No Load Detector and Cut-off Circuit for Inverters

The post discusses a relay cut-off circuit which may be included in inverters to ensures that under a no load at the output the condition is quickly detected and the supply cut off, preventing the inverter from operating unnecessarily. The idea was requested by Mr. Rajath.

The Circuit Request

Good Morning Sir, I need to adopt a no load auto cutoff system into my inverter, do you have any suitable design, which could help me. or else can you give any idea on how to achieve ,as i need to shut down the output of the inverter when ever there is no current drawn from it. please help me ,here.

Regards Rajath


 The Circuit Design


No load cut off circuit for inverters for safeguarding inverters from no lad conditions

In a few of ay previous posts we have learned how to make overload cut off circuit such as:

Low Battery Cut-off and Overload Protection Circuit.

Motor over current protector circuit

However, the present concept deals with an opposite situation wherein a no load condition is supposed to be detected and cut off for persisting, that is we discuss a circuit for preventing a no load condition for inverters.

As shown in the above figure, a no load detector and cut of procedure can be initiated by incorporating this design in any inverter circuit.

The operational details may be understood with the following explanation:

The circuit comprises two stage, namely the current amplifier and sensor stage using the T3/T4 Darlington pair, and a simple delay ON stage using T1, T2 and the associated components.

As soon SW1 is switched ON, the delay-ON timer counting is initiated through C1 which begins charging via R2 and D5 keeping T1 switched off in the process. With T1 switched T2 is switched ON which in turn switches ON the relay.

The relay enables the positive from the battery to get connected with the inverter so that the inverter is able to start and generate the required AC mains to the intended appliances.

With the presence of a load at the output the battery undergoes a proportionate amount current consumption, and in the course Rx experiences a current flow through it.

This current is transformed into a proportionate amount of voltage across Rx which is sensed by the T3/T4 Darlington pair and it is forced to switch ON.

With T3/T4 switched ON, C1 is instantly inhibited from getting charged, which leads to an immediate disabling of the delay ON timer, making sure that the output of the inverter continues to supply the voltage to the load.

However, suppose the output of the inverter is devoid of any load (no load condition), T3/T4 is then unable to switch ON, which allows C1 to get charged gradually until the potential across it becomes sufficient to trigger T1.

Once T1 is triggered, T2 is cut off and so is the relay. With the relay contacts cut off and shifted from N/O to the N/C contact, the positive to the inverter is also cut off, the system comes to a stand still.

50 thoughts on “No Load Detector and Cut-off Circuit for Inverters

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hello sir,
    Nice and simple concept
    I have a few questions though. While mind simulating the circuit I want to ask
    1) "under signigicant load" when huge currents flow through Rx which converts to proportional voltage and then the Darlington pair(T3 and T4) is triggered with sufficient voltage at its base this should ground the voltage at the base of T1 which leads to having a voltage at the base of T2 which triggers T2 and therefore stops the relay from working right?
    2) how will the capacitor trigger T1 though the diode

    • Hello Michael,


      1) with T1 switched OFF, T2 is allowed to be switch ON, which in turn keeps the relay ON. The relay contacts in this situation connect across the N/O allowing the inverter to operate.

      so in the presence of load, the relay is allowed to remain activated.

      2) the capacitor while charging pulls the T2 base voltage to the ground level, until it has charged to a point where the base voltage at T1 is able to reach above the 0.6V mark, and at this point T1 switches ON.

  3. Hello Sir,
    So if am right by your explanation then it means that
    When the circuit is on T1 base voltage is grounded due to the capacitor charging and this allows a base voltage at T2 thereby switching it on and allowing the really to be switched on.
    Now when load is significant voltage is present at T3 and T4 base which then halts the capacitor charging.
    2) the delay time formulae is t = 1.1RC right?
    3) I also noticed that C has to be large enough to effectively ground voltage at base of T1?

    • Hello Michael,

      yes that's correct, but a "significant" voltage or current is not required, the operation can be assumed to initiate even with a small load at the output, and that's what the circuit is intended to be like. This can be set by calculating Rx correctly.

      I have not yet checked the formula, it could be correct, but is not important as the value of R2, C1 can be fixed with a little trial and error.

      C does not have to be large, because we want the changeover to take place quickly so basically it should be relatively smaller.

  4. Hello sir
    I am looking at this IC hip4081A it's a full bridge driver as I don't want to use two ir2110.
    Sir do you have any experience with this driver?
    It can give out up to 2.5A peak current.

    • Hello sir
      Question about gate resistors.
      If I use say a 10 ohm resistor say for a push pull and I have just one resistor on each side.
      Now if I parallel say 3 resistors on each side does that mean I use 30 ohm resistors for each mosfet so that the each three MOSFETs in parrell now have 10 ohm resistors when we use the parallel resistor formulae.

