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pure sine wave inverter circuit design

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Topic:

Topic author: tsikscar
Subject: pure sine wave inverter circuit design
Posted on: Mar 08 2011 6:50:41 PM
Message:

Hi all,
I need help with my design. I am designing a pure sine wave inverter to produce 60 volts ac from 12 volts dc. I used a wien-bridge for the oscillator section and calculated the capacitance and resistor values to give me a pure sine wave output @ 51 hz. My problem now is how to proceed from here. I tried connecting a transformer directly to the output of the oscillator circuit and connected a resistor as load for simulation purposes. The amplitude and voltages have gone down.
Also when i was running the oscillator simulation on it's own, it takes some time before it gradually builds up the amplitude until it gets to a pure sine wave, why is this?
Thanks.

Replies:

Reply author: audioguru
Replied on: Mar 08 2011 9:56:32 PM
Message:

A Wien bridge oscillator needs something to stabilize its output amplitude so it doesn't keep rising until it saturates and produces a square-wave instead of a sine-wave. Many years ago an incandescent light bulb was used because its resistance changes with the AC voltage level that drives it but a Jfet is also used.
With the stabilizing circuit then the gain of the circuit can be high so that it gets up to normal amplitude wquickly. Without the stabilizing circuit then the gain is turned down so it takes a long time to reach max level but then the clipping distortion is low.

You are making a linear inverter with an oscillator, a power amplifier and a transformer. They are never made because they waste a lot of power making heat. Manufactured pure sine-wave inverters use pulse-width-modulation at a high frequency so that the power devices switch comletely on and off without making much heat but the duty-cycle of their switching produces a pure sine-wave.

Yout output level drops when loaded because you need an amplifier that has enough power.

Reply author: tsikscar
Replied on: Mar 09 2011 08:50:40 AM
Message:

Thanks very much Audioguru. I have followed your advise to others on various forums and i hope you will also help me.I have attached the wien-bridge oscillator circuit and the multisim simulation results i got. Could you help me with the next stages of the circuit in order to get an ac voltage output of less than 60v and a current not exceeding 2A. The idea is to build a perfectly working low power inverter circuit.
Please may you also point out the mistakes i have made so far?
Thank you.

Reply author: tsikscar
Replied on: Mar 09 2011 09:31:04 AM
Message:

Here are the Circuits Audioguru. I thought it would be an easy attachment. Thank you once again.

Download Attachment: sinewave2.jpg
25.19 KB

Reply author: tsikscar
Replied on: Mar 09 2011 09:50:48 AM
Message:

This is the sine wave i was getting before connecting and loading the transformer.

Download Attachment: sine wave 1.jpg
47.46 KB

Reply author: audioguru
Replied on: Mar 09 2011 1:04:25 PM
Message:

I already replied to you today on another website.
Your simulation results here are too small to be seen. Also, you saved them as a fuzzy JPG file type instead of as a very clear PNG file type.

You have a very weak oscillator that does not have enough output current to drive a transformer. Its max output current is only about 20mA. You need it to feed a power amplifier that drives the transformer. If the power amplifier is analog then it will waste a lot of power making heat. A class-D switching power amplifier uses PWM and will create much less heat and the battery charge will last longer.

Your circuit will be missing a voltage regulator so the voltage will be too high when the battery is fully charged and/or when the load is light. The voltage will be too low when the battery is running down and/or when the load is heavy. Maybe the Jfet stabilizing parts in the oscillator can be used for voltage regulation by sensing attenuated output voltage.

What do you have that will run from only 60VAC? Isn't your mains 230VAC?
I think somewhere you said that 60V is safe. No it is not. If you are damp or sweating then 60VAC will kill you. Its peak voltage is 85V.
In Canada, PA systems in schools are limited to only 25V so that they don't kill kids who are crawling around in the ceiling.

Reply author: tsikscar
Replied on: Mar 09 2011 5:51:30 PM
Message:

Thank you very much Audioguru. I will now work on the power amplifier. Any ideas of implementing the pwm in the amplifier?

Reply author: audioguru
Replied on: Mar 10 2011 2:47:08 PM
Message:

Usually a PWM controller IC like a TL494 makes the PWM in an inverter. Or a microcontroller IC.
The "amplifier" is two or more power Mosfets used as on-off switches. The Mosfets drive a stepup transformer.

