3 Tested 220V High and Low Voltage Cut OFF Circuits Using IC 324 and Transistors

The article describes 3 accurate automatic over and under voltage cut out circuits can be made at home for protecting the domestic appliances from sudden dangerous high and low voltage influxes. The first designs explains a LM324 transformer based circuit, the second circuit uses a transformerless version, that is it works without a transformer, while the third concept explains a transistor based cut off circuit, all of which can installed at home for controlling over and under voltage cut off protection.


The AC mains high and low voltage cut off circuit explained in this article is very easy to build and yet very reliable and accurate. The circuit utilizes a single IC LM 324 for the necessary detection and instantly switches the relevant relays so that the connected loads are isolated from the dangerous inputs.

The circuit also provides visual indications of the respective voltage levels during any instant.

The following circuit utilizes a transformer for powering the circuit

Circuit Diagram

Parts List for the proposed high, low mains voltage protector circuit.

R1, R2, R3, R4, R5, R6, R7, R8 = 4K7,
P1, P2, P3, P4 = 10 K Presets
C1 = 1000 uF/ 25 V,
OP1, OP2 = MCT 2E, opto coupler
Z1, Z2, Z3, Z4 = 6 Volts, 400 mW,
D1, D2, D3, D4 = 1N4007,
D5, D6 = 1N4148,
T1, T2 = BC547B,
LED = RED, GREEN as preferred,
Transformer = 0 – 12 V, 500 mA
Relay = SPDT, 12 Volt, 400 Ohm

Circuit Operation

In one of my previous posts, we saw a very simple yet effective design of a mains over voltage and low voltage cut off circuit, which is able to switch and cut off the mains power from reaching the connected appliances once the input voltage crossed over or below the dangerous thresholds.

However due to the over simplicity of the design, involving just a couple of transistors, the circuit has its own limitations, the major limitation being less accuracy and considerable hysteresis, resulting high threshold gap of more than 60 volts between the high and the low limits.

The present design of a high voltage and low voltage cut off circuit is not only highly accurate but also provides visual indications regarding the relevant voltages insteps. The accuracy is so high that virtually the thresholds can be separated and sensed within 5 volts range.

The incorporation of op amps in the circuit equips it with the above feature and therefore the whole idea become very much reliable.

Let’s understand the circuit in details:

How the opamps Operate as Comparators

The op amps, A1, A2, A3, A4 are obtained from a single IC 324, which is a quad op amp IC, means consists of four op amp blocks in one package.

The IC is outstandingly reliable and easy to configure and hardly poses a problem with its functioning, in short it has robust specs and is too flexible with most of the configurations.

The four op amps are rigged as voltage comparators. The inverting inputs of all the op amps are clamped to a fixed reference value of 6 volts which is done through a resistance/zener network for ech of the op amps discretely.

The non-inverting input of A1 to A4 are connected to the power supply of the circuit through a voltage divider network formed by the presets P1, P2, P3 and P4 respectively.

The presets can be adjusted as desired to flip the outputs of the respective op amps when the relevant input level crosses the reference level set over the inverting inputs of the respective op amps.

The outputs of A1 to A4 are integrated to LED indicators in a rather special way. Here instead of following th conventional method of connecting the LED cathodes to the ground, it is connected to the output of the preceding op amp’s output.

This special arrangement ensures that only one relevant LED is switched ON in response to the rising or falling voltage levels from the op amps.

How the optocouplers function

Two opt couplers are introduces in series with the uppermost and the bottom LEDs so that the optos also conduct with the relevant LEDs during high and low voltage levels, specified as dangerous thresholds.

The conduction of the opto couplers instantly switches the internal transistor which in turn toggles the respective relay.

The poles of the two relays and the poles of the relays are connected in series before supplying the output through them to the load.

The series connection of the contacts ensures that if any one of the relay conducts, cuts of the mains supply to the load or the connected appliance.