    • Hello Michael,

      you'll need to connect 10 ohm individual resistors in series with each gate of the mosfets…it does not have to be 30 ohms

      780X are rated to deliver 1 amp…so you can can divide this with IC's current specs to get the

    • Hello Micheal, no that'll not happen…as long as the input is at least 3V above the set output voltage, you can be sure of the getting a constant voltage as per the set level.

  5. Hello sir
    For this delay circuit I can use a 24vdc input I will only increase the relay rating and double the value of the resistors to limit current?
    I then use a 12v regulator to power the circuit after the relay.

  6. Hello sir
    Also I have been trying to see if I can come up with an overload/ surge circuit using the delay circuit concept above. Here what I intend to achieve
    1) the circuit should have a standard 1500w (full load)reference current or voltage.
    Then when we power up a load that is higher than 1500w say 1505w or 1550. As a matter of fact anything from 1501w, instead of indicating an overload immediately it should allow for a minimum amount of time maybe 10secs and then if the load is still at a wattage higher than 1500w it then trips off.

    Remember I said I wanted surge ability for loads that require more starting current so now if we power a load that will go as high as say 3000w surge technically it is higher than 1500w and so the circuit delays tripping off overload signal for maybe as usual 10secs and then if after 10secs the load has not droped to its normal power consumption of say 500w then it trips off.
    Hope you get what am trying to do here.
    Am hoping the circuit can have like a reset button too.

  7. Hello sir
    Can I pass a weak pwm signal into a MOSFET driver like a tc4427 and then use it to drive another full bridge driver IC like the ir2110.
    Basically can I drive a MOSFET driver with another MOSFET driver?

    • dear chandan,

      10000/180 = 55 amps, therefore the switch and the relay wil need to be rated at 60 amp minimum…T2 will need to be upgraded to TIP122

      Rx will depend upon the intended minimum wattage at which the inverter may be required to be operative.

  8. hello sir, i would like to ask some few question about this project
    1)the darlington transistor are connected at the negative terminal of the battery how is that possible to work? i cant a clear picture about the current flow of this circuit
    2)if there are designing calculation for the circuit analysis pleas help me with them
    thanks in advance

    • hello Bady, the Rx resistor needs to be in series with the supply, regardless of the position a voltage will be developed across it whenever the load is switched ON, in the shown design since the current is entering from top of the Rx, the (+) voltage is developed at the base of the associated T3 transistor.

  9. Hi, I came across this post and the concept of your circuit is exactly what I am looking for. I have a 4k 110v AC inverter powered by 4 6v batteries wired in series for 12v. The inverter has a power saver mode built in (25 watts) but it will sense and turn on my 110v AC refrigerator. However, when I open the door, the light bulb is enough to trigger the inverter to power on. Also, the power sensor pulses every 3 seconds which I'm afraid might damage the compressor. So my question is, is the load in your circuit adjustable and how would I do that to sense a low load. Thank you in advance for your reply.

    • Hi, the resistor Rx value can be altered as per the required specifications, and the inverter can be enabled to switched ON or OFF at any desired load conditions. Higher values will allow the inverter to operate with higher loads only and vice versa,

    • sorry I don't sell parts
      the parts are already specified in the diagram, please click to enlarge it, and copy down the numbers as mentioned in the diagram…all the parts are standard type and does not need any consideration.

  10. Hi Swagatam,

    I’ve built a large powerbank from 16850 Lithium cells for my laptop.

    I understand how to use the formula to calculate the desired Ω for the RX resistor.
    However, I am totally not sure about the required wattage the resistor needs to be able to handle.

    Assuming my powerbank powers an 80Watt laptop, then RX needs to be for example a 100Watt capable resistor?


    • Hi Rick,

      the formula for calculating RX is T3/T4 minimum triggering level divided by the desired current threshold, and the wattage will be the desired current threshold multiplied by the triggering level.
      I hope you got the relationship

    • for example let's assume the current threshold to be 100mA
      T3/T4 triggering potential to be 0.2V, the Rx will be

      0.2/0.1 = 2 ohm

      wattage will be 0.2 x 0.1 = 0.04 watts

    • sorry, the answer should be 0.02 watts

      …however 0.1 amps cannot be considered for calculating the Rx wattage, because 0.1 is the minimum current value, we would want it to be multiplied with the max current limit, which could be say for example 2 amps.

      so Rx wattage for this example would be 0.2 x 2 = 0.4 watts

  11. Hello Swagatam,

    Everything is 100% clear now!

    Thanks a ton for your time and dedication to explain the formula in detail.

    Very much appreciated!

    Success and regards,

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