Reply author: tsikscar
Replied on: Mar 11 2011 03:41:09 AM
Message:

Audioguru, sorry for sounding so silly. If i use the tl494 do i just feed the output from the oscillator into the input of the ic and the output of the ic into the transformer? Is it possible to use a pulse transformer instead of the Tl494? If so do i connect it as i described? I am relly stuck thats why i am asking questions you may find silly or trivial. Will i need a filter circuit for this?
Thanks for your help so far.

Reply author: audioguru
Replied on: Mar 11 2011 08:37:46 AM
Message:

The TL494 is a high frequency oscillator and it produces the PWM waveform. Your sine-wave generator feeds the TL494 as modulation. The TL494 feeds power Mosfets that drive the transformer. The transformer must have a good enough high frequency response to pass the high frequency PWM signal. The high voltage output of the transformer has the high frequencies removed with a small LC filter.

Simply buy a cheap Chinese pure sine-wave inverter and look at the circuit inside.

Reply author: tsikscar
Replied on: Mar 11 2011 1:47:54 PM
Message:

Thanks Audioguru for the valuable information. I think i know the way now. You have been a massive help.

Reply author: tsikscar
Replied on: Mar 13 2011 12:16:17 PM
Message:

Audioguru, i was looking at the TL494 pins. What is now confusing me is that it has 2 input test pins and i have 1 output from my oscillator. Can you help me with a circuit showing how i should connect my oscillator output to the TL494 and the outputs to the power mosfets. Does it make sense to use a pair of darlingtons as the amplifier driver for the mosfets in the output stage? Thank you once again for your help so far.

Reply author: tsikscar
Replied on: Mar 14 2011 5:26:57 PM
Message:

Audioguru, can you please help me with my request?
Thank you.

Reply author: audioguru
Replied on: Mar 14 2011 11:04:20 PM
Message:

quote:
Originally posted by tsikscar

Audioguru, i was looking at the TL494 pins. What is now confusing me is that it has 2 input test pins and i have 1 output from my oscillator. Can you help me with a circuit showing how i should connect my oscillator output to the TL494 and the outputs to the power mosfets.

I don't know and I do not need to know because my electricity is reliable, I don't go hunting and fishing and I don't go to school anymore. If I want a pure sine-wave inverter then I would buy one because I probably cannot find the special parts.
I have never seen a TL494 IC.

quote:
Does it make sense to use a pair of darlingtons as the amplifier driver for the mosfets in the output stage?

No.
I guess you know nothing about darlington transistors and Mosfets.

A darlington transistor can provide a high output current when it has a low input current. But how much current does the gate of a Mosfet draw??

Reply author: pebe
Replied on: Mar 15 2011 02:31:04 AM
Message:

Here is an application note for the TL494. Perhaps that will help.

http://focus.ti.com/lit/an/slva001d/slva001d.pdf

Reply author: tsikscar
Replied on: Mar 15 2011 3:46:53 PM
Message:

Thank you very much Pebe.

Reply author: tsikscar
Replied on: Mar 26 2011 06:07:44 AM
Message:

Hello people, i am back again with the same problem. My wien-bridge osillator is taking hours to build it's oscillations. i have connected the output of the wien-bridge to the non-inverting pin of a comparator and an output of a triangular generator to the inverting output of the comparator. The problem is with the wien-bridge taking long although i have used a JFET for AGC. Please help me speed up the oscillator. Thanks.

Reply author: pebe
Replied on: Mar 26 2011 09:54:43 AM
Message:

You have got two cascaded RC time constants in the feed back loop from D3 to Q1 base - I can read C5 and C3 but cannot read the resistor numbers (the drawing is fuzzy because you saved it as a .jpg file - you should have saved it as a .gif file).

I don't understand why you have the feedback path R?/C5 from Q1 drain to gate. try removing it so the gate will get an immediate response to changes in the voltage across C3. BTW, C3 is shown with wrong polarity.

What are the values of C3 and its shunt resistor?

Reply author: audioguru
Replied on: Mar 26 2011 10:33:25 AM
Message:

Echo, echo.
I answered this thread a few minutes ago on another website where the schematic is very clear.
It was saved an an awful DOC file instead of as a GIF or PNG file.

I think a copy of the Texas Instruments Sine-wave Oscillators tutorial was posted on the other website so I don't know why this circuit didn't copy it.

