Piezo Element Energy 2

[swertel]

Please help out a mechanical engineer interpret this electrical verbiage.

I have an assembly that uses a Piezo element in it to excite a component upon impact. The specification requires that it

shall supply electrical energy equivalent to that discharged by a 0.005 microfarad capacitor that has been charged to 9 volts, minimum, when impacted by a 1.94 ounce steel ball dropped from a height of no greater than two inches (2”)

What is that in terms of Voltage, Amps, or Watts?

The disc is .7 inch diameter by .1 inch thick per MIL-P-12815 if that helps.

I'm trying to test the component that gets energized by the piezo without destroying a bunch of piezo elements in the process. Thus, I need to know how much energy the piezo actually gives off so I can reproduce it using another supply source.

--Scott

http://wertel.eng.pro

 

[Comcokid]

Well, a 0.005uF capacitor charged to 9 volts, W=1/2*C*V^2, or energy equivalent to 0.405 micro-coulombs. One coulomb is 1 watt-second.

A piezo under flexing may produce a voltage greater than 9 volts. A 1.94 oz steel ball hitting a piezo from 2" will produce a transient voltage of very short duration. Normally a piezo being struck will produce a voltage of one polarity as it is struck, and reverse polarity as it relaxes back to it's original position, but in this case the piezo will produce a voltage until it fractures.

If I was to non-destructively duplicate the condition, I would first verify the spec by setting-up the piezo with the circuit and an oscilloscope, perform the steel ball drop and record the waveform, look at the voltage/time given the circuit impedance - just to make sure I'm duplicating reasonably the 0.405 micro-couloumb value. -THEN- I would use the waveform I recorded, and play it back through a arbitrary waveform generator (taking into account possible differences in source impedance of a piezo vs the generator) to duplicate the conditions to check the operation of the circuits.

 

[geekEE]

Hey Comco, I think you skipped the divide by two. Also, I think you mean joules rather than coulombs. I come up with 2.025E-7 joules.

Glenn

 

[IRstuff]

Jiminy!! ;-) A coulomb is an amp-second


I also get 202.5 nJ of energy.

One might hope that the specification has a more direct meaning; that the discharge of a 9V impression on a 5 nF capacitor actually behaves like the piezo element:

http://www.piezo.com/prodbg7qm.html

Based on that webpage, one might just try to see if a 5 nF cap with 9 V behaves like the piezo element.

TTFN

FAQ731-376

 

[Comcokid]

DOH!!

Your right geekEE. That's what I get for replying at the end of the work day when my brain is half fried.

 

[swertel]

Thanks, Comco for the detailed explanation. And thanks, Glenn, for the errata. I'd thank IRstuff, but I have other dealings with him online and he needs no applause.

--Scott

http://wertel.eng.pro