Why the opamps Comparators Arranged in Series

At normal levels opamp A1, A2 or even A3 may be conducting because all these are arranged in an incremental order and go on switching in sequence in response to gradually rising voltages and vice versa.

Suppose at certain normal levels A1, A2 and A3 are all conducting (outputs high), and A4 not conducting, at this point only the LED connected to R7 would illuminate, because its cathode receives the required negative from the output of A4, whereas the cathodes of the lower LEDs are all high because of the high potentials from the above op amps.

The LED connected to R8 also remains shut off because A4’s output is low.

The above results appropriately influence the respective opt couplers and the relays such that the relays conduct only during dangerous low or dangerous high voltage levels detected by only A1 and A4 respectively.

Using Triac instead of Relays for the cut off

After some analysis, I realized that the above high, low mains voltage cut off protector circuit could be simplified into a much easier version using a single triac. Please refer to the diagram given below; it's self-explanatory and very simple to understand.

However if you have problems understanding it, shoot me a comment.

Using Triac instead of Relays for the cut off

Modifying the Design into a Transformerless Version

The transformerless mains high low voltage cut off circuit version of the above explained design can be visualized in the following diagram:

Warning: The below shown circuit is not isolated from mains AC. Handle with extreme caution to avoid a fatal mishap.

transformerless high low voltage auto cut off circuit

If a single relay is intended to be used instead of a triac, the design could be modified as shown in the following figure:

Please use a 22uF/25V capacitor across the transistor base and ground, just to make sure the relay does not stutter during the changeover periods...

Using PNP Relay Driver

As shown in the given mains AC high, low voltage protector circuit, we can see two op amps from the IC LM 324 are used for the required detection.

The upper op amp has its non inverting input rigged to a preset and is terminated to the supply DC voltage, pin #2 here is provided with a reference level, so that as soon as the potential at pin #3 goes above the set threshold (by P1), the output of the op amp goes high.

Quite similarly the lower op amp is also configured for some voltage threshold detection, however here the pins are just reversed, making the op amp output go high with low voltage input detection.

Therefore, the upper op amp responds to high voltage threshold and lower op amp to low voltage threshold. For both the detections, the output of the respective op amp becomes high.

Diodes D5 and D7 make sure that their junction produces a common output from the op amp output pin outs. Thus whenever any one of the op amp output goes high, it is produced at the junction of D5, D7 cathodes.

Transistor T1’s base is connected to the above diode junction, and as long as the op amps output remain low, T1 is allowed to conduct by getting the biasing voltage through R3.

However the moment any of the opamp output goes high (which may happen during abnormal voltage conditions) the diode junction also becomes high, restricting T1 from conducting.

Relay R1 instantly switches OFF itself and the connected load. Thus the connected load remains ON as long as the op amp outputs are low, which in turn can only happen when the input mains is within the safe window level, as adjusted by P1 and P2. P1 is set for detecting high voltage levels while P2 for the lower unsafe voltage level.

Mains High Low Voltage Cut Off Circuit, Using IC 741

Pin Details of IC LM 324

IC LM324 pinout diagram

Parts List for the above mains high, low voltage protector circuit

R1, R2, R3 = 2K2,
P1and P2 = 10K preset,
C1 = 220uF/25V
All diodes are = 1N4007,
T1 = BC557,
Relay = 12 V, 400Ohms, SPDT,
op amps = 2 opamps from IC LM 324
Zeners = 4.7 volts, 400mW,
Transformer = 12V, 500mA

PCB Layout

Mains high low voltage cut off circuit PCB layout

So far we learned an IC version of the circuit, now let's see how a mains 220V or 120V operated over voltage and under voltage protection circuit can be built using just a couple of transistors.

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 Transistors

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 to 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.

