Power/Voltage question

Discussion in 'General Electronics Chat' started by codemastercool, Mar 8, 2012.

  1. codemastercool

    Thread Starter New Member

    Jan 23, 2012
    Hey everyone,

    I have a doubt regarding some basic concepts for a project I'm working on.
    I'm doing a project on energy harvesting using piezos. Now, the theoretical values show an output peak to peak voltage of 2.2V, and with a load of 10MegaOhms, an output power of .2uW.

    Now, what I want to do is try and portray another idea to my classmates, and that is - using the energy harvested, I want to give them an idea of the quantity by using day-to-day gadgets they use, for example, "With this circuit, you can charge an iPhones/iPods battery in X amount of time".

    Could someone just give me a basic theory explanation of how to do this?

    Some more background info:

    The paper from which we're referencing uses the piezo to generate AC voltage, but I want to put in a bridge rectifier to change that to DC.
    So am I right in saying -
    1. I calculate the voltage output of the rectifier, with an AC input of 2.2 Vpp.
    2. Try and find out the gadgets battery specs (perhaps the resistance) so that I can calculate the current flowing through? And with that, relate to charging capacity or something?

    I'm not sure as to how to go about the second part of step 2 (if that's right).
    I did a Google search, and one replacement battery for the iPhone 4 shows specs of 3.7V and 1420mAh.

    I would really appreciate if someone could help me out here.
  2. Audioguru

    New Member

    Dec 20, 2007
    What activates the piezo? If it is very loud continuous sound then the rectified output will be 1.1V peak minus 1.4V for a fullwave rectifier leaving you with absolutely nothing to charge a battery with.

    Why not connect six piezos in series so their peak voltage is 6.6V? Then it can be fullwave rectified into 4.9V and can charge a cell phone.

    A cell phone has a 1420mAh battery. The output current from the piezos is almost nothing which is about 10nA. It will take about 142,000 hours to charge the battery. That is 16.2 years.
  3. codemastercool

    Thread Starter New Member

    Jan 23, 2012
    It is activated by air flow. Now, what we're doing is demonstrating the energy harvesting properties of the piezo, we're using a pretty small piece as is.
    We are not physically charging any battery - I just want to do some of the math to give them an idea of *how* much current/power is being generated. This class consists of a majority of mechanical and civil engineering students, so I don't want to just say " Our setup presents us with x amount of current", I want to say "The setup gives us this much current, which could charge up an AA battery in so many hours, or light up an LED for so long".