What happens when DC is applied to a piezo crystal?

Question

I've heard that when an alternating current is passed through a piezo crystal it vibrates. But what happens if a direct current is passed through the crystal? I'm assuming depending on the direction of the DC applied the crystal either compresses or expands. Am I right? I'm interested in compressing a cuboidal shaped piezo crystal. How do I do it with DC. I haven't bought a crystal yet so I would like to know theoretically what direction of DC will cause the crystal to compress or expand?

Answer 1

How do I do it with DC

For a very short period of time is the short answer.

Longer answer - a piezo is basically a capacitor in terms of equivalent circuit and, to push a constant current through that capacitor requires a ramp voltage applied to the terminals. This follows from the basic capacitance equation: -

$$ I = C\dfrac{dv}{dt}$$

In other words, to keep current at a constant value dv/dt needs to be constant and that means a voltage waveform that is a ramp. Pretty soon you will have raised the ramping voltage so high that you have thousands of volts across the piezo and it's going to breakdown due to over-stress voltage.

so I would like to know theoretically what direction of DC will cause the crystal to compress or expand?

Assuming you can overcome the above difficulty by only applying the ramp for a short period of time, the data sheet for the piezo will tell you the answer.