Modified Sine Wave Inverter Circuits using IC 555 and 4017

When an inverter with square wave AC output is modified to generate a crude sinewave AC output, it is called a modified sine wave inverter.

The following article presents interesting modified sine wave inverter designs with exhaustive descriptions regarding its construction procedure, circuit diagram, waveform output and detailed parts lists. The designs are intended for learning and building experimental projects by engineers and students.

Here I have explained different varieties of modified designs ranging from a modest 100 watt to a massive 3 Kva power output model.

How Modified Inverters Work

Folks who are new to electronics may get a bit confused regarding the difference between a square wave and a modified square wave inverter. It may be understood through the following brief explanation:

As we all know an inverter will always generate an alternating current (AC) similar to our domestic AC line voltage so that it can replace it during power failures. An AC in simple words is basically a rise and fall of voltage of a particular magnitude.

However, ideally this AC is supposed to as close as possible to a sinewave as shown below:

Basic Difference between Sine waveform and Square Waveform

This rise and fall of voltage happens at a particular rate i.e. at a particular number of times per second, known as its frequency. So for example a 50 Hz AC means 50 cycles or 50 ups and downs of a particular voltage in one second.

In a sine wave AC as found in our normal domestic mains outlet the above rise and fall of voltage is in the form of a sinusoidal curve, i.e. its pattern gradually varies with time and thus is not sudden or abrupt. Such smooth transitions in the AC waveform becomes very suitable and a recommended type of supply for the many common electronic gadgets like TVs , music systems , Refrigerators, motors etc.

However, in a square wave pattern the voltage ups and downs are instant and sudden. Such immediate rise and fall of potential creates sharp spikes at the edges of each wave and thus becomes very undesirable and unsuitable for sophisticated electronic equipment. Therefore it is always dangerous to operate them through a Square weave inverter supply.

Modified Waveform

In a modified square wave design as shown above, the square waveform shape basically remains the same but the size of each section of the wave-form is appropriately dimensioned so that its average value matches closely to an AC waveform’s average value.

As you can see there's a proportionate amount of gap or null areas between each square blocks, these gaps ultimately help to shape up these square waves into sinewave like output (albeit crudely).

And what is responsible for adjusting these dimensioned square waves into sinewave like features? Well, it is the inherent characteristic of the transformer's magnetic induction which effectively carve the "dead time" transitions between the square wave blocks into a sinewave looking waves, as shown below:

In all the designs I have explained below we try to implement this theory and ensure that the RMS value of the square waves are appropriate controlled by chopping the 330V peaks into 220V modified RMS. The same can be applied for 120V AC by chopping down the 160 peaks.

How to Calculate through Easy Formulas

If you are interested to know how to calculate the above modified waveform so that it results in an almost ideal replication of a sinewave, then please refer to the following post for the complete tutorial:

Calculate Modified Square Wave RMS Sine Equivalent Value

Using IC 4017

Let's investigate the first modified inverter design which is rather simple and uses a single IC 4017 for processing the required modified waveform.

If you are looking for an easy to build modified sine wave power inverter circuit, then perhaps the following concept will interest you. It looks astonishingly simple and low cost with an output that’s to a very extent is comparable with other more sophisticated sine wave counterparts.

We know that when a clock input is applied to its pin #14, the IC produces a shifting cycle logic high pulses through its 10 output pins.

Looking at the circuit diagram we find that the pin outs of the IC are terminated to supply the base of the output transistors such that they conduct after every alternate output pulse from the IC.

This happens simply because the bases of the transistors are connected alternately to the IC pin outs and the intermediate pin-out connections are just eliminated or kept open.

The transformer windings which are connected to the transistor’s collector respond to the alternate transistor switching and produce a stepped up AC at its output having a waveform exactly as shown in the diagram.

The output of this Modified sine wave power inverter is although not quite comparable to the output of a pure sine wave inverter but definitely will be far better than that of an ordinary square wave inverter. Moreover the idea is very easy and cheap to build.  

WARNING: PLEASE CONNECT PROTECTION DIODES ACROSS THE COLLECTOR EMITTER OF THE TIP35 TRANSISTOR (CATHODE TO COLLECTOR, ANODE TO EMITTER)

UPDATE: As per the Calculations presented in the this article, the IC 4017 output pins could be ideally configured for achieving an impressive looking modified sinewave inverter.

The modified image can be witnessed below:

WARNING: PLEASE CONNECT PROTECTION DIODES ACROSS THE COLLECTOR EMITTER OF THE TIP35 TRANSISTOR (CATHODE TO COLLECTOR, ANODE TO EMITTER)

A MOSFET version of the above design can be witnessed in the following diagram. Using MOSFETs IRF3205 can allow the inverter to handle above 400 watts.

