# What happens when DC is applied to a piezo crystal?

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?

• PI Ceramic has a pretty good overview about Piezo technology on their website here – Arsenal Apr 28 '20 at 11:03

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.

• Where can I find a data sheet for piezo crystal? When I search the internet all I can find is data sheet for piezoelectric sensors. – Somanna Apr 28 '20 at 8:24
• I'm no expert in using piezos but there are some around because there have been other questions on this site that linked to them. Try doing a search on this site. For instance this one - scroll up to the question and you'll find a link. But remember, this is a Q and A site and not a shopping site. – Andy aka Apr 28 '20 at 8:28
• @Somanna I got several hits searching for "piezo block": you could search further on terms you find in these hits, like "discrete piezoelectric stacks", "Piezo Ceramic Block" – Huisman Apr 28 '20 at 8:55