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I am a high school student and have no experience with piezoelectricity but I’m working on a piezoelectric shoe.

My plan, initially, was to connect piezoelectric transducers in parallel and solder the two end points to a full wave bridge rectifier and the DC current would be used to charge a rechargeable battery pack or the sort.

enter image description here

(Courtesy of the Internet)

But after searching on the internet ive been told that I’d require a separate bridge rectifier for each transducer (because of actuation problems)!!

  1. Is this true?
  2. So the following setup wouldn’t work?

Thicker red and black wires from transducers :-

enter image description here

The aim of my project is to create electricity and though the output current may be low, I’m not really concerned about that.

The justification I was given for having separate bridge rectifiers for each transducer was that they can, but don’t always, cancel each other’s power outputs (and this was from a person who has done it, I think he has an instructable on this)

From this link I gather that the piezo elements have to have the same frequency and voltage and should be in phase. I presume the frequency refers to the resonant frequency but frankly I have no idea.

And how would it be possible to modulate all of their frequencies, wouldn’t they be slightly different as pressure is applied to them? I think I am misunderstanding something fundamentally and would like clarification.

Ps: the transducers have the same (resonant) frequency

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    \$\begingroup\$ If you've been told something won't work then what was the question you asked - in other words what are you trying to achieve and what was is the validity of the person saying it wouldn't work - i.e., what were their reasons? \$\endgroup\$ – Andy aka Dec 6 '19 at 13:03
  • \$\begingroup\$ The parallel solution might result in some of the transducers absorbing a portion of the power generated by more active ones. Also, what kind of voltages are we talking about? Will it overcome the diode drop? \$\endgroup\$ – Cristobol Polychronopolis Dec 6 '19 at 13:18
  • \$\begingroup\$ are you trying to detect steps with the shoe, or are you trying to measure weight, or center of mass? \$\endgroup\$ – Scott Seidman Dec 6 '19 at 13:44
  • \$\begingroup\$ Do you even need a rectifier? \$\endgroup\$ – jusaca Dec 6 '19 at 14:05
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    \$\begingroup\$ The output is essentially AC but the charge that is produced with each deflection is tiny, normally you'd be exciting these at a highish frequency to produce any worthwhile output - which even then is not likely to be enough to charge a battery. See this one for an idea of the scale 0.5mW @100KHz steminc.com/PZT/en/piezo-ceramic-generator-40x11x17mm so if you're looking to harvest the energy from each footfall, maybe an electromagnetic solution would be better. \$\endgroup\$ – Phil G Dec 6 '19 at 14:23
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As they are all in parallel, any single piezo element that generates voltage drives three other piezos, so paralleling these will get little energy out.

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  • \$\begingroup\$ So it’s alright then? I wouldn’t face any problems because of the phase mismatch between the transducers and cancelling of power outputs that I’ve been told about. I can’t actually test this since the elements have not arrived yet.. \$\endgroup\$ – Serapion Dec 6 '19 at 21:24
  • \$\begingroup\$ No, exactly the opposite. I just explained if one piezo experiences movement/vibration so it is generating energy, the energy is immediately used to move/vibrate the other piezo elements. \$\endgroup\$ – Justme Dec 6 '19 at 21:33
  • \$\begingroup\$ And if you have them on different spots on your shoe, then if you're pushing down on one at the same time you're lifting off of the other, that's a phase mismatch. \$\endgroup\$ – TimWescott Dec 6 '19 at 23:15
  • \$\begingroup\$ @TimWescott yes precisely so it won’t work? Because shouldn’t it be in phase? \$\endgroup\$ – Serapion Dec 7 '19 at 6:33
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I think the comments are all gently suggesting that you have picked a project with very little chance of anything resembling success. Just looking at the piezo transducers you have chosen provokes all sorts of negative thoughts such as how they will survive with flexing in a shoe and how to accommodate the wiring while providing good mechanical contact with the surfaces. The primary use for devices you have chosen is as a transducer to move air in the audible frequency range - a beeper.

The comments describe how simply paralleling the transducers won't work because if one is generating the others will be absorbing power. In addition, while you may - with the right mechanical arrangement - be able to generate significant voltage this won't translate into useful power unless it can provide significant current at the same time. (P = V × I.)

If you really wish to study these devices and try to generate some power then I suggest you try to make a mat that will harvest energy from people walking over it. This will have a much simpler construction, wiring and testing. You can get repeatability in testing by dropping a weight onto the device from a fixed height, etc., something that will be very difficult to repeat in a shoe. You can also very easily experiment with single and multiple bridge rectifiers.

The transducers have the same (resonant) frequency.

That won't be relevant as they won't be oscillating. They will be driven at the frequency of the mechanical system causing the flexing.

The voltage created by piezo elements is appreciable and can overcome the diode voltage drop for forward bias.

Notice that the photo of your test setup is measuring voltage with no load. Try repeating the test with a 10 kΩ, 1 kΩ and 100 Ω resistor across the leads to get some idea of useful power. (P = V2 / R.) I suspect that you will be disappointed. Is it enough to blink an LED?

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  • \$\begingroup\$ Thank you for your immensely helpful feedback. You suggest making a piezoelectric doormat, although it wouldn’t face problems such as flexing and wire accommodation and all, wouldn’t that face the same problems of phase mismatch, which is most worrying for me. I guess I’ll have to experiment with the diodes. Oh and on a side note, though it may be a very dumb question, what about connecting ac sources in series? \$\endgroup\$ – Serapion Dec 7 '19 at 10:29
  • \$\begingroup\$ Yes, it would face the same problems of phase mismatch but the repeatability would make it much easier to study with and without rectifiers. The "in-series" solution will have problems too. To drive a current from one it will have to go through all the others which will absorb much of the energy and give off vibrations. It would be a bit like trying to power something with a string of four AA batteries three of which were flat. \$\endgroup\$ – Transistor Dec 7 '19 at 10:36
  • \$\begingroup\$ Well alright, thank you. I guess I’ll experiment around a bit. Do you suggest connecting the piezo elements for the mat in parallel or series and then trying to distribute the force?Hopefully I’m not being too annoying \$\endgroup\$ – Serapion Dec 7 '19 at 11:04
  • \$\begingroup\$ Start with one and characterise its output. Try a bridge rectifier and 100 uF capacitor with no load and measure the voltage you can get on the capacitor. Measure the energy it can supply into various loads and what type of activation or vibration gives a useful result. Then, if there is still hope, investigate how to scale it up. \$\endgroup\$ – Transistor Dec 7 '19 at 11:39

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