Video Demo:

Minimum Specifications

• Input: 12V from Lead Acid Battery, for example 12V 7Ah battery
• Output: 220V or 120V depending on transformer rating
• Waveform : Modified sinewave

Feedback from one of the dedicated viewers of this blog, Ms Sarah

Hello Swagatam,

This is what I obtained from the output of IC2 post resistors R4 and R5. As I earlier said I expected to have a bipolar wave. One in positive and the other in negative . to simulate an ac wave cycle. I hope this picture will help. I need a way forward please.

Thanks

My Reply:

Hello Sarah,

The IC outputs will not show bipolar waves since the signals from these outputs are intended for identical N type transistors and from a single supply....it's the transformer which is responsible for creating the bipolar wave at its output since it's configured with a push-pull topology using a center tap ....so what you are seeing across R4 and R5 are correct waveform. Please check the waveform at the output of the transformer for verifying the bipolar nature of the waveform..

Using only 3 IC 555

The following section discusses best 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.

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 D/C)Are these the expected results, My concern is that it is a modified sine where you can vary the

RMS,not a pure sine

Regards

Robin 

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.Regards.

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.

Regards

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:https://www.homemade-circuits.com/2013/04/how-to-modify-square-wave-inverter-into.html

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, and with an output capacitor, 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 confidence)I added that 2200uf cap between c/tapp and ground but that made no difference to the output waveform.

I 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 loadWhen 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.

Automatic Feedack

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.

The above modified inverter design by Theofanakis inverter was tested and approved by one of the avid followers of this blog, Mr. Odon. The following test images by Odon confirm the sinewave nature of the above inverter circuit.  

Heavy Duty 3Kva Modified Inverter Design

The below explained content investigates a 3kva sine wave inverter circuit prototype made by Mr. Marcelin using only BJTs instead of the conventional mosfets. The PWM control circuit was designed by me.

In one of my previous posts I have explained a 555 pure sine wave equivalent inverter circuit, which was collectively designed  by Mr.Marcelin and me.

How the Circuit was Built

In this design I have used strong cables to sustain the high currents, I used sections of 70 mm2, or more smaller sections in parallel. 3 KVA transformer is actually as solid weighs 35 kg. Dimensions and volume is not a drawback for me. Photos attached to the transformer and installation in progress.

The following assembly nearing completion, based on the 555 (SA 555) and CD 4017

On my first try, with mosfets, earlier this year, I used IRL 1404 which Vdss is 40 volts. In my opinion insufficient voltage. It would be better to use mosfets with a Vdss at least equal to or greater than 250 volts.

In this new installation, I foresee two diodes on the transformer windings.

There will also be a fan for cooling.

TIP 35 will be mounted by 10 in each branch, like this:

Complete Prototype Images

Finalized 3 KVA Inverter Circuit

The final circuit design of the 3 kva modified sine wave inverter should look like this:

Parts List

All resistors are 1/4 watt 5%, unless specified.

• 100 Ohms - 2nos (value can be between 100 ohm and 1K)
• 1K - 2nos
• 470 ohms - 1no (can be any value upto 1K)
• 2K2 - 1nos (slightly higher value will also work)
• 180K preset - 2nos (any value between 200K and 330K will work)
• 10K preset - 1no (please 1k preset instead for better outcome)
• 10 Ohm 5 watt - 29nos

Capacitors

• 10nF - 2nos
• 5nF - 1no
• 50nF - 1no
• 1uF/25V - 1no

Semiconductors

• 2.7V zener diode - 1no (upto 4.7V can be used)
• 1N4148 - 2nos
• 6A4 diode - 2nos (near transformer)
• IC NE555 - 3 nos
• IC 4017 - 1no
• TIP142 - 2nos
• TIP35C - 20 nos

• Transformer 9-0-9V 350 amps or 48-0-48V / 60 amps
• Battery 12V / 3000 Ah, or 48V 600 Ah

If 48V supply is used then make sure to regulate it to 12V for the IC stages, and supply the 48V only to center tap of the transformer.

How to Safeguard the Transistors

Note: In order to safeguard the transistors from a thermal runaway, mount the individual channels over common heatsinks, meaning use a long single finned heatsink for the upper transistor array, and another similar single common heatsink for the lower transistor array.

Mica isolation would be fortunately not required since the collectors are joined together, and the body being the collector would get effectively connected through the heatsink itself. This would actually save a lot of hard work.

In order to obtain maximum power efficiency, the following output stage is recommended by me, and must be employed with the above explained PWM and 4017 stages.

Circuit Diagram

Note: Mount all the upper TIP36 over a larger finned common heatsink, DO NOT use mica isolator while implementing this.

The same must be done with the lower TIP36 arrays.

But make sure these two heatsinks never touch each other.

The TIP142 transistors must be mounted on separate individual large finned hearsinks.