Piezoelectric effect -- How long does a stressed crystal produce a DC voltage?

[Sasho Andonov]

I need help regarding piezolectric effect...
If I add DC voltage to piezoelectric crystal it will change its dimension (length) and vice versa: if I put pressure on piezoelectric crystal it willl produce DC voltage on its surfaces...
But I am wondering how long this voltage will last? If I keep pressure 10 minutes will my voltmeter keep the voltage on the same value...?
THANK YOU VERY MUCH!

 

[sophiecentaur]

Quartz is quite a good insulator so, as long as the PD is low enough to prevent a discharge, I would have thought the meter reading would stay the same.

 

[Sasho Andonov]

PD...? Could you please explain what is this...?
Anyway, THANK YOU VERY MUCH!
This information is very important to me! :-)

 

[sophiecentaur]

PD is Potential Difference or Voltage.
Be careful with Piezoelectricity, you can get a nasty shock. You need to read around about the subject if it is "very important" to you. Asking a few questions is not really enough.

 

[anorlunda]

If you use the voltage to cause a current to flow, the voltage must vanish. The reason is conservation of energy.

When you press, you are doing work which can be converted to electric power. But if you then maintain the pressure with no more motion, you are not doing work any more so conservation of energy tells us that you can't continue generating electric power.

 

[sophiecentaur]

Is the polarisation not just a manifestation of Potential Energy due to displacing charge? You are implying it's an AC coupled thing and I think it is DC coupled. Must read it up again tomorrow. Bed time and a spot of fiction reading for me now.

 

[sophiecentaur]

If you use the voltage to cause a current to flow, the voltage must vanish. The reason is conservation of energy.

When you press, you are doing work which can be converted to electric power. But if you then maintain the pressure with no more motion, you are not doing work any more so conservation of energy tells us that you can't continue generating electric power.

Is the polarisation not just a manifestation of Potential Energy due to displacing charge? You are implying it's an AC coupled thing and I think it is DC coupled. Must read it up again tomorrow. Bed time and a spot of fiction reading for me now.

We're not exactly disagreeing here. If you are looking with a voltmeter (ideal or even just a DMM) I would reckon not enough current would flow to discharge the induced charge. If there's a resistive path then it will discharge - but the resistor would need to cover the area of the end of the crystal to collect all the charge. As usual - it all depends.

 

[anorlunda]

@Sasho Andonov , is your original question asked with energy harvesting in mind, or just the properties of materials?

 

[Sasho Andonov]

@Sasho Andonov , is your original question asked with energy harvesting in mind, or just the properties of materials?

I am asking because I am investigating use of piezoelectric sensor... I would like to know will the generated voltage exist enough time to measure it by voltmeter...

 

[sophiecentaur]

If there is a PD drop, over the period of the experiment, by gathering enough data of V against t, you can correct for any sag with t. The secret would be to take your measurements quickly and over the same time for each value. You would need to plot a graph to find the time constant and to make your correction based on that.

 

[Sasho Andonov]

If there is a PD drop, over the period of the experiment, by gathering enough data of V against t, you can correct for any sag with t. The secret would be to take your measurements quickly and over the same time for each value. You would need to plot a graph to find the time constant and to make your correction based on that.

Thank you! This is not problem for my application!

 

[sophiecentaur]

Thank you! This is not problem for my application!

From the questions you have been asking I would have thought it could be a serious problem. You wanted to wait 10 minutes for the reading. That could be a very long discharge time for a small crystal. If you don't want to get a reliable answer then why ask the question in the first place?

 

[Sasho Andonov]

From the questions you have been asking I would have thought it could be a serious problem. You wanted to wait 10 minutes for the reading. That could be a very long discharge time for a small crystal. If you don't want to get a reliable answer then why ask the question in the first place?

I put 10 minutes as example and I was expecting numerical answer, let's say: No, 10 minutes is to much, but in 20 seconds the charges will disappear... :-)
Unfortunatelly, no one privide such a type of answer. Anyway, the responses (which I have received) give me a confidence that my idea would work...
THANK YOU VERY MUCH! :-)

 

[sophiecentaur]

Unfortunatelly, no one privide such a type of answer.

Until you say what size of crystal you are planning to use and the resistance of your meter, it is impossible to say. That's why the (very carefully) PF responses were non specific.
Edit: I now 'remember' that piezoelectric devices are mostly ceramic as their piezoelectric constant is two orders of magnitude higher than quartz. I am looking for a good equivalent circuit that will suggest a value for the equivalent capacitance of a typical sensor.