Transformerless Modified Sine wave Inverter Circuit

In this post we are going to construct a transformerless inverter circuit which can be power via solar panels and also using batteries. The proposed transformerless inverter design is a modified sine wave type which is better than square wave counterpart. We will learn the different stages of this inverter in-depth.

We will see:

  • Full Circuit Diagram.
  • Stages of this Transformerless inverter circuit.
  • Difference between Square wave and Modified Sine Wave.
  • How to test this Transformerless inverter circuit.
  • Advantage and Disadvantage of this inverter circuit.

Transformerless Inverter Circuit Diagram:

Transfomerless Inverter Circuit
Transfomerless Inverter Circuit

Circuit Description:

The circuit consists of commonly available components like IC 555 and IC 4017 and some passive components like resistors and capacitors. The IC 555 and IC 4017 constitute the oscillator stage which outputs modified sine wave. The MOSFETs does the job of switching the high voltage which is configured as H-bridge. The MOSFETs are rated above 400V which can drive 230V load without any issue. Since this inverter doesn’t boast a transformer unlike traditional inverters which can step-up the low voltage, we have to apply 310 VDC to the MOSFET stage.  The MOSFETs will convert the high voltage DC to nominal 230 VAC, the waveform and frequency is determined by the oscillator stage.

Now let’s learn how each stage of this inverter functions in-depth:

The proposed inverter has 3 stages:        

  • Oscillator stage.
  • MOSFET / Switching Stage.
  • Power Input Stage.

These three stages will determine the output frequency, voltage and the quality (wave form).

Oscillator Stage:

The frequency and the wave form are determined in this stage. The IC 555 and IC 4017 combinedly make the oscillator stage and IC 555 is the heart of this project as it generates the pulse for the inverter. The IC 555 generates square pulse at 200Hz at pin #3; the frequency is determined by the network of RC component connected to IC 555. We can also see a diode connected across pin #6 and #7; this is for generating pulse at 50% duty cycle.

The role of IC 4017 is to convert the square wave generated by IC 555 to 50 Hz modified sine wave. Let’s have a look at how a modified sine wave looks like compare to square wave.

Square wave:

Square wave
Square wave

Modified Sine wave:

Modified Sine wave
Modified Sine wave

The modified sine wave is technically a better waveform than square wave and it is less noisy. Modified sine wave is used in many cheap commercially made inverters to power home appliances, but this doesn’t mean that modified sine wave is perfect.

Calculation of IC 555:

To get the desired frequency of 50Hz, the IC 555 has to generate 200Hz and the IC 4017 must divide the wave from by four. The IC 4017 can divide the wave form without any external components. To get 200Hz and 50% duty cycle from IC 555, we can use the frequency formula:

F = 1.44 / (R1 + R2) x C

F = 1.44 / (36000 + 36000) x 0.1 x 10^-6

F = 200Hz (exactly)

So, using two 36K resistor and 0.1 microfarad capacitor we got precisely 200Hz.

Let’s check the same using an oscilloscope at pin #3 of IC 555:

IC 555 output
IC 555 output

We got 197Hz and 52% duty cycle which is close enough; the slight variation is due to the tolerance of the resistors and capacitor.

Now let’s convert this square wave into modified sine wave using IC 4017.

Pin configuration of IC 4017:

IC 4017 Pin Diagram
IC 4017 Pin Diagram

IC 4017 is a decade counter which can count from Q0 to Q 9 (10 outputs). For successive pulses applied at pin #14, successive outputs (Q0 to Q9) gets high. Here we don’t need all the 10 outputs, we just need 4 outputs so that we can divide the 200Hz input to 50Hz output.

To enable only four outputs we have connected the reset pin #15 to pin #10 (Q4 – 5th output), by doing so we are disabling outputs Q4 to Q9. Now the outputs get HIGH and LOW from Q0 to Q3 (First four outputs).

Out of the four outputs we are utilizing only 2 outputs which give the modified sine-wave shape. In the wave form we can see couple of pauses at 0V twice in a cycle this is because of the unused remaining two outputs which ultimately gives the required waveform.

Now let’s hook the oscilloscope and see the waveform at the outputs of pin# 3 and #4 of IC 4017:

Readings of Modified Sine Wave
Readings of Modified Sine Wave

As we can see, we got the exact anticipated waveform at 50Hz and 50% duty cycle. Now this waveform need to be amplified, to do this in the next stage we are using some powerful MOSFETs which will drive the connected 230VAC gadgets.

