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Load Independent/Output Corrected Inverter

The common problem with many low cost inverters is their incapability of adjusting the output voltage with respect to the load conditions. With such inverters the output voltage tends to increase with lower loads and falls with increasing loads. The circuit explained here can be added to any ordinary inverter for compensating its varying output voltage conditions in response to varying loads.


The circuit was requested to me by one of my friends Mr.Sam, whose constant reminders prompted me to design this very useful concept for inverter applications.

The load independent/output corrected or output compensated inverter circuit explained here is quite on a concept level only and has not been practically tested by me, however the idea looks feasible because of its simple design.

Simulation and Working

If we look at the figure we see that the entire design is basically a simple PWM generator circuit built around the IC 555.

We know that in this standard 555 PWM design, the PWM pulses can be optimized by changing the ratio of R1/R2.

This fact has been appropriately exploited here for the load voltage correction application of an inverter.
An opto-coupler made by sealing an LED/LDR arrangement has been used, where the LDR of the opto- becomes one of the resistors in the PWM "arm" of the circuit.

The LED of the opto coupler is illuminated through the voltage from the inverter output or the load connections.

The mains voltage is suitably dropped using C3 and the associated components for feeding the opto LED.

After integrating the circuit to an inverter, when the system is powered (with suitable load connected), the RMS value may be measured at the output and the preset P1 may be adjusted to make the output voltage just suitable enough for the load.

How to Set Up

This setting is probably all that would be needed.

Now suppose if the load is increased, the voltage will tend to fall at the output which in turn will make the opto LED intensity decrease.

The decrease in the intensity of the LED will prompt the IC to optimize its PWM pulses such that the RMS of the output voltage rises, making the voltage level also rise up to the required mark, this initiation will also affect the intensity of the LED which will now go bright and thus finally reach an automatically optimized level which will correctly balance the system load voltage conditions at the output.

Here the mark ratio is primarily intended for controlling the required parameter, therefore the opto should be placed appropriately either to the left or the right arm of the shown PWM control section of the IC.

The circuit can be tried with the inverter design shown in this 500 watt inverter circuit

Parts List

R1 = 330K

R2 = 100K

R3, R4 = 100 Ohms

D1, D2 = 1N4148,

D3, D4 = 1N4007,

P1 = 22K

C1, C2 = 0.01uF

C3 = 0.33uF/400V

OptoCoupler = Homemade, by sealing an LED/LDR face to face inside a light proof container.

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. I can just buy any optocoupler and place in this circuit right

  2. no, it's a homemade LED/LDR opto, not the regular ones that are available in the market

  3. Hi Swagatam, have you tested this circuit?

  4. Hi Wade, I have not yet tested it.

  5. Sir, I have bought the components of this circuit to give it try with the suggested circuit of IC4047 and I will update you on it very soon.

    Thanks for your effort to help us Sir.

  6. Aminu, I hope you have understood the working of the circuit and will be able to optimize the results correctly...otherwise it could be difficult to get the intended output.

    Wish you all the best

  7. Sir, I built this circuit. But I did not used IC4047, as suggested, I used IC4093 as in this inverter:

    And is it necessary to use C3 i.e 0.33uf 400v?
    Can I used any of 0.39uf 400v or 0.47uf 400v or any other one?
    Thanks Sir.

  8. Aminu, C3, and R7 are responsible for generating the basic 50Hz frequency, you can replace C3 with any other capacitor but then you will have to also change R7 value proportionately so that the 50Hz frequency is not disturbed.

  9. Thanks for the help Sir.
    But I dont mean C3 of IC4093 of the shared link. I mean C3 of the above IC555.

  10. you can use 0.47uF also for C3.

  11. Good day Sir.
    Sir could you help me with a timing alarm circuit.
    I want the project to work as follow:
    1st, to ring an alarm at 35minuts intarval for atleat 3 times.
    2nd, to ring an alarm at 35minuts intarval for atleast 2 time.
    3rd, to ring an alarm at 35minuts interval for atleast 2times.
    Again, I want another one to work at 30 minuts interval with the above discriptions, that is 1st, 2nd and 3rd.

    Thanks for the help Sir.

  12. Aminu, you can try the following concept and adjust the stages accordingly as per your need

    limit the number of timer stages as per your need.....

  13. Hi swag i build inverter circuit using ic sg3525 successfully but only problem there is big delay when city power go off the inverter oscillator take time to switch on and this cause my computers to go off. How should solve this problem please swag!!!

  14. Hi Moses,

    Please post this question under the same article which you are referring to, so that I can see the schematic and understand the issue you are facing at the moment...


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