Transistorized Mains Over Voltage and Under Voltage Cut of Circuit


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

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97 thoughts on “3 Tested 220V High and Low Voltage Cut OFF Circuits Using IC 324 and Transistors”

  1. Hi,
    Can you please explain where the connection goes after R8. Why does it go to the output of A1 . Doesnt it short. Should it go to the ground? Also R1 to R8 is mentioned as 4K7 resistors. CAn you be morespecific please like is it 47K and at what wattage?
    Thank you

  2. Hi Sir,

    If I am opting for the transformerless version, what all changes would I have to make in order to cut off the circuit at 150V and 370V AC.

    • Hi SST, you can build the second last design as shown in the diagram. Make sure to connect LEDs between A1 output and ground, and A2 output and ground…both the LEDs should have its own 1K series resistors.

      initially keep the preset to ground level…then feed 370V and adjust the upper preset until the A1 LED just lights up
      Next, feed 150V at the input, adjust the lower preset until the A2 LED just lights up..that’s all the set up is complete.

      The 100K resistor should be rated at 3 watts, and the 105 capacitor should be rated at 600V

      Make sure the positive pin Vdd of the opamp gets a 12V fixed supply from a 7812 IC.

      • Sir, I am getting output acroos my Triac for any voltage below 150 and nothing across the load. (is this normal). I just wanted to check if there was any voltage across the loads for a lower voltage.

        • SST, first you must follow the setting up procedure that I explained, and make sure the triac is responding correctly to the opamp switching, at the moment I am unable to understand what exactly you have done and how you are testing it?

      • Hi Sir,

        I configured the lower opamp circuit, and it is working for 150V, but when the output is reduced below 150V it takes some time for the LED to completely go out.

  3. Hi Sir,

    I am currently trying to implement this circuit to cut off the voltage for below 150V and 370V. My step down transformer produces an output of 75V and 175V. I used a voltage divider circuit in order to provide an input of 12V to the IC and also made changes to the zener diode circuit. But the entire voltage is appearing across the triac for voltage as low as 1V.

    • Hi SST, your step down transformer is supposed to produce 12V DC in response to a 220V input AC, how can it produce 75V?

      And if you are using a transformer for powering your circuit, it is supposed to be a a 0-12V/220V transformer. Make sure you are doing the procedures with proper understanding.

      • I used a 440V/ 220V transformer which belongs to another part of my circuit that I am currently working on. This part works fine, so I can’t change the transformer at that part. in order for cut off I used a voltage divider circuit such that it would produce 12V.

        • you can follow the same proceduers as explained above.

          Just forgot to tell you:

          Make sure the positive pin Vdd of the opamp gets a 12V fixed supply from a 7812 IC.

          And remember the whole set up is not isolated from mains AC and can be extremely lethal to a human touch

  4. Hi Sir,

    I would like to design a circuit for high and low voltage cut off. Suppose if the input is as high as 400V and on the lower side 160V, can I use this circuit? will transformers be available to step it down to a low level.

    • Hi gks,

      yes you can use this circuit for any desired cut off thresholds. But this will not stabilize the output instead will only cut off the mans supply at the specified levels. If you are referring to the input transformer, it will need to be specially made to order for withstanding 400V input, or you can opt for the capacitive version and employ a 600V capacitor for achieving the same.

      • Sir,

        If I am using a voltage divider in place of that preset, I can get voltage levels at 6V. Will that be adequate enough? ( a 10K series resistor with 420 ohm in series, and taking the output across 420 ohm will give 6V)

        • gks, That may be possible but only after setting-up the opamps practically by feeding 160V and 400V and adjusting the presets appropriately for getting the required cut offs on these voltage levels..
          Once this is achieved you can measure the adjusted values of the preset and replace them with fixed equivalent resistors.

      • Hi Prasanth, the second last circuit in the above article shows the transformerless version. You can implement it with appropriate setting up procedures.

  5. Hi Sir,
    I want to design a circuit such that it should cut off the input supply voltage if input voltage is below 150v or above 370v. Sir, can u please design the above circuit for this specification?

