Piezoelectric dilemma!

Question

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.

(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 :-

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

Answer 1

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.

Answer 2

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?