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How do you extract the maximum amount of energy from a piezoelectric element? (For energy harvesting purposes.)

The maximum power theorem would seem to say that you need to load it with a resistance equal to the piezo's internal resistance, but a piezo's internal impedance is capacitive rather than resistive. So the load would have to be an inductor? Or a resistance tailored for a specific frequency of vibration? How would you use this to do something more normal, like charge a capacitor for powering other circuitry?

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May want to peek at this: link – tyblu Dec 11 '10 at 5:39

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If you want to charge a cap in a very simple manner you can just run the piezo output into a rectifier (or doubler if you want the extra voltage and don't mind the reduced charge rate) and then connect the DC output straight across a capacitor. As the piezo vibrates you will see the voltage on the cap slowly increase - if you have a high enough input impedance volt meter that is - there are some good papers by Ferrari et al that cover the basics. Shad Roundy's PhD thesis also covered this rather well I think but it is a trifle long.

If you want to get really fancy with the inductance you could look into a technique called SSHI (synchronized switch harvesting on inductor) the maths is heinous but it is clever.

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For purposes of this question, I do want to get really fancy. Can you elaborate on SSHI? Any relation to en.wikipedia.org/wiki/Active_rectification? – endolith May 5 '11 at 16:04
Not really related, although Ferrari did present a paper at the 2008 IEEE Sensors conference where they attempted something similar - replacing passive diodes with switched FETs. Have a read of jim.sagepub.com/content/17/8-9/831.abstract or "Toward Heat Energy Harvesting using Pyroelectric Material" DANIEL GUYOMAR or iopscience.iop.org/0964-1726/16/6/028 what is your end application? It's worth bearing in mind that any clever processing/control added in needs to use less power than it creates – SimonBarker May 5 '11 at 16:44
My end application is curiosity. :) Thought I guess I'm imagining things powered by human movement. But other methods (like in self-winding watches) may be better than piezos, this was just a tangent. – endolith May 5 '11 at 18:46
Cool, well MIT (and maybe Berkley if my memory is correct) have done work into the idea of bionic soldiers powering their own kit. Piezoelectric backpack straps and knee braces have been produced so far. MIT also made a piezoelectric shoe heel insert a couple of years back now. Have fun! – SimonBarker May 6 '11 at 5:24
up vote 4 down vote

Maximum power specifies that you should load it with a load equal in magnitude, but opposite in phase. So if it is capacitive, you need an inductive load to cancel the capacitive components, then a resistor equal to its resistance to get max power.

Yes, the load would be tuned for a specific frequency.

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Otherwise known as conjugate impedance matching. – tyblu Dec 11 '10 at 5:06
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@tyblu, always the showoff. Yes, this is how you max a complex load. You want the C and L to resonate and the Rs to match. – Kortuk♦ Dec 11 '10 at 21:51

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