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ronald1234512345
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Mains electricity in your home is made with a mechanical windmill, water tubine in a river or steam turbine heated with coal, natual gas or nuclear. Its waveform is a smooth sine-wave at the low frequency of 50Hz or 60Hz.

An inverter operating from a battery can use a sine-wave oscillator and a linear power amplifier but the amplifier will waste 30% to 40% of the battery power by making heat.

An old cheap inverter used a square-wave oscillator driving power transistors or Mosfets.
Its output transistors switch completely on and off therefore do not waste much power making heat. But many electronic products and motor speed controls won't work. It uses a huge heavy expensive low frequency transformer to stepup the voltage.

An inexpensive fairly modern inverter uses a "modified sine-wave" waveform that is a square-wave with two steps in it. Most electronic products work with it but cheap inverters do not have voltage regulation so the voltage is too high when the load is not heavy and/or the battery is fully charged then the voltage is too low when the load is heavy and/or the battery charge is running down.

Good modern inverters use a Pulse-Width-Modulation circuit to synthesize a pure sine-wave by switching at a high frequency. The high frequency allows a small lightweight inexpensive transformer to stepup the voltage. They have voltage regulation and a meter to show the battery voltage and actual power output. The circuit is complicated and the transformer is not sold anywhere.

My electricity is reliable and I do not go in the widerness to do hunting or fishing so I have never used an inverter and I don't need one.

There are two square-wave inverters in our projects section. They do not work.

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The simple schematic for a pure sine wave inverter has acceptable efficiency. It looks to be about 80%.

The same basic circuit is used for a modified sine wave inverter. But there is no voltage output from the transformer most of the time. It would seem better to filter or clamp the output, but it would have poor power transfer or still not be a sine wave.

A pure sine wave inverter might be a little inefficient, but the modified sine wave inverter of the same basic design is not a legitimate power supply. A well designed modified sine wave inverter would need to be purchased.

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Good modern inverters use a Pulse-Width-Modulation circuit to synthesize a pure sine-wave by switching at a high frequency.


Or pulse amplitude modulation. I doubt using circuit components after the output transformer because of the high voltage and power transfer problems. So the output of a good inverter is still only compatible with simple electronic loads.
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You are wrong, Kevin.
PAM wastes a lot of power. PWM does not.
I worked for 15 years with a very high quality Norwegian very large intercom system. It used PAM and was called Pamex. The amplitudes of the audio was sampled at a high frequency and the samples had their own time slots. Each audio channel used one of those time slots. I modified it a little so it met FM broadcast radio station standards (audio frequency response, distortion and signal to noise ratio) so it was used by Canadian airports for their PA. Competitor systems were not even close and sounded awful.

The output of a modern inexpensive pure sine-wave inverter is a pure sine-wave. Their efficiency is about 90% and could be higher with better and more Mosfets because their output Mosfets switch completely on and completely off. The duty-cycle of the pulses, not the voltage determines the average output voltage. The switching frequency is high so the output filter is simple.
Any AC load works from the pure sine-wave inverter if the power from the inverter is high enough.. 

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Could pulse postition modulation more accurately describe the circuit. The filter type circuit following the output transformer cannot be very simple and will reduce power transfer. A square wave will not be output by a transformer but for a short time of the voltage transistion. Also, the voltage that is transfered will be reduced by the same magnitude on the next transistion.

The modified sine wave shown in textbooks is only two square pulses, making it a modified sine wave.

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Could pulse postition modulation more accurately describe the circuit?

No.
The datasheet for a 555 shows that Pulse Position Modulation also changes the frequency while changing the pulse widths. PWM is not supposed to change the frequency.

The filter type circuit following the output transformer cannot be very simple and will reduce power transfer.

No.
The frequency of the PWM pulses is very high so a simple LC 12dB/octave filter smooths the pulses into a pure sine-wave. The loss is very low because the inductor is small and has very low resistance.

The modified sine wave shown in textbooks is only two square pulses, making it a modified sine wave.

I call it a modified square-wave. Its output is full of harmonics.
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The frequency of the PWM pulses is very high so a simple LC 12dB/octave filter smooths the pulses into a pure sine-wave. The loss is very low because the inductor is small and has very low resistance.


So then a good inverter is two pulses followed by a simple filter? If there are many pulses used to comprise a single sine wave cycle, as I would assume the intention of the design to be, the negative transistions of the pulse will negate the positive transistions unless there were more than a simple LC filter.
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So then a good inverter is two pulses followed by a simple filter? If there are many pulses used to comprise a single sine wave cycle, as I would assume the intention of the design to be, the negative transistions of the pulse will negate the positive transistions unless there were more than a simple LC filter.

You don't understand that the waveform of a modified sine-wave or modified square-wave inverter is simply a square-wave with a peak on the positive-going portion and another peak on the negative going portion of a square-wave. The peaks are to make the average peak voltage the same as the peak voltage of a sine-wave. The off-times are to allow the total average AC voltage to be the same as the mains AC voltage.
You cannot filter away the low frequency most significant harmonics of a modified sine-wave.
It is simple to filter away the very high frequency PWM carrier in a pure sine-wave inverter.

Here is the waveform of a 120VAC pure sine-wave (its peak is 170V), a 120VAC modified sine-wave (its peak is at 150V) and a 120VAC square-wave (its peak is at 120V):

post-1706-14279144292611_thumb.png

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  • 1 month later...

Hi ronald1234512345,
Check my article- Power Inverter on http://www.simplecircuitsandprojects.com/circuits/power-inverter.html. I hope it will be helpful as it contains basic information needed to build an inverter by yourself. Of course, you will see inverter circuits that you can use or modify to suit your taste or need on the site.
Good luck

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It looks like a good article.

I like the way you figured out you need to use a modified sinewave and that the peak voltage needs to be the same as the mains. If you use a ready made transformer, you can use 9V-0-9V for 12V input and 18V-0-18V for 24V input.

The only thing I have to pick you up on is you say it's 25% duty cycle which is not quite true. It's really 50% dyty cycle, as current flows when the current is both positive and negative.

Have you considered making a switched mode inverter yet?

You need an isolated DC-DC converter to convert the battery voltage to a higher DC voltage equal to the mains peak voltage, followed by an h-bridge to convert it to AC. This is obviously much harder to do but it has the advantage of being smaller, more compact and efficient than using a bulky transformer.

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@Hero999,
Thanks for the comment. You are right and i was not wrong as well. I only need to make myself clearer: there are two duty cycles in a complete cycle one in each half(+ve & -ve) and each makes 25% of a complete cycle.

Switched mode inverter? yeah, i am aware of its advantages. i have not tried it out as i have not been able to get transistor that can handle that voltage in our local market.

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Switched mode inverter? yeah, i am aware of its advantages. i have not tried it out as i have not been able to get transistor that can handle that voltage in our local market.

The IRF750 springs to mind.
http://www.ic72.com/pdf_file/i/49649.pdf

The hardest part of building a switched mode inverter will be controlling the MOSFETs across the isolation barrier. You can use a pulse transformer for this but it needs to be able to work down to 50Hz.
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