where the DP outputs were connected earlier meaning to the rest of the house wiring.

      Please do not do these if you are an expert in the field.

  14. Hi, after installing a tankless water heater my washer turns on & off when I am using hot water. I tested the voltage on the outlet where the washer is plugged into and every time the heater turns on the voltage drops about 1 volt.

    Will this circuit help me keep the voltage steady?

    Thank in advance


    • Hi, the above circuit is not designed to compensate voltage drops, you may have to use a voltage stabilizer circuit for this, however the solution could be something else, much simpler than this.

  15. hello sir,
    please suggest me a circuit for mobile signal booster for my house.
    it should receive signal through antenna outside house and transmit through antenna inside house.i am looking for a solution from you for low signal coverage.

  16. hello sir..
    i want ask you..
    if i dont want use a transformer and rectifier to under and overvoltage can or not??
    just connet with ac supply 240V..
    and what type and range suitable to ecah component used
    please help me

    • hello Nick,

      the circuit will fry and turn into a ball of fire if you connect it to 220V mains….so a transformer will be required as shown above.

      the range is standard for all the components.

  17. Ok. I am building a circuit such that at T2 I will have a led indicator telling me the transistor is on and will also be able to drive two relays, one relay switches to Mai while the other switches off alternative power supply. Also in case the voltage is too high or too low, will I can I an led indicator to show that.
    Then, can I use a transformerless circuit instead of the usual step down transformer. Thanks

  18. Hi Swagatam Majumdar

    I want to on and off the input voltage(like 12v to 24 v) based on the time like 1.3minutes on and 0.30seconds off the supply voltage(the time of on and off should variable i.e adjustable), let me know the sample circuit or solution for that.
    And one more thing temperature based control, if temperature reach 75 degree cut off the input voltage and temperature decreases to 45 degree turn on the input voltage, i want to maintain the temperature in between minimum45and maximum75 degree,please send me the circuit if available.

    thanks in advance

  19. Hi Swagatam Majumdar

    I have been following you for a while ,love your work, have alredy build three or your circuit, and they are working fine without issues, now I am required to solve a situation I believe you can help me solve, I have few off-grid Solar system powering university student residence, each room have a distribution box connected to my inverter, I require an overload cut off where I can measure maximum load threshold in Amps , to keep the system alive,the circuit above address my situation but I will need to be able to calculate R1 to get the required load not voltage .i will appreciate your help , thanks in advance

    • it can be done using a variable power supply…..apply the DC equivalent of 190V to the circuit and adjust the left preset to cut-off the relay at N/C.

      then apply the high voltage DC equivalent of 260V and adjust the right side preset to trigger the relay at the N/O side

      for a 12V supply the equivalents can be calculated by the following cross multiplication formula:

      12V/x = 220/260

      12/y = 220/190

    • Thanks for the reply. If I want to set the low and high points at 200 and 240 respectively, then will the same equation apply OR some other. Kindly advise

    • the same equation can be used for calculating any desired threshold level, provided the 220 = 12V reference is correctly matched, otherwise this must be verified first and then suitably used in the formula.

      for example if 220V gives 12.5V for your supply, then the 12V in the formula should be replaced with 12.5

    • first determine the DC thersholds that correspond with the mains thersholds, using the following formula:

      mains normal/mains Hi or low = DC normal/DC hi or low

      suppose using a 0-12V transformer you get 14V for 260V, and 10V for 200V,

      now adjust both the presets to the ground side.

      next apply 10V from a adjustable power supply to the circuit, slowly adjust the right side preset until the relay just clicks ON.

      now increase the voltage to 14V…now adjust the lefts side preset until the relay just clicks OFF..

      you may need to tweak this a little more until the most favorable response is achieved.

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