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Old 17th July 2009, 07:56 AM   #11
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Default sine wave inverter design

my problem just lies on the coupling tx otherwise I still have hopes of succeeding in one. let powerloss not be a factor.
I do not have a software to show you the cct that I'm working on should i get, i would draw it thanks.




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Old 17th July 2009, 12:22 PM   #12
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Well, do read the product folders on the Linear Tech devices -- a sine inverter will be much less efficient than a pulse width modulated one -- as you seek to reduce the noise in the LLTC devices heat issues start to dominate the economics.

Reading the product folders will get you much further along in not reinventing the wheel.
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Old 17th July 2009, 01:07 PM   #13
star882 is offline star882  United States
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And low frequency (a few kHz) PWM is not very hard to implement. Some microcontrollers have PWM hardware built in, so a complete implementation would use a DDS in software.

I wonder what modifications, if any, would be required to drive the output H bridge of a common modified sine inverter with PWM at a few kHz...
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Old 9th August 2010, 12:42 PM   #14
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bumping the thread.....

I'm planning on making a pure sine wave inverter.

plan is 220V 60Hz. power is 300watts more or less and I will use readily available parts.

so far, my idea is to use a 12V to 300VDC DC-DC converter (no problems there, been making car amp SMPS's for a while now) then use a triangle wave + comparator PWM circuit and feed it from a sine wave oscillator.

here's where my questions start. I'm thinking below 100kHz switching frequency good enough for 60Hz output? probably 50kHz to make things a little easier on layout?

H bridge will probably be needed. it's easier to handle a single 300V rail than a 600V bipolar supply.

planning on useing IRF740 mosfets due to availability and low cost.

I have a few spare MPP toroids that I could use for the output filter. how do you calculate the inductance/capacitance needed for the output filter? its my first time to design a class D based PWM output stage from scratch.
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Old 9th August 2010, 01:55 PM   #15
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all repeating signals are made up of the fundamental and higher order harmonics. square, triangle, very distorted sines etc. If you want to clean up your line voltage. You can take the several hundred dollars that will cost to build or buy an inverter and spend that money on a very good line filter. Not an audiophile golden ears filter with cyro oxygen free silver coated wire and organic materials, but a series of pi filters tuned to 60 hz. There was a series of articles in audio express a while back that dealt with this problem. Basically you want to build a 60hz filter with a high Q. You must include the power transformer inductance in your calculations. If you eliminate/filter out the high harmonics you get a pure sine.
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Old 9th August 2010, 02:44 PM   #16
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sorry, I wasn't clear on my application. I have a 12V 200Ah battery under my workbench and a 40watt and a 75watt solar panels on the roof. I would like to power some bench tools off of it. they don't run that well on my modified sine wave inverter so I want to run a pure sine wave inverter.

I doubt cheap ones would do the work well enough. besides, this is DIY, I want to try build one myself.
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Old 9th August 2010, 03:27 PM   #17
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Karakacha,
You must have a very small power application, otherwise you wouldn't be interested in doing this linear.

You probably shouldn't bother trying to make your own power transformer for line frequencies. It is too effective to buy one, especially below a couple hundred watts. If your application power is above that then linear is arguably a poor choice. I wouldn't use linear in any case unless I actually needed the sine for something. Since all the loads I drive that actually care about supply cleanliness really need DC anyway, a sine inverter is impractical. In most cases it indicates a needless conversion step.

With what are you loading this inverter? What is the power? What is your inverter input voltage?

Once you have all that straight you just need a transformer of right size with the proper primary and secondary RMS voltages. Your supply voltage will need to be over about 1.5 times higher than the primary RMS voltage, if you intend to hold the secondary at rated voltage through a half bridge running in linear mode with any regulation headroom. (.75 for push-pull)

Last edited by Andrew Eckhardt; 9th August 2010 at 03:31 PM.
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Old 10th August 2010, 03:24 PM   #18
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some bench tools only draw 200 watts or less, some draw more than 1KW under load. If your inverter is 100% efficient you will be drawing about 100 amps from your battery. Think of the size of the wires you will need. A 50/60hz sine generator feeding a 1-2kw car stereo driving a transformer backwards might work. You would have to know what the maximum output voltage of the car stereo is ie 2kw mono amplifier driving 2 ohms would provide about 64 volts ac rms, so you would need a 220 volt to 60-65 volt transformer at 1-2kw
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Old 11th August 2010, 02:47 PM   #19
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300 watts should be enough. just a standard mini drill press, maybe a small 62" band saw or a belt sander. they won't be running all at the same time. just one at a time coz I'm the only one that uses them.
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Old 13th August 2010, 02:35 PM   #20
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You may look at the power consumption on your tools. My small belt sander draws 900 watts, small drill press 800 watts, small hand drill 600 watts. Skill saw 1500 watts. Router 1500 watts. Remember volts times amps. Plus more for start up surge.
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