circuit wiring

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.

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.
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 article.

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.