    • Hi RSG, first find out the equivalent DC value of your power supply with respect to the input AC level…. I am referring to the power supply that you would be using for powering the above circuits.

      Let me know this then I’ll tell you how to proceed further.

        • Hi RSG, I meant the DC voltage that you would be using to operate the circuit and the IC. 75V and 185V cannot be used for operating the circuit, it has to be in the range of 12V

  6. Hi Sir,
    This question is a little bit of topic, apologies. Will it be possible to design a booster circuit to boost 24V DC to 240V DC. I designed a booster circuit, but I am unsure if it will be practical and compact.

  7. Hello Sir,
    I want to build a system that cuts off the supply for lower level at 150V and higher value at 370V AC. I don’t have a clarity on what the DC Voltage corresponding to this will be.

    • Hello SST, you can find the equivalent values through simple cross multiplications, just check your input AC and the equivalent DC from the bridge rectifier of your power supply.
      Comparing them you can find other equivalent values, as shown in the following example:

      Suppose you find 12.8V DC at 230V AC input, other values could be found by solving the following expression:

      12.8/y = 230/z
      Here “y” will be the required unknown DC value, and “z” will be the intended AC low or high cut off value

  8. Hello yonder,
    I found all your circuit and projects interesting but please I need a special assistance. I want to build a Low and high battery full cutoff that can handle about 360VDC (30 Batteries in series) such that when battery is full at 405VDC charging Voltage will cutoff and when battery drop to like 325VDC it also cutoff battery low.
    Please, do share this experience with me.

  9. Hi Sir,
    In the above first circuit (Highly accurate over/under voltage CKT.)

    can u pls tell me connections of OPTO coupler,

    Pin 1 = anode (connected in CKT)
    pin 2 = cathode (connected in CKT)
    pin 3 = NC
    pin 4 = Emitter (connected in CKT)
    Pin 5 = collector (connected in CKT)
    then where to connect pin 6 = Base of OPTO…..?

    and Bc547 given in the components list, are not needed in the circuit. Right….?

    Thanks in Advance…..:)

  10. hi once again,how r u hope fine
    i want to use reliable transformerless power supply with auto cut off circuit as u published
    any modifications?and in this page there is also a second ckt which should b used and i am asking for transformerless power supply because the transformers i get are getting hot they will fail after some time and also in high voltages there is maximum chance of burning of transformer and if transformer fails the relay denergize and the connected appliance would burn then the cut off ckt will b safe but the main target the appliance will burn off
    so please give me a ckt with a reliable reliable psupply which can bear high voltages, fluctuations ,no zenner shorted and no heating of components which should run continiously for example : i am using to led street lights with day night ckt ,the supply should b always connected if any small problem comes the main lights will surely b in trouble and u know how expensive are these
    please dont mind for so long detailed approch thanks and i admire for ur great hobby
    which is not only helping the persons like us and also ur providing good knowledge for the society
    hats off for u .god bless u

    • Dear Anioko,

      The first diagram is correct in all respects, you can use it for your project but it's unnecessarily complex, the lower one will give the same results and is more compact, a relay can also be used instead of triac in the second design.

    • Dear Nisha, build the second circuit it's much easier, the circuit is a simple comparator based design and there's no way it won't function if you did everything correctly.

      Connect an LED in series with R3 in the second diagram and check the response by moving the presets

  11. sir i want to use this circuit https://homemade-circuits.com/2011/12/simple-mains-high-and-low-voltage.html as a final year project. i submited the project as electronic wacthdog control system to the supervisor and he is requesting for the abstract, please sir can U assist me with the written materials for the project and what is the difference between this circuit and that of the stabilizer. thanks as i awaite your quick response. my email is canioko@yahoo.com

    • Anioko, all the data are provided in the article explanation, you can rephrase the details appropriately as per your requirement.

      The abstract material can be found at the beginning of the article.

      a stabilizer will automatically correct the voltage fluctuations by supplying extra voltage to the appliance or by deducting it as per the mains situation.

