Modifed Sine Wave Inverter

Discussion in 'The Projects Forum' started by aliman1, Dec 8, 2010.

  1. aliman1

    Thread Starter Member

    Mar 22, 2008
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    Hi everyone, I have built a Square wave DC/AC Inverter. It runs at 60 Hz, 130V but it's only a Square Wave ,so I don't dare run any delicate electronics on it, just lights. My electronic skills are at the beginner/intermediate level, I can build circuits from schematics and breadboard with some proficiency.
    I have attached two diagrams of the square wave circuit which drives a basic Mosfet Output stage. A very stable Square wave is generated by the 4069 Cmos Hex Inverter then fed to two pairs of High current Mosfets. This combination works Great!
    I wanted to improve on the Inverter by adding a PWM circuit to achieve a Modified Sine Wave so I can run virtually anything on it. I have also added a schematic of a PWM circuit I found on the Internet. Its an ingenious design using Two cascaded 4017 Decade counters, achieving frequency division and a very nice wave form, also a diagram of that.
    I first built the PWM circuit on a Breadboard and verified that it does produce the specified wave form.
    Then I built the circuit onto a PCB. In doing so I basically put the PWM circuit just after the Output of PIN #2 of the 4069, grounded out PIN #3, then put the Buffer Transistors TR1 & TR2 , 10K & 1K Bias Resistors to the end of the Diode Junction Outputs.
    I had to modify the Output of Square wave generator so that it produced a Frequency of 1.086 KHz ,this frequency is sent to the Two 4017's which have Ten Outputs each for a total of twenty, which act as Frequency Dividers. Theorecticaly a Input frequency of 1200 Hz would give you a final Output frequency of 60 Hz. For some reason in my actual circuit, I need an Input frequency of 1086 Hz to have a final Output frequency 60 Hz.
    I have included a screen shot of the wave form captured on my Laptop Oscilloscope.
    Now the BIG problem is that when I remove my Original Square wave generator and replace it with the New PWM Square wave circuit in place of it, when I turn the Inverter ON, it blows a Mosfet in the Output stage, and I have to quickly turn the Inverter OFF. This is what is confusing me. I know for a fact that the square wave circuit works and so does my Output stage.
    I can't figure out Why I keep destroying Mosfets, the circuit should work.
    Each of the 4017's fires in tandem, first IC1 then IC2. I've tested this with two LED's, one at each of the Diode Junctions and confirmed that they do indeed fire in tandem, not at the same time.
    I know theres someone out there that can help me.

    Any help would be Great!

    Sincerely Aliman:)
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I don't see a MOSFET in your schematics.

    Are you using the "Magic Sine Wave" PWM?
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You need to probe this with the 'scope. I dearly hope it has two channels. They waveforms from the PWM oscillator may be over lapping or just coincident with each other. Switching a fet on before the other switches off.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    An "iffy" circuit at best.

    The 4017 reset inputs aren't Schmitt triggers. On power-up it would be very easy for the 4017's to be out of sync with each other, causing problems.

    4017s are not synchronous internally; they're ripple counters. The higher in the count (from O1 to O9) cycle you get, the greater the propagation delay. This variation in propagation delay may or may not be significant due to the relatively low speed of the circuit.

    I see no benefit in this circuit over the plain square wave version. There are plenty of disadvantages; the added complexity leaves plenty of opportunities for failures; the MOSFETs will spend much more time in the linear region due to the higher switching rate, and timing requirements will be more critical while introducing non-synchronous components - and the output will still not resemble anything close to a sine wave.
     
  5. aliman1

    Thread Starter Member

    Mar 22, 2008
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    lowcost800wmodifiedsine.gif Thanks for everyone's help. So what your saying SgtWookie is that this circuit does not work. I was thinking of adding a Base to Ground Resistor.
    I tested the current draw of my original complete square wave Inverter and it draws 2 Amps, now when I replace the square wave circuit with the PWM circuit it draws 14 Amps then blows a Mosfet. I was going to place a 12V sealed beam Car headlight in series with the positive post of the battery then connect my Amp meter in series with the headlight, then the Inverter after the Amp meter, and add a 10K Dual Pot across the Bases to Grounds on the buffer transistors, and adjust the Pot until the Amp meter shows 2 amps.
    Do you think this will work, or am I just wasting my time.
    SgtWookie you said that this wave form is not much better then the square wave, can you please explain ?
    I've also added schematic of a different Inverter design, can you please tell me what you think of it ?