MOSFET / Switching stage:

In this stage the generated weak signal from the oscillator gets amplified. We are using 4 MOSFETs, two N-channels and two P-channels in H-bridge configuration.

H-bridge is the best way to alternate the polarity (converting DC to AC) across the connected load. To do this we need to switch the four MOSFETs in the following sequence.

H bridge
H bridge

Now let’s consider the diagram on left side, the S1 and S3 (MOSFETs) are closed, now look at the polarity across the load, on left we have +Ve and on right we have -Ve. Now let’s open these two switches (MOSFETs) and close S2 and S4, now the polarity cross the load is reversed (diagram on the right side). To get 0V across the load, we just open all the MOSFETs, this sequence will repeat.

Alternate H-bridge configuration:

You can also build an H-bridge using 4 N-channel MOSFET with bootstrapping.

High Voltage H-Bridge Circuit
High Voltage H-Bridge Circuit

Specifications of MOSFETs:

IRF740 N-Channel MOSFET:

  • Voltage Drain to Source (Vds): 400V
  • Voltage Gate to Source (Vgs): +/-20V (Max)
  • Continues Drain Current: 10A (Continuous)

IXTP10P50P P-Channel MOSFET:

  • Voltage Drain to Source (Vds): -500V
  • Voltage Gate to Source (Vgs): +/-20V (Max continuous)
  • Continues Drain Current: -10A (continuous)

The MOSFETs used here can handle voltage as high as 400VDC and 310VDC from solar panel should not be an issue for these MOSFETs. If you could not find MOSFETs of these part numbers, you may utilize any other N and P channel MOSFETs with similar specifications.

Power Source:

The power source we are going to use is arrays of solar panels, the combined series output voltage around of 310 VDC. You can also use bank of batteries to get similar voltage level.

Solar panel
Solar panel

You need to get help of professionals to install solar panels. In real life solar panels are not directly connected to an inverter, they are installed with batteries with battery management system. 

Why do I need around 310 VDC input to get 230 VAC output?

The modified sine wave need 310V peaks to get 230VAC RMS output. Ok, what do I mean by that? If you hook an oscilloscope to AC mains of our home, you will see that peak to peak voltage is 325VAC and not 230VAC.

The 230VAC is called the RMS (root mean square) and 325V is called peak to peak voltage. The RMS is the average voltage, since the voltage is varying with time from 0 to 325V and vice-versa; we cannot conclude a voltage measurement at any given instant. So, we are measuring the average voltage of the AC sinusoidal cycle, our multimeters and volt meters designed to measure RMS.

This is same for modified sine wave; we need around 310V peaks at 50Hz to get average of 230VAC.

But in the case of square wave the RMS = Peak voltage, meaning if we apply 230 VDC to the input we will get 230 VAC RMS. You can learn about such square wave transformerless inverter here.

How to Test this Transformerless Inverter Circuit:

Once you complete the construction of this inverter circuit, you need to connect the oscillator to a 12V power source. You also need a high voltage DC source which can output 310 VDC, you may use high voltage DC lab power supply or you can connect 34 nine-volt batteries in series to obtain 310 VDC as a cheap testing solution.

Battery for transformerless inverter
Battery for transformerless inverter

Connect a tungsten bulb of 40 watt across the output terminals of the inverter, power the oscillator first and connect 310 VDC to MOSFETs second. As soon as you connect the high voltage DC to MOSFETs, the bulb should glow instantly at full brightness. Now your inverter is working fine.

How much power can it deliver?

From our estimation, it can deliver about 1000 watt or 1KW with MOSFETs cooled adequately. You need to apply 310VDC at 4A or above from solar panel / batteries to this inverter.

Pros of this inverter:

  • Simple design with reasonable output quality.
  • High Efficiency as the loss due a transformer doesn’t exist.
  • Dirt cheap price to construct and zero maintenance for the inverter.
  • It can power most of the home appliances.

Cons of this inverter:

  • Need High voltage DC as input.
  • Modified sine wave is not suitable for medical equipment.
  • Installing solar panels and battery banks are expensive.
  • No automatic voltage regulation is present to stabilize the output AC.

If you have any questions regarding this project, comment your questions, you can anticipate a guaranteed reply from us.