Simple Modified Sine Wave Inverter Circuit

In this article we will build a modified sine wave inverter circuit utilizing IC 555 and IC 4017. Modified sine wave inverter occasionally also referred to as modified square wave is an higher segment inverter design, over a simple square wave type.  We shall take a look at the proposed inverter circuit stage by stage.

We will find out:

•    What is modified sine wave inverter?
•    Difference between square wave and modified sine wave inverter.
•    Circuit Diagram of Modified Sine Wave Inverter.
•    In Depth Analysis of individual stages.
•    Prototype images.

Exactly what is Modified Sine Wave Inverter?

Modified sine wave inverter is actually a category of power inverters whose wave form and power quality is more advanced than standard square wave inverter. The modified sine wave inverter’s wave form continue to be at zero volt for a while prior to any change in polarity.

Difference between Square wave and modified sine wave:

Below shown waveform is assessment between square wave and modified sine wave:

Modified Sine Wave

The waveform in green colour signifies square wave and the red color symbolizes the modified sine wave.

The square wave varies its polarity suddenly, however the modified sine wave continues to be at an intermediate point (zero volts) prior to any kind of variation in polarity.

Simply by having the waveform at zero point for a few milliseconds it is going to minimize noise at the output, however modified sine wave should not be compared with pure sine wave.

Now you have understood the core difference between square wave and modified sine wave.

Circuit Diagram for Modified Sine Wave Inverter:

Reverse Engineering of a modified inverter circuit:

The above inverter produces modified sine wave at the output. The proposed circuit can be divided into the following stages:

•    Oscillator (IC 555)
•    Wave shaping circuit (IC 4017)
•    Switch circuit / MOSFET stage
•    Transformer / step-up stage

Oscillator Stage:

The oscillator circuit generates the required clock signal for this inverter, and we may consider this stage as the core section of the inverter.

The 555 timer is surely an classic IC and we are applying it as an oscillator. Our necessity is to generate 200Hz square wave at 50% duty cycle which is to be divided by 4 by IC 4017 to obtain 50Hz AC modified sine wave output.

IC 555 is linked to an RC network of a couple of 36K ohm resistors and another 0.1 uF capacitor and a diode is attached around pins 6 and 7 to obtain 50% duty cycle from IC 555.

We are able to determine the frequency of IC 555 using a diode across pin 6 and 7 through the following equation:


Frequency = 1.44 / (R1 +R2) * C
•    Duty Cycle = R1 / (R1 + R2)

By executing the values in the circuit diagram in the above formula we reach the following data,

Frequency = 1.44 / (36000 + 36000) x 0.1 x 10^-6
Frequency = 200 Hz
Duty Cycle = 36000 / (36000 + 36000) = 0.5
Duty Cycle = 0.5 x 100% = 50%

Let’s check this on the scope:

As we are able to see on the oscilloscope that mathematics did comply, we are obtaining values in close proximity to our specifications. The output values are somewhat off with a little margin because of tolerance of the components.

Important Notice: You should never neglect the 0.1 uF capacitor at pin #5 of IC 555 that is helping to keep stability from external noises. Not including  may alter the frequency and duty cycle from anything you might have calculated.   
Waveform shaping stage:

I'm implementing the 50Hz, 50% duty cycle signal to IC 4017 that is a decade counter, which could generate the modified sinewave, let us check it out in more detail.

IC 4017 pin diagram:

The IC 4017 has 10 output pins and one input pin (14). When we put on clock signal to input pin, each one of the 10 output pin tend to get high in a serially incrementing manner.

As an example when we apply 5 clock pulses, the 5th output could get HIGH while remaining input pin will get LOW; when we clock 3 input pulses the 3rd subsequent pin will receive HIGH and remaining pins will continue to be LOW.

Pin 15 could be utilized for resetting the count number back to zero when we do not wish to work with all the 10 outputs. For instance:

In this article we are working with just 4 output pins pin #3, #2, #4, #7 which are outputs Q0, Q1, Q2, Q3. The pin #15 is linked to output number Q4 or pin #10, hence the output pin #3, #2, #4, and #7 will remain active, other 6 output pins continue to be inactive.

Now the 200Hz signal is given to clock pin of the IC 4017, we are applying just the two alternative outputs Q0/pin #3 and Q2/pin #4 and the other a couple of pins are retained unconnected.

Now by hooking up the oscilloscope to pins 3 and 4 we are able to see this waveform:

As we are able to observe, we are getting modified sine wave across these two pins which are finally applied to MOSFETs for current boosting.

Values of the generated wave form:

Readings of Modified Sine Wave

From the oscilloscope analysis we are able to observe that we achieved results close to the 50Hz and 50 % duty cycle prerequisite, this really is adequate for a low power inverter.

MOSFET Stage:

So ar  we have 50 Hz, 50% duty cycle signal in modified sine waveform, the output from the IC is actually weak to deal with the transformer, and we have to reinforce the signal to ensure that we are able to drive the transformer.

To get this done we could get assistance from MOSFETs which could move huge current to transformer’s winding with low power signal from the IC. We are making use of IRF540N which is sufficient for handing 200 watts of load.

MOSFET mounted on a heat sink: