Modified Sine Wave Inverter Circuit with Waveform Images Verified

The following post discusses a simple modified sine wave inverter circuit with waveform images, confirming the credibility of the design. The concept was designed by me, the waveform being confirmed and submitted by Mr. Robin Peter.

The discussed concept was designed and presented in a few of my previously published posts: 300 watt sine wave inverter circuit, and 556 inverter circuit however since the waveform were not confirmed by me the relevant circuits weren’t completely foolproof.

Now it’s been tested, and waveform verified by Mr. Robin Peter, the procedure revealed one hidden flaw in the design which has been hopefully sorted out here. Let’s go through the following email conversation between me and Mr. Robin Peter.

Hi Swags,

I built the simpler modified sine wave alternative version IC555’s,with no
transistor. I changed some of the values of the resistors and caps
and did not use[D1 2v7,BC557,R3 470ohm]

I joined Pin2&7 of IC 4017 together to get the required
waveform. IC1 produces the 200hz 90% duty cycle pulses(1 image), which
clock IC2 (2-images) and therefore IC3(2 images, min duty cycle & max
Are these the expected results, My concern is that it is a modified sine where you can vary the RMS,not a pure sine

Hi Robin,

Your modified sine wave circuit diagram looks correct but the waveform isn’t, I think we’ll need to
use a separate oscillator stage for clocking the 4017 with frequency fixed at 200Hz,
and increase the frequency of the topmost 555 IC to many kHz,
then check the waveform.


Hi Swagatam

I have attached a new circuit schematic with the changes you suggested along with the resultant wave forms.
What do you think of the PWM waveform,the pulses don’t seem to go all the way down to ground level.

Hi Robin,

That’s great, exactly what i was expecting, so it means a
separate astable for the middle IC 555 must be employed for the intended
results….by the way did you vary the RMS preset and check the
waveforms, please do update by doing so.

So now it looks much better and you can go ahead with the inverter design by connecting the mosfets.

….it’s not reaching the ground due to the diode 0.6V drop, I assume….

Thanks very much


Actually a much easier circuit with similar results as above can be built as discussed in this post:

More Updates from Mr. Robin

Hi Swagatam

I varied the RMS preset and here are the attached waveforms.
I would like to ask you what amplitude of triangle wave can you
apply to pin 5,and how would you synchronise it so that when pin 2 or 7
go + the peak is in the middle

regards Robin

Here’s some better modified sine waveform, maybe the guy’s will understand them easier. It’s up to you whether you publish them.

By the way i took a 10uf cap from pin2 to 10k resistor to .47uf cap
to ground.And the triangular wave looked like this(attatched).Not too
triangular,7v p-p.

I will investigate the 4047 option

cheers Robin

Output Waveform across Transformer Mains Output (220V)

The following images show the various waveform images taken from across the output mains winding of the transformer. Courtesy – Robin Peter

No PWM, no Load

No PWM, with load

With PWM, without load

With PWM, with load

The above image magnified

The above waveform images looked somewhat distorted and not quite like sinewaves.
Adding a 0.45uF/400V capacitor across the output drastically improved the results, as can be witnessed from the following images.

Without load, with PWM ON, capacitor 0.45uF/400v added

With PWM, with load, this looks very much like an authentic sinewaveform.

All the above verification and testing were conducted by Mr. Robin Peters.

More Reports from Mr. Robin

Ok,I did some more testing and experimenting last night and found
that if I increase the batt voltage to 24v the sinewave did not
distort when I increased the duty/cycle.( ok,I’ve regained my
I added that 2200uf cap between c/tapp and ground but that made no difference to the output waveform.
noticed a few things that were taking place,as I increased the D/C the
trafo makes a noisy humming sound(as if a relay is vibrating back and
forth very quickly),The IRFZ44N’s get hot very quickly even with no load
When I remove the cap there seems to be less stress on the
system.The humming noise is not so bad and the Z44n’s don’t get so
hot.[of course no sinewave}

The cap is across the output of the trafo not in series with one leg.
I took (3 different windings) round inductors{I think
they are toriodal} out of a switch-mode power supply.The result was no
improvement in the output wave(no change),The trafo output voltage also dropped.

Adding an automatic load correction feature to the above modified sine wave inverter circuit idea:

The above shown simple ad-on circuit can be used for enabling automatic voltage correction of the inverter output.
The fed voltage across the bridge is rectified and applied to the base of the NPN transistor.
The preset is adjusted such that at no load the output voltage gets settled at the specified normal level.
To be more precise, initially the above preset should be kept at the ground level so that the transistor says switched OFF.
Next, the 10k RMS preset at pin#5 of the PWM 555 IC should be adjusted to generate around 300V at the transformer output.
Finally, the load correction 220K preset should be realigned to bring down the voltage to may be around 230V mark.

Done! Hopefully the above adjustments would be enough for setting up the circuit for the intended automatic load corrections.

The final design might look like this:

Filter Circuit

The following filter circuit can be employed at the output of the above inveter for controlling Harmonics and for enhancing a cleaner sinewave output

More Inputs:

The above design was studied and further improved by Mr Theofanakis, who is also an avid reader of this blog.

The oscilloscope trace depicts the modified waveform of the inverter across the 10k resistor connected at the mains output of the transformer.