The article which discusses a 300 watt pure sine wave inverter circuit with automatic output voltage correction, is a modified version of one of my previous posts, and was submitted to me by Mr. Marcelin. Let's learn more about the converter implementations.

How it Works

The presented design  actually produces a modified sine wave output, but the waveform is highly processed and constitutes an exact equivalent of a sinusoidal waveform.

A single IC 556 forms the heart of the circuit and is responsible for manufacturing the required PWM controlled modified sine output waveform.

One half of the IC on the left is configured as a 200Hz frequency generator, this frequency is used for providing the required square wave clocks to the preceding monostable which is formed by wiring up the other half of the 556 IC.

The clocks are received from pin#5 and applied to pin#8 of the IC. The right hand side section of the IC does the actual processing of the above square wave by comparing it to the triangular waves applied at its pin#11.

The result is an output at pin#9 which is a  PWM, varying in accordance with the amplitude of the triangular waveform.

Ideally the triangular waves can be replaced with a sine waveform,  however since triangular waves are easier to generate, and also appropriately replaces the sine counterpart, its been employed here.

R1, R2, C1 should be appropriately selected so that pin#5 produces a 50% duty cycle, 200 Hz frequency.

The 200 Hz is not critical here, however it becomes critical for the IC 4017 stage and that's why it's been selected to that value.

The modified sine wave PWM generated by the IC556 is next applied to the switching stage comprising the IC 4017 and the relevant output mosfet devices. Let's see how it's done.

Parts List

IC1 = 556
R1,R2,C1 = select to generate 50% duty cycle
R3 = 1K
C2 = 10pF.

The output stage

The diagram given below shows the output stage configuration where the IC 4017 takes the center stage. Basically its function is to switch the driver transistors alternately so that the connected mosfets also conduct in tandem for inducing the required mains AC output into the transformer.

The IC receives the clock pulses from the above explained 556 circuit (pin#5/8) and its outputs sequence across the connected transistors alternately as discussed above.

Until here the circuit behaves like an ordinary square wave inverter, however the introduction of D1/D2 with the pin#9 of the 556 transforms the circuit into a full fledged pure sine wave inverter.

As can be seen, the common cathodes of D1/D2 are integrated with the processed PWM pulses from the above 556 stage, this forces D/D2 to conduct only during the negative pulses from the generated PWM blocks.

It simply means that when D1/D2 are forward biased, T1 and T2 are inhibited from conducting since their gates become grounded through D1/D2 into pin#9 of the IC 556, which make the mosfets respond exactly to the PWM pattern.

The above process generates an output across the transformer secondary that's perfectly chopped and processed and equivalent to a sine waveform.

Parts List

IC2 = 4017

all resistors are 1K

D1,D2 = 1N4148

T1,T2 = IRF540n

Transformer should be also appropriately rated as per the requirement.

The Triangular Wave Generator Circuit

The entire modified sine PWM waveform construction and implementation is dependent on the fed triangular waves at pin#11 of the IC556, therefore a triangle wave generator circuit becomes crucial and imperative.

However there are many types circuits that will provide you with the required waveform inputs, the following is one of them which incorporates yet another IC555 and is pretty simple to configure.

The output from the below given circuit must be fed to pin#11 of the IC556 for enabling the proposed sine wave inverter functioning.


A simpler alternative to the above design is shown below, the configuration would produce same results as explained above:

A Practical 300 watt Inverter Design

The idea was inspired by the design as discussed in the above section, however Mr.Marcelin has refined it considerably for better efficiency and reliability.

To me, the modifications and the implementations done  look great and feasible.

Let's understand the design elaborately with the following points:

IC2 and IC3 are specifically configured as the PWM generator stage.

IC2 forms the high frequency generator required for pulsing the PWM waveform which is processed by IC3.

For processing the IC2 pulses, IC3 needs to be fed with a sine wave equivalent information at its pin#5, or the control input.

Since creating sine waveform is a bit complex than a triangular waves, the later was preferred as its easier to make yet performs as good as a sine waveform  counterpart.

IC1 is wired up as the triangular wave generator, whose output is finally fed to pin#5 of IC3 for the generating the required RMS sine equivalent at its pin#3.

However the above processed PWM signals needs to be modulated over a push-pull kind of arrangement so that the waveforms are able to load the transformer with alternately conducting current.

This is necessary for achieving an output mains consisting of both positive and the negative half cycles.

Simulation and Working

The IC 4017 is introduced just for implementing this action.

The IC generates a sequentially running output from its pin#2 to pin#4, to pin #7, to pin#3 and back again to pin#2, in response to every rising pulse edge at pin #14.

This pulse is derived from the output of IC2, which is  set to 200 Hz strictly so that the outputs of IC4017 results in a 50 Hz across the sequencing from the above discussed pin outs.

Pin#4 and pin#3 are purposely skipped, for generating a dead time across the gates triggers of the respective transistors/mosfets connected to the relevant outputs of IC4017.

This dead time makes sure that the devices never conduct together even for a nano second at transition zones, and thus safeguard the health of the devices.

The sequencing positive outputs at pin#2 and 7 trigger the respective devices which in turn force the transformer to saturate with the alternating battery power induced in the respective winding.

This results in the generation of around 330+ V AC at the output of the transformer.

However this voltage would be a square wave with high RMS if it wouldn't be processed with the PWM from IC3.

Transistor T1 along with its collector diode is fed with the PWM pulses such that T1 now conducts and grounds the base trigger voltages of the outputs devices in accordance with the PWM content.

This results in an output that's an exact replica of the the fed PWM optimized input..... creating a perfectly carved pure sine wave AC equivalent.

The circuit has additional features such as a manual output voltage correction circuit.

The two BC108 transistors are stationed for controlling the gate drive voltage levels of the mosfets, the base current of these transistors are derived from a small sensing winding on the transformer which provides the required output voltage level information to the transistors.

If the output voltage goes beyond the expected safe level, the base current of the above transistors may be adjusted and reduced by varying the 5K preset, this in turn brings down the conduction of the mosfets, ultimately correcting the output AC to the required limits.

The BD135 transistor along with its base zener provides a stabilized voltage to the associated electronics for sustaining constant PWM output from the relevant ICs.

With IRF1404 as the mosfets, the inverter would be able to generate anywher around 300 to 5000 watts of pure sine wave output.

Many drawbacks and flaws were detected while assessing the above circuit details. The finalized circuit (hopefully) is presented below.

The above circuit may be further enhanced with an automatic load correction feature as shown below. It is implemented by the inclusion of the LED/LDR opto-coupler stage.

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