Reply author: tsikscar
Replied on: Apr 12 2011 5:16:37 PM
Message:

Hi people, i have a few questions which i know some of you may find annoyingly simple, but hey that's why i am seeking help on these forums.
I have got to a stage on my design where i have a PWM signal from discrete components. Where i am a bit lost is: i now want to drive a resistive load with a pure sine wave. My initial idea(which was not very successful) was to drive a complimentary pair of BJT's as a class B amplifier, then use an LC low pass filter. What i am getting out is just a negative square wave output. My questions are: Can you drive BJT's with a PWM signal? If so how do you connect them in a circuit? My sine wave reference signal is 50Hz, Triangular wave is 1.92KHz and PWM signal is 1.92KHz. Is this frequency enough? I need help on the amplifier circuit using BJT(they are available to me) and filter stage.Any ideas?
Thanks

Reply author: audioguru
Replied on: Apr 12 2011 10:37:31 PM
Message:

Pure sine-wave inverters use a PWM frequency of 20kHz to 900kHz. Then the LC filter is small and inexpensive.
Complementary Mosfets are used as the switching output transistors.

Besides the very low switching frequency you are using, you didn't post your PWM oscillator circuit nor your transistor output circuit so I have no idea what is wrong.

Reply author: tsikscar
Replied on: Apr 19 2011 07:37:20 AM
Message:

Hi Audioguru, Here is my schematic.This is the onlf file format i know how to upload. I have also included anothe circuit i found on these forums which i want to know if it can work on my circuit as an out put stage. Will i need to add or subtract anything. Please help. I want to try and get the prototype up and running by the end of this weeekend.

Thanks.

Download Attachment: untitled.GIF
71.26 KB

Download Attachment: mosfets.GIF
26.74 KB

Reply author: tsikscar
Replied on: Apr 19 2011 07:47:58 AM
Message:

I have resubmitted schematic using a different format now.

Download Attachment: untitled 1.PNG
52.02 KB

Reply author: audioguru
Replied on: Apr 19 2011 08:33:04 AM
Message:

I saw your very fuzzy DOC file on another website.
Did you know that a lousy old 741 opamp is 43 years old and has trouble above only 9kHz? Most opamps made in the past 30 years go perfectly to 100kHz.

In your schematic I cannot see at what DC voltage the inputs of the 741 opamp are biased.

It looks like your PWM carrier is only 2kHz (so it will be very noisy) instead of 20kHz to 200kHz.

Your circuit with Mosfets and an extremly slow LM324 opamp will not work.

Reply author: tsikscar
Replied on: Apr 19 2011 4:04:27 PM
Message:

What can i do Audioguru please?i want to finish this circuit.

Reply author: audioguru
Replied on: Apr 19 2011 8:48:05 PM
Message:

1) Throw away the old 741 opamp and replace it with a proper comparator iC like an LM393 dual comparator.
2) Increase the triangle waveform to 30kHz using a faster opamp like a TL081 or MC34071.
3) The output of the comparator can drive complementary Mosfets that feed the LC filter.

Reply author: tsikscar
Replied on: Apr 20 2011 5:44:34 PM
Message:

Thank you very much Audioguru.

Reply author: tsikscar
Replied on: Apr 24 2011 3:40:37 PM
Message:

Hi Audioguru, i have tried the mosfets in the configuration shown but it seems not to work.I calculated the frequency for 30KHz but on simulating can only get about 8KHz. But being low power i suppose this will have to do? Can you please sent me a circuit that works i can use for the output stage( from comparator into the mosfets/amplifier? Also i have at my disposal 2 BUZ11 mosfets and a TIP 2955/3055 complmentary BJT pair. Can i use them in anyway in the output stage.Does this stage require a transformer? Please help me cross the finishing line. Anymore advice is welcome. Thank you very much, you have been a massive help so far.

Download Attachment: mosfets1.PNG
62.42 KB

Reply author: audioguru
Replied on: Apr 24 2011 5:56:54 PM
Message:

You need to read about how a Mosfet works. Both of your Mosfets are upside down (they don't do anything and their internal diode is conducting all the time) and have nothing driving them.
You also need to read about the LM339 quad comparator or LM393 dual comparator because their output is just the collector of an NPN transistor that pulls down a negative voltage and you must add your own pullup resistor.

