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12 volts to 120 volts the easy way?

  1. Mar 28, 2015 #1
    Living in a remote desert location, I have to generate my own power from solar. Big loads require big inverters, and for me sine wave is a must, not modifieed sine. Higher voltage inverters allow more batteries in a string meaning more storrage capacity in that string and since multiple parallel strings don't charge well, a large capacity single battery or string is better than smaller parallel arrays.

    Big sine wave inverters are expensive. So while considering the problem, I hit upon what at first glance seems to be a simple and effective solution. Namely, put sufficient batteries in series to reach the desired a/c voltage and then using a simple and inexpensive waveform generator, drive a large power transistor or series of them to provide the wattage capacity desired. It should be clean enough as far as power goes to be usable without much waveform conditioning I would think.

    With 12 volt batteries, for example, 10 of them when fully charged will be at 126 volts and when drained still be at 108 volts, nearly 110 volts, though one wouldn't want to let them get that far drained. Probably cut out when they reach 11.2 volts or so, or 112 volts on a chain of 10. With my 24 volt inverter, the maximum number of 12 volt batteries I can put in a string is 2, with 6 volt batteries, 4. With this method I can use ten 12 volt batteries or 20 6 volt!

    OK so what am I missing? I get a sneaking suspicion that someone is going to mention the down sidde that I'm overlooking. Ok so have at it! Is this a good solid solution or should it be filed in file 13?
     
  2. jcsd
  3. Mar 28, 2015 #2

    nsaspook

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    Lets start with a very simplistic circuit.
    First to generate a good sine wave directly from the batteries the total voltage will need to be 2* the peak voltage of the output AC waveform. You then need to isolate the DC average voltage from the AC voltage with a large capacitor to get that pure AC sine wave output. This means using a class A power circuit.

    It might work but will be very inefficient unless you also need a space heater.
    amplifier19.gif
     
    Last edited: Mar 28, 2015
  4. Mar 28, 2015 #3
    Beginner engineering student here. Have you thought about current?

    Wouldn't a 120V supply supply different current than the current from a 12 volt supply?
     
  5. Mar 28, 2015 #4

    phinds

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    Yes, but so what? What's your point? The actual current drawn depends on the voltage source and the load.
     
  6. Mar 28, 2015 #5
    Potential is at 120 volts already, ten 12 volt batteries in series, for example. Simply turning an scr on and off 120 times per second reversing polarity every other on cycle gives me a 60 hz square wave, which is what a lot of cheap inverters actually produce. Seems a simple capacitance circuit might shape the square wave into the sine. Or maybe someone makes a cheap square wave to sine wave converter. I mentioned the power transisttors or maybe power FETs as just a way to make the low power of my cheap waveform generator output the power as a sine wave. In other words use the low power sine wave generator to drive the base of the transistor. The load will determine the current drawn through the transistor or whatever.
     
    Last edited: Mar 28, 2015
  7. Mar 28, 2015 #6

    phinds

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    This method will not produce what is normally mean by "120V" because that is an RMS value whereas you are talking about peak value. Reread nsaspook's post #2
     
  8. Mar 28, 2015 #7
    Sorry I was thinking about 240 when I was writing 120. 120 is 60 volts peak, 120 volts peak to peak. But that's fine then 240 volts a/c from the 10 batteries. I guess I am most interested if there is a relatively simple way to do this that isn't too ineffficient. No I don't need a space heater in the summer in Arizona.
     
  9. Mar 28, 2015 #8

    phinds

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    You need to check your math.
     
  10. Mar 28, 2015 #9

    nsaspook

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    There really is no efficient way to directly generate the sine wave directly from battery voltage. Using a push-pull transformer solves the 2* peak voltage problem but if you stick with direct analog generation for the pure sine wave you will be stuck at <50% efficiency. A efficient device is a complex device as this project demonstrates.

    https://www.wpi.edu/Pubs/E-project/Available/E-project-042507-092653/unrestricted/MQP_D_1_2.pdf
     
  11. Mar 29, 2015 #10
    Simple way to get around the RMS vs. peak voltage problem: add 4 more batteries. That brings you to a total of 168 volts, which is really close to 170 volts, which is the peak voltage out of your standard 120 volt wall socket. With that out of the way, the only other problem I see with this is that you're assuming the transistors will behave in a completely linear manner in response to switching voltage. I have no idea if that's true, but I doubt it. You better check that out before you blow some appliance up with a malformed waveform.
     
  12. Mar 29, 2015 #11

    jim hardy

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    i dont know of any household appliances that are picky about sinewave purity.
    What is your thinking?
    I've seen a square wave inverter with just a third harmonic filter, a brute force L-C series resonant circuit tuned to 180 hz , but the waveform is still rough...


    Mr Spook's link is a most interesting approach . I never saw that triangle wave - comparator trick before in that application.

    Another method is here...
    http://www.tinaja.com/glib/msinexec.pdf
    sort of a a twist on Fourier
    synthesize a sinewave as a sum of square wave harmonics

    old jim
     
    Last edited: Mar 29, 2015
  13. Mar 29, 2015 #12
    How big are your loads and why do you need such a clean voltage, (your not Blofeld setting up a secret base are you). Have you considered an MG setup?
     
  14. Mar 29, 2015 #13

    sophiecentaur

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    This is a statement that I have to question. What is so special about a 'pure' sine wave? If you consider that 'the Mains' would do your your purpose then I suggest you look at the actual waveform the supply company gets to your house. It is only, ever, a rough approximation to a sine wave and it varies as other loads are added and removed. So, if you reconsidered your requirement, life could be a lot easier.
    Alternatively, you could perhaps modify the equipment you are supplying with the 120V AC and make it less fussy. That could be a much easier way through this problem of yours.
     
  15. Mar 29, 2015 #14

    Svein

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  16. Mar 29, 2015 #15

    sophiecentaur

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  17. Mar 29, 2015 #16

    rollingstein

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    Do motors run hotter? I thought they did. Not sure.
     
  18. Mar 30, 2015 #17

    Svein

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    I just wanted to draw attention to it. I am repeatedly getting warnings for being too close to the answers...
     
  19. Mar 30, 2015 #18
    What about using a Boost Converter or a cascade of these to get your voltage really high? Then build the inverter.
     
  20. Mar 30, 2015 #19

    sophiecentaur

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    A few words to help direct the reader to the appropriate section can help without 'giving away' anything. But, in any case, it was not a homework type question so it deserves loads of helpful answers.

    Possibly, but that's no reason to have to produce a 'perfect' sine wave. (How perfect does that mean, in any case?)
    I asked the question about required quality of sine wave because it is the first thing an Engineer should do - i.e. define the specific problem rather than come in half way through with an answer that takes ones fancy.
     
  21. Mar 30, 2015 #20

    sophiecentaur

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    What would be the point of changing the input DC voltage when a transformer will do the job (probably) cheaper and more efficiently, once you have your AC?
     
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