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Full wave rectifier for piezoelectric stack

  1. Feb 15, 2009 #1
    Hi

    My project involves rectification of a voltage produced by a piezoelectric stack subjected to an impact. The voltage shall be rectified using a full wave bridge rectifier.

    I aim to use a capacitor (in position of the smoothing capacitor) to store the energy harvested from the impact (simply because the device must fall from a certain height so it cannot be connected to any measuring equipment during its descent). The capacitor shall be disconnected following impact and connected to a volt meter so stored energy can be calculated.

    My question is: does anyone have any suggestion as to how I can calculate required capacitor value? I have no idea of the frequency, current or voltage of the piezoelectric stack output, and these are likely to vary for different impacts.

    I do know the voltage and current I require from the rectifier output - if that helps!

    Any help would be greatly appreciated

    Thanks

    L
     
  2. jcsd
  3. Feb 15, 2009 #2

    dlgoff

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    Science Advisor
    Gold Member

    The biger the capacitor the more you can store.
     
  4. Feb 15, 2009 #3
    capacitors are rated for voltage, so you'll need to know the maximum rectified voltage and maybe go a % higher for a safety margin. and you'll need to know the maximum energy output of the stack (or maximum signal applied if it's less than the stack capacity) to choose the total farads. you can use multiple caps in parallel if necessary.

    edit: could it be that this is a system identification problem and that you'll have to do some lab work and analysis before you get down to designing the test instrument ? that's just the way it goes sometimes, you've got a black box and you've got to figure out how it works.
     
    Last edited: Feb 15, 2009
  5. Feb 16, 2009 #4

    berkeman

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    Staff: Mentor

    Don't use a full-wave rectifying bridge. That would be an error in either the project definition, or in your interpretation of the project definition.

    Tell me why I make that statement. And tell me what kind of *single* diode you should use in this project, and why.
     
  6. Feb 17, 2009 #5
    I'm afraid I can't work out what you're getting at. Having tested the piezo stack using an oscilloscope, it clearly shows an AC signal when an impact is applied. As far as I can tell, the only way to extract maximum available power is to rectify the output.

    Please let me know if I'm getting confused here :)

    Thanks
     
  7. Feb 17, 2009 #6

    berkeman

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    Staff: Mentor

    Well, if it's truly an AC output, with significant power in the negative excursions, then okay, a full-wave bridge might be justified. I'd assumed that the main power was in the initial compression of the piezo stack, which would put the main power in the initial voltage spike. Having just one Schottky diode drop versus two seemed like a good increase in efficiency, depending on the voltage levels you are getting from your stack.
     
  8. Feb 17, 2009 #7
    it is certainly a valid question whether it's a true AC signal with a zero-volt average, or a DC signal with AC components (which would probably be a reflection of vibrations/bounce from the impact)
     
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