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Thread: PWM that swings both ways

  1. #1
    ElementalPhase Guest

    Lightbulb PWM that swings both ways

    It has been 10 years since I last worked in electronics. But I wonder if anyone has come across this implementation of the PWM? All that I have seen with the PWMs is that when the signal goes high the power transistor turns on and then turning it off when the signal goes low. Why not have two generators and two transistors, one transistor that opens and turns on generator [[a]] when the signal goes high and the other transistor that opens when the signal goes low turning of [[a]] and turning on generator [[b]]. This way you always have a generator in the on state-they are just alternating. You would theoretically double your output.

    The change to the circuit would be minimal when dealing with a true 50% duty cycle, though I know that is not the case in most implementations. The two transistors/generators would operate in parallel.

  2. #2
    sp1r0 Guest
    Theoretically sounds like a good idea. I wonder if it's as easy as using 1 PWM and hooking up 2 electrodes. One hooked to + and the other hooked to -. The one hooked to + would energize when the signal went low, and the one hooked to - would energize when the signal went high.

  3. #3
    ElementalPhase Guest
    This pic should give a general idea of the principle. Though you most likely would not use silicon PNP and NPN transistors but power mosfets but the principle is the same. I just wish I had the money.

  4. #4
    sp1r0 Guest
    Not sure if this would work, but I think we're on the same page...

  5. #5
    Join Date
    Jul 2008
    Posts
    627
    Sp1r0: In your setup only the generator on the right would work. The mosfet works by either allowing the +12 to flow to ground or it is shut off. ElementalPhase's design is closer even if it isn't complete. I am not sure if there is a mosfet that works on a negative bias voltage though. Maybe one of our EEs can chime in.
    --
    Some days I get the sinking feeling that Orwell was an optimist!

  6. #6
    sp1r0 Guest
    Yeah, so there would have to be two output stages ,one is inverted...Would it work though. I mean could it increase efficiency?

  7. #7
    Join Date
    Sep 2008
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    I am playing with just that Idea. An h bridge is the answer. It will require 4 fets instead of only one and you will have to slow the frequency of the pulse way way down to allow the reverse spike to dissipate before re-charging the cell with opposite polarity. I have some devide by n logic chips coming to slow down the pwm and will give it a try.

  8. #8
    ElementalPhase Guest
    Well, I am not sure what an "H-Bridge" is, but this two cycle idea is only the first step. The final stage of my hypothosis is you first figure what your brute force power requirements are for your given plate design. Next, you find out what the best frequency of charge, period as well as pulse width. Finally with this you can figure out how many generators you can drive. So you may be only able to go two with your plate pattern or you may be able to go eight. I just see it as a waste of "opportunity/production" to see a cycle go unused.

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