    Cheers Aliman:)
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    No, I said it was "iffy at best." It's not going to produce anything like the sine wave they overlaid on the output.

    I think your circuit has an imbalance of current in the primary of the transformer. This can lead to a phenomenon known as "flux walking". You wind up with a large DC current in one direction, that eventually blows the MOSFETs.

    Well, with only 2A current in the primary, you would have about 24 Watts of power. That's not much; about 200mA RMS at 120vac.

    You want just 24 Watts output?

    I'm leaning towards the "wasting time" suggestion.

    It's not better; it's worse. The possible power out has been reduced with no benefit and with considerably increased complexity; the output waveform will not resemble a sine wave no matter how neatly they drew a sine wave over the gate trigger plot.

    It's not going to output a sine wave either, and the output voltage will not be regulated. However, it will likely be very resistant to the aforementioned flux walking, as each pair of MOSFETs will have a 25% ON duty cycle. The output will probably resemble more like a triangle wave than anything else. It would be an improvement over a plain square wave output.

    I haven't looked at the opamp portion of it; not quite sure what he's doing with that.

    He's tied the 4017 output pins 3 and 4 to ground via diodes; that is not good. Unused outputs should be left floating. CMOS ICs have very low output current, and IRFZ44's have quite a large gate charge. Might get by with that at such a low frequency; but even for a few hundred HZ you'd need a better gate driver arrangement.
     
  7. aliman1

    Thread Starter Member

    Mar 22, 2008
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    Thanks for your input SgtWookie
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The original of the "new" modified square-wave inverter was designed and discussed last year on another website then was discussed on this website. Its schematic here seems to have been deleted. Its peak voltage is the same as the peak voltage of a sine-wave from the mains. It didn't have outputs from the CD4017 grounded with two diodes.
    It regulated its output voltage by varying the PWM as the battery voltage dropped.
     
  9. aliman1

    Thread Starter Member

    Mar 22, 2008
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    Hi Audioguru, are you talking about this design?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    No.
    I am talking about the one that Juan Dela Cruz made.
     
  11. aliman1

    Thread Starter Member

    Mar 22, 2008
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    Ok , this one.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    No.
    This one has been modified and has some serious problems like two outputs of the CD4017 shorted to ground by two diodes.
     
  13. aliman1

    Thread Starter Member

    Mar 22, 2008
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    What if those two diodes were removed, like SgtWookie said, if outputs on the 4017 are not used, they should remain floating.
    What other problems do you see?
     
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Take a look at the "Magic Sinewave". It is a microcontroller controlled output waveform for an inverter that gives a good approximation of a sinewave on the output.

    They sell the pre-programmed uC as a product on that page, it is capable of 3 phase output.
     
  15. aliman1

    Thread Starter Member

    Mar 22, 2008
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    I don't have Pic programmer, suks.
     
  16. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    They sell the pre-programmed PIC, it only needs power and a crystal for the desired frequency, just like wiring in a Logic IC or other IC.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    The two diodes were used correctly on the original schematic.
    I found this schematic on the other website but I don't know if it works properly:
     
  18. aliman1

    Thread Starter Member

    Mar 22, 2008
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    Do you know if this circuit produces a stepped square wave like this ?

    I also found this schematic, is this it?
     
    Last edited: Dec 10, 2010
  19. Audioguru

    New Member

    Dec 20, 2007
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    Yes.
    If the circuit works properly then it produces a "stepped" modified sine-wave which is really a modified square-wave.
    I don't know which schematic is the best one and I don't know if the circuit works.
     
  20. aliman1

    Thread Starter Member

    Mar 22, 2008
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    Ok, do you know what that Op Amp in the center is, is it the CA3140?
     
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