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Piezoelectric generator wiring question

  1. Jan 13, 2012 #1
    Hello, I am working on science fair project in which I am harvesting kinetic energy and converting it into electrical energy to charge an electrical device. However, i would appreciate another opinion on the setup of my circuit. Currently, my setup is as follows:
    I am using a piezoelectric ceramic disk transducer and am using my own weight to compress the disc. The disc is then connected to a full wave bridge rectifier to convert the AC current to DC. Next, I will store the DC current in a 400 MF 25V capacitor. Then i want the capacitor to be connected straight to the electrical device (3.7 V battery cell phone) via a wire soldered to the phone specific charging wire head (the AC adapter will be removed). I would like to know how i can attach another transducer in parallel and whether the rest of the design will work. Or will a transformer be necessary before the rectifier? I am unsure of the necessity of a transformer since on average 2 volts of electricity will be produced from one compression. Also, if there are any other errors, please let me know. Respond as soon as possible, please and thank you!
     
    Last edited: Jan 13, 2012
  2. jcsd
  3. Jan 13, 2012 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Since this is a science fair project, we need to ask you to do the bulk of the work. Having said that, I have one tip and one tutorial question for you.

    First the tip. Be careful having your energy harvesting device deposit energy in a large capacitor. Large capacitors tend to have significant reverse leakage current, so you will likely lose all of the energy you put in. Do a little searching and reading about "low leakage capacitors", to help you optimize your low-power design.

    And the probing question -- what advantage and disadvantages do you think there are for putting a transformer into the design? What would be the task of the transformer? What kind of specifications would it need to have? What could you gain by using a transformer?
     
  4. Jan 13, 2012 #3
    PFmentor, I know the transformer is for the purpose of stepping down voltage, so as not to overload the cellphone batter and cause damage, however that is when high voltage is involved and since the average output for one compression of a piezo disc is roughly 2 AC which the rectifier would convert to something significantly smaller in DC. So I do not think it will be necessary to step down the AC voltage since it is such a small voltage, however, i was just asking since my teacher had mentioned that it may be needed.
    As for what you mentioned about the capacitor, my teacher had also mentioned the "bounce-back" effect, but when you say "large capacitor" are you referring to the voltage or capacitance? And what voltage/capacitance would be considered a large capacitor?
    Thank you for answering!
     
  5. Jan 13, 2012 #4

    berkeman

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    Staff: Mentor

    Hmm, I guess I misinterpreted what you were intending with the transformer. I thought you might be considering using it for stepping *up* the AC voltage transients that you get out of your Piezo...

    And on the capacitor, what you wrote "400 MF 25V capacitor" could either mean 400mF (milliFarad) which is huge, or 400uF (microfarad) which is still very big for such a small power application. Anything over about 1uF ceramic capacitor will generally involve either a tantulum dielectric or electrolytic dielectric, both of which have enough reverse leakage current to cost you most or all of your harvested energy.

    Have you done much reading about how others have interfaced Piezo energy harvesting mechanisms to things like battery chargers? There are some techniques that I can think of that would be fairly efficient, but rather than us telling you, you should hopefully be able to find them with some good web searching...
     
  6. Jan 13, 2012 #5
    I tried connecting the circuit with a 47 uF capacitor and it produced about 5-8-V AC (before going through the rectifier), then after going through the rectifier it produced about .016-.08 V DC. However, after the current was passed through the capacitor the voltmeter read 0-V DC. So the current is not registering in the capacitor. I will try this again using a 1 uF capacitor and see if that helps and possibly a step up transformer as it may be necessary since so little is produced. What were your concerns/thoughts regarding using a step up transformer?
     
  7. Jan 14, 2012 #6

    NascentOxygen

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    Staff: Mentor

    You may find it's better to connect the piezo element through just a single diode then into the capacitor. Using a bridge rectifier causes 2 diode drops, so you'll lose some of the voltage, and until you know how much voltage you're getting, you aren't able to say whether you can afford to lose two drops each of 0.7 volts.

    Sorry, but IMO you have not a snowflake's chance in hell of being able to power a phone from this. The best you can hope for is after multiple compressions, you might be able to flash a LED momentarily.

    Don't think about a transformer, it won't do anything useful here.

    Although your meter may be giving a reading, it should be regarded with healthy skepticism. The reading is probably meaningless for a number of reasons, principally that the meter is not being used under conditions for which it was designed. The DC voltage across the capacitor may be useful, but even here the meter may drain too much current.

    What is the piezo element you are using; what was it designed for? Does it have a rating of some description?
     
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