I don't use an inverter because my electricity is reliable. I have never seen a schematic of a good pure sine-wave PWM inverter.

Reply author: wasssup1990
Replied on: Apr 25 2011 01:29:09 AM
Message:

John, don't you get sick of repeating yourself? I mean... Every time I come up here it seems like the same thing is happening over and over again. You have a lot of patience with these noobs.

Reply author: tsikscar
Replied on: Apr 25 2011 04:03:20 AM
Message:

It's sad that you feel that way Wassup1990. What Audioguru and a lot of other people are doing on these forums is very much appreciated by many people. We are all not gurus in electronics but through these forums and sometimes making a fool of one's self one will end up understanding much much more of this vast field of electronics.These forums should be for electronics lovers, both experts and novices, i believe.
Audioguru and others, i am not alone in appreciating what you do on these forums. Please don't be discouraged, you are doing a great job.

Reply author: wasssup1990
Replied on: Apr 25 2011 07:47:47 AM
Message:

quote:
Originally posted by tsikscar

It's sad that you feel that way Wassup1990. What Audioguru and a lot of other people are doing on these forums is very much appreciated by many people. We are all not gurus in electronics but through these forums and sometimes making a fool of one's self one will end up understanding much much more of this vast field of electronics.These forums should be for electronics lovers, both experts and novices, i believe.
Audioguru and others, i am not alone in appreciating what you do on these forums. Please don't be discouraged, you are doing a great job.

I agree and doubt he will get discouraged. He knows how much I appriciated his help several years ago when he was helping me on a project. He has a lot of patience. I like electronics as well, but there is a point where I would get tired of being asked the same questions. It just seems that some people who come here are just plain lazy and can't be bothered to look up the information for themself. Audioguru isn't the source of all information. You've got millions of webpages to look at. He also seems to get a little annoyed with some of the new commers here, where I see him repeating himeself. Now does that sound sad, or does it sound more like I am supporting him?

Reply author: tsikscar
Replied on: Apr 25 2011 08:13:12 AM
Message:

You may have a point but sometimes some people find it easier to understand from a person explaining than reading text because they will be responding to specific questions. Some people take longer than others to fully comprehend hence the likes of you may find them annoying.But i don't believe anyone goes out to deliberately annoy people in a public domain like this one. On the contrary they come for whatever assistance they can get, myself included. Hope you understand.

Reply author: wasssup1990
Replied on: Apr 25 2011 08:56:24 AM
Message:

quote:
Originally posted by tsikscar

You may have a point but sometimes some people find it easier to understand from a person explaining than reading text because they will be responding to specific questions. Some people take longer than others to fully comprehend hence the likes of you may find them annoying.But i don't believe anyone goes out to deliberately annoy people in a public domain like this one. On the contrary they come for whatever assistance they can get, myself included. Hope you understand.

Oh well if they are specific questions, then that's fine. But Audioguru himself told you to go and read a book. I mean if you are putting your MOSFETs in upside down then it shows you haven�t bothered to understand the basics of how a MOSFET works. That's something you don't need a forum for.

I haven�t found anyone deliberately annoying people in a public domain, except on YouTube.

I remember when I was doing my Advanced Diploma in Electronics Engineering. I enjoyed helping others as I tended to be so far ahead of everyone else that it was a nice feeling to offer my help when I wasn't busy. However I also know what it's like to be asked the same questions repeatedly. It's friggin annoying!.. but you get a laugh out of it, like I admittedly do here.

Poor Audioguru.

I have to say that the teachers really appreciated my help.

I'm just sticking up for Audioguru, that's all.

I can only offer my help sometimes regarding electronics on this forum because I have other things to do that keep me very busy, but most of the time I am at a computer. I don't have the time to go and do someone else�s research.

Reply author: tsikscar
Replied on: Apr 25 2011 12:41:04 PM
Message:

Fair enough.

Reply author: elective
Replied on: Jun 28 2011 05:57:19 AM
Message:

Dears infact TL494 is not meant for pure sine wave inverter this may use for modified sine wave so dont wast your time, for pure sine wave you need SPWM technique

Reply author: elective
Replied on: Jun 28 2011 06:03:13 AM
Message:

Dears infact TL494 is not meant for pure sine wave inverter this may use for modified sine wave so dont wast your time, for pure sine wave you need SPWM technique

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