      The above circuit will not do this rather cut off the mains supply to the appliance if it tends to shoot above or below a certain predetermined danger level

    • the voltage step-up transformers will need to be built using ferrite transformers involving complex switching circuits, just as we have in compact inverters.

  12. ok sir thank you for the respond,sir there's something confused me a bit how will i know if the triac is off? sir how can one voltage supply circuit be use at the same time to feed low and high voltage to the transf? or should i connect the supply out put to the rectified and adjust the supply variable to high and low ?can you please provide me what current the triac need before it can switch off? thank you.

    • you can put a LED in series with the triac gate for the indications.
      yes you can use a variable power supply for quickly varying the supply high to low and oppositely for confirming the results.
      the opamps will take care of the triac switching as per the settings.

  13. ok sir please can i use this circuit
    How to Make a Simple 220V Transformerless Power Supply Circuit Using a Single MJE13005 Transistor to set this last circuit?

    • feed the lower threshold AC to the trafo and adjust P1 to just switch OFF the triac.
      Next feed the higher threshold voltage to the trafo and adjust P2 to just again switch OFF the triac.
      the triac will remain switched ON only in between the above high/low thresholds,

      your circuit is set now.

      you will need a variac for doing this.

  14. Sir, can you please explain this more clearly = i.imgur.com/ejlMplj.jpg
    Moreover in your new diagram, where I have to wire relay to get DC power?

    Thanks in advance 🙂

    • Unknown, your questions suggest that you are very new in the field and in that case the above circuits are not recommended for you, anyway here are the answers:

      (+) of the IC will go to the positive of the bridge rectifier.

      the load is supposed to be a 220V load that's why it's connected across the mains via the triac.

      the left side TR1 stands for trafo, while the right side TR1 indicates the triac, these are accidentally taken identically

      R5 is connected to the gate of the triac TR1.

    • Sir, yes I'm quite new to this field and will be more careful to apply this diagram.

      1) Can you please tell me why should I add preset instead of 10k resistor?
      2) Where should I wire 12v DC relay?
      3) If I want to add 2 second delay timer by 555 IC how can I do that?

    • 1) presets are used for setting the cut-off thresholds by adjusting them appropriately.
      2) triac and R5 can be removed and R4 replaced with a relay (with bocking diode)
      3) for switching delays add a 220uF/25V capacitor across transistor base and positive.

    • Sir, please let me know if this OK = i.imgur.com/oWIZzrK.jpg

      a) I have removed 220v AC line across PCB.
      b) Wired (+) of the IC to the positive of the bridge rectifier.
      c) Added 12v DC relay as per your instruction.

      If anything goes wrong it will be helpful if you correct this diagram for me 🙂

  15. sir the first circuit
    Highly Accurate, Mains High and Low Voltage Cut OFF Circuit, Using IC 324 Explained can i replace 4n35 optp in place of MCT 2E,opto coupler,so this circuit also need to be sets by variable power supply? sir i have a 12 transformer 2000mA can i use it to power the circuit? thank you .

    • bianzz, you can use MCT2E, however the last circuit is much simpler and sensible so you could try the last one.
      a variable PS is a must for setting up the circuits.

      if your relay is 12v, then you can use a 12v trafo for powering the circuit

  16. according to me to activate an electromagnet so that it pulls an iron shaft we would need a DC. so I assume the AC being converted to DC inside the relay

    You can connect a bridge rectifier externally and supply the relay coil through this bridge rectifier and then use many 10uF/250V caps across the relay coil, this would make sense.

  17. a relay coil will always respond to DC and not AC, yet I will suggest that you first measure the voltage across the relay coil using a good quality multimeter, and then procure the matching capacitor as specified earlier by me.

  18. Put 3 or 4 10uF/250V capacitors in parallel right across the coil of the relay, this will restrict the buzzing of the relay and will simply not allow the relay to activate below a certain minimum level.

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