simple a/c capacitor circuit

Discussion in 'The Projects Forum' started by mikereth, Apr 14, 2015.

  1. mikereth

    Thread Starter New Member

    Apr 14, 2015
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    Hello,

    I have a material that produces an a/c wave of around 100 volts and .0002 amps (those values can range depending on the frequency.) I want to create a simple circuit to test the charge and discharge time of a capacitor using d/c rectifier. Also, I would want to create an analytical description of the circuit. Can anyone give me some info i need and where to start.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What value/type cap do you intend to use? Its leakage will need to be << 200uA for your measurement to be reasonably accurate.
     
  3. #12

    Expert

    Nov 30, 2010
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    Start with Vcap = Vmax x e^(-t/RC)
    That equation describes the time and voltage of a capacitor charging through a resistance.
     
  4. mikereth

    Thread Starter New Member

    Apr 14, 2015
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    Alect: Im really not sure what capacitor I will use. The charge time that I have available for the capacitor is anywhere from 1 hour to 12 hours. The frequency is usually low though (mechanical induced voltage from piezoelectric strips). I want a significant amount of charge though, maybe eventually connecting it to a battery for small electronic devices.

    #12: Do I need to use a resistor, or will the capacitors resistance be fine?
     
  5. mikereth

    Thread Starter New Member

    Apr 14, 2015
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    I have taken one simple circuits class in my mechanical major, so I am relearning and just plain leaning this as I go. This is a side project to capture mechanical vibrations that have a low frequency (1-15 Hz). Any opinions or advice you have is welcome. Teach me!
     
  6. #12

    Expert

    Nov 30, 2010
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    Two different questions. How to characterize the charge on a capacitor and how to scavenge micro-watts of energy. To use the equation, there must be a resistance or the answer will be e to the negative infinity. What's a nice word for B.S.? Undefined?

    A typical capacitor should provide very close to zero resistance. It's difficult to measure the time required to discharge through zero resistance so you will have to add a resistor. This is in contradiction to harvesting tiny amounts of energy because all resistors turn energy into heat. Do you want heat? I don't think so.

    You will just have to do one job at a time. Harvest first, measure how capacitors work second.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Have you done the maths? Charge = current x time. Even if you could sustain a 200uA charge rate continuously for 12 hours (is that likely?) you won't be harvesting a great deal.
     
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