piezoelectrics and vibrations/frequencies

Factual Questions
1

I’ve heard of piezoelectrics since I was a kid

certain minerals, like quartz, produce a voltage when “squeezed”, and conversely contract when a voltage is applied.

But when I read other stuff, like the wikipedia article, it talks about using this property to create very high frequencies, either acoustic sound or electric frequencies.

What gives? What does the one property have to do with the other? If I in real life apply a voltage to a piezoelectric piece, does it stay contracted, or does it vibrate? If it’s the latter, is it because once the contraction happens, the effect is lost momentarily, and then the crystal springs back out again, only to feel the voltage again and then contract again?

Ohhhhhh, is it that they put the wires with a slight gap from the piece so that when it contracts it loses the voltage and then expands again and thus cycles like that?

What the dilly, yo?

2

When they are used to make sound, piezoelectric crystals are driven with alternating current to make them move back and forth at the sound frequency desired.

The resonant properties of piezoelectric crystals are used to create oscillator circuits like those used in clocks. For this they are generally used in a feedback circuit to generate the oscillation and the mechanical resonance of the crystal determines the oscillator frequency.

3

If you apply a steady voltage, it will stay in its distorted shape. Piezoelectric crystals are used in the control mechanism for diesel fuel injectors, where they allow precise control of the duration of the fuel injection events - up to and including multiple injection events per combustion cycle. Apply your control voltage for x milliseconds, and the injector stays open for that long; the crystal doesn’t revert back to its at-rest shape until the voltage is removed.

Piezoelectric crystals are also found in active sonar transducers for marine use. You can hit the crystal with an oscillating voltage to generate an outbound sound wave, and then immediately after that record the voltage signal that comes from the crystal as it gets buffeted by the sound waves reflected from distant objects.

Ultrasonic flow meters employ piezoelectric crystals in a similar way.

4

Here’s something I don’t understand

Taken from wikipedia:

How does the frequency of the current change?You put intypical 60hz power,and the crystal vibrates at 60hz

or are they sayng the power’s frequency is changed first based on the crystal’s resonant frequency, and then resonant-tuned-adding of the voltage allows the voltage to step up?

Speaking of the latter, how does that work exactly, because I’ve heard Tesla Coils bump up voltage using resonance this way. A normal transformer I understand, but why should the power “add up” just because you hit the resonant frequency? The power is coming OUT o the power source, the power source is the only source of power, how could any power be added without another power source?

5

Yes, if you put in 60 Hz the crystal will vibrate at 60 Hz. It’s already a common trick in conventional power supplies to rectify and filter the incoming AC and then use a high frequency oscillator (typically somewhere in the range of 40 kHz to 100 kHz), which then goes through conventional transformers to boost the voltage. They do this because then you can make the transformers much, much smaller and lighter.

They are talking about the same basic idea with the piezoelectric transformer. The incoming AC is filtered and rectified and that is used to drive a high frequency oscillator which is tuned to the crystal’s resonant frequency. An easy way to visualize how they are using the crystal in this case is to imagine holding a long, flexible rod in your hand. When you hit the resonance frequency of the rod, shaking it back and forth, the ends of the rod are going to swing much farther than the movement of your hand in the middle. This is the basic idea behind driving the crystal with a frequency at one end then getting a higher magnitude voltage swing out at the other end.

Just like in a conventional transformer, the amount of power in equals the amount of power out (ignoring the relatively small losses of your transformer, of course). So if you increase the voltage by a factor of 10, the current on the high side is reduced by a factor of 10. As Scotty says, you cannae break the laws of physics, Jim. You can’t just pull power out of thin air. Power in has to equal power out.

Tesla coils work much differently than that. In a Tesla coil, you charge up a capacitor until it breaks down the air resistance in the spark gap, kinda like a miniature lightning bolt of sorts. As the energy is released, it flows through the primary coil and induces a voltage in the secondary coil. The resonance of the coils ends up sending the energy sloshing back and forth between the two coils, which charges up the second capacitor. When this second capacitor reaches its limit, then it discharges, releasing your high voltage, low current electricity. It’s basically just a really fancy way of charging a capacitor then using that capacitor’s voltage.

6

Not being a jerk here, just following along: initiating the wave from the end of the rod and its following events is different than an instantaneous “wave” initiated in the center of the rod.

Rod–>taught string…flex–>strike or pluck

Does considering this case have any bearing or visualization of related? I’m familiar with the idea of the resonant frequency and the simultaneous lower-power natural and unnatural harmonics simultaneous in the flexion.

7

Maybe I’m not explaining it very well, but I was trying to show how when you get a resonance, relatively small movements where you apply the energy end up creating larger movements elsewhere. The same basic thing happens if you shake it by the end, too, but while i think a lot of people have picked up sticks or flexible rods and have shaken them and naturally found the resonance of it, I don’t think too many people have done the same thing by shaking it from the end, so that’s why I used the example I did.

This would be really easy to demonstrate if you were standing in front of me and I could shake the rod to show you what I mean. Describing it in words isn’t so easy, though. But, if you could cut the rod in half at that point and you could hold what is now the end (formerly the midpoint) firmly enough, then you would be shaking the rod from the end with the same resonance, so it is exactly the same thing.

But anyway, since I am probably explaining it very poorly and/or I’ve chosen a bad example, I went poking around on youtube and found this example of resonance.

Now imagine instead of using a shaking table you have an electrical current pushing the end of the crystal back and forth, and at the other end of the swinging part you are picking up the voltage from the much wider swings of the crystal, and that's basically how the piezoelectric transformer works.

I hope that’s a better explanation.

While that’s the basic idea of it, real world piezoelectric transformers these days use much more complex shapes than a simple bar. With some of the designs, it really is more like shaking the bar in the middle and picking up the wider vibrations on either end.

This paper does a decent summary of piezoelectric transformers, but it does assume a fair knowledge of electrical circuitry (also, pdf warning):

docs.faceco.com/FE/KH/MRS03.pdf