Easiest way

Discussion in 'General Electronics Chat' started by shortbus, Dec 11, 2011.

  1. shortbus

    Thread Starter AAC Fanatic!

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    What would the easiest way to PWM a sine wave be? Without using a micro controller. I want to replicate a sine wave to switch a mosfet on and off.

    Google searches all come back with using a micro. It had to be done before PICs and such, right?

    Thanks for any answers.
     
  2. spinnaker

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    Are you certain you don't want a square wave? If so you can use a 55 timer.
     
  3. MrChips

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    You cannot PWM a sine wave. If you did it would no longer be a sine wave. You may alter the period of the sine wave.
     
  4. praondevou

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    I am not sure that I understand this question.
    Can you explain in a little more detail what you want to achieve?
     
  5. SgtWookie

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    I think you're talking about something like a Class D amplifier?

    What is it you're trying to do, make a true sine wave converter?
     
  6. thatoneguy

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    I mentioned it in another thread as a joke, not sure if it is the same thing OP is talking about.

    Sinewave, same shape, except every other mS is turned off, or somthing, so it still looks the same on the scope, but all chopped up. Cut in more than one place, such as a Triac would do.

    More for novelty than actual function.
     
  7. Markd77

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  8. thatoneguy

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    Like that, but chop up the actual AC sinewave PWM style, so it still goes to 170V peak and back through zero to -170V, just with several zero periods.

    Think of a Triac that would only conduct when there was voltage at the gate (instead of staying on for the rest of the cycle), and put the output of a PWM on the gate of the Triac.
     
  9. THE_RB

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    It can be done in analogue but a micro is by far the best choice.

    Here's one I did that generates a 1kHz sine wave from a PIC using PWM from a lookup table;
    http://www.romanblack.com/onesec/Sine1kHz.htm

    In your case based on your previous threads about motor controllers I'm assuming you are making some type AC motor controller?

    For that you don't want to make a sine and feed it to the gate of a FET. If you need a sine voltage at the output of the FET you should generate a sine, then use a closed loop circuit like an opamp etc so the output of the FET matches the original sine. So you generate a 2v sine with the micro, then the FET generates a 100v sine because it is driven in a closed loop system. :)
     
  10. Wendy

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  11. shortbus

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    @spinnaker - well, basically the sine will become a PWM square wave. to replicate the voltage of the sine wave.

    @MrChips - its, done in inverters and VFDs, just with micros and DSP. Don't want to learn programing though.

    @prandevou- I'm trying to make a inverter/VFD/motor controller. the taking DC and making it in to simulated AC to run an induction motor.

    @SgtWookie - you hit it right on the head! But instead of starting from 12V the plane is to start with 160V DC and eliminate the first section of the inverter.

    @MarkD77 - Missed that on the Falstad site. But that is what I'm after! Thank you.

    @thatoneguy - Mark's circuit will work if the out put is fed to a H-bridge to get the AC. Right? Like in a bipolar stepper driver.

    @THE RB- yeah, to drive an induction motor. The PIC or some other micro would be easier but at my age learning to program would take longer for me than I have left in my life :).

    @Bill Marsden - looked at that way first. But to chop the sine wave to a PWM type wave, like Mark showed, wouldn't I need to have several comparators set at the different voltage levels?

    What I am trying to accomplish is an induction motor controller. My first thought was to use a bipolar stepper circuit to switch a H-bridge. A stepper motor in micro-step mode is like an AC motor in its wave form. But all the circuits I find use either a chip that has built in H-bridges that aren't rated to fit my needs. Or there micro controller based that needs programing.

    I don't have a oscilloscope yet so can't check the actual wave form from a step driver, but from the data sheets it looks like there is no zero voltage between steps. The wave form in the data sheets has more of a stair step look to it, and that would put the mosfits in a linear mode rather than a PWM type switching.

    Thanks for all the answers and please feel free to make suggestions. And I am going to get a scope.
     
  12. praondevou

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    Why don't you go the classical sinewave with triangle comparator way?

    You need a sinewave generator, an inverter for the sinewave, a triangle generator, 2 OPAMPs to compare the sinewave with triangle, the output of the OPAMP will be PWM. This is fed to a H-bridge. For 3-phases there would be 3x 120 degree phase shifted sinewaves necessary of course.

    The most cumbersome part maybe the sinewave part.

    For a good sinewave with low distortion we used a "digital" sinewave that was stored in an EPROM... But any sine wave generator with low distortion will do it.
    However I'm thinking more of a general DCAC converter, not one for motors, extrem low distortion is probably not necessary in your case.

    If it's for asynchronous AC motor it needs to be adjustable in both frequency and voltage...

    If necessary can I draw something up.
     
    Last edited: Dec 12, 2011
  13. thatoneguy

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    Yes, but it RB's circuit would need to be modified to output 50-60Hz, making the filter inductors and capacitors much larger, and also leaving you with using linear outputs, like a very big, 60Hz only audio amplifier.
     
  14. shortbus

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    Its for an asynchronous/induction motor. A square wave is to abrupt but a PWM of 4 or 6 steps would work great.

    Would it be a lot of trouble to together a schematic? The motor is going to be a two phase instead of three phase, so the phase is only shifted by 90 degrees. Any help will be great.
     
  15. praondevou

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    Can you provide a link with a datasheet of the motor you are going to use? .... Two-phase motor ...

    Also have a look here.
     
    Last edited: Dec 12, 2011
  16. thatoneguy

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    How about The RB's linstepper? Run two of them 90 degrees out of phase?

    Good for 1A each, 2 if you have a good heatsink. Check it out in his sigline.

    The driving wouldn't be for CNC stepper motors, but to create to phases, 90 degrees from each other, at whatever DC voltage you'd like
     
  17. shortbus

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    Thank you for that link to the AAC thread. On a quick look (I'll read it in depth) that looks to be what I need.

    I'm going to rewind a three phase motor to be a four pole two phase. The motor rewind won't be bad, its the electronics thats the problem for me.
     
  18. shortbus

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    My original thought was to use a bipolar stepper circuit for this. At one time there were all kids of different discrete component circuits on the Web for stepper circuits, but can't find one now for micro stepping(1/4 step is what I need). All the driver ICs that are for micro stepping have internal H-bridges built in. I could try to switch my mosfet drivers with one of them, but don't know if it would work. What do you think?

    I looked at RB's Linistepper but its only for unipolar. And the AC wave form is bipolar.
     
  19. strantor

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    can you do a little writeup on the motor rewinding once you're done please?
     
  20. praondevou

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    Ok, for the non-motor experts: Will this work the same way as for a 3-phase AC motor VFD, i.e. the voltage increases with frequency (Sine wave voltage not PWM) and there is an IR compensation at low frequencies to provide some boost?

    Btw, the 90 phase shift at changing frequencies makes the circuit a little bit more complex. If one only could use a uC... :D
     
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