AVR Freaks

The small ultrasonic cleaners I've seen all used piezoelectric crystals, although I'm sure other approaches exist.

If one physically deforms the crystal, (either with a sound wave, or a hammer), it generates a voltage.

If one applies a voltage across the crystal, it deforms the crystal, generating motion, and hence a pressure wave.

(In this case the crystal is usually connected to a diaphragm, think of an old fashioned "crystal microphone", used backwards.)

The two common ways to generate the ultrasonic signal are to apply a drive signal to the crystal.

The drive signal can be set to the crystal's resonant frequency, so you get a continuous and good size physical vibration in the crystal, or one just applies a big voltage pulse to the crystal, to deform it, and it will ring at its resonant frequency.

Perhaps a material science engineer can chime in, (any of those on Freaks?).

I think the limiting factors on the ultrasonic cleaner are both the electrode bond to the crystal, and the destruction of the crystal by its internal (shear ? ) forces with higher drives.

Last Fall I spent a little time trying to measure the impedance of a small ultrasonic transducer as a function of the applied frequency.

I clearly wasn't doing it like the manufacture's set up. 

My results didn't generate the nice graph in the data sheet...

I did, however, see several mini-resonant frequencies, from say a longitudinal wave vs a transverse wave in the crystal.

Very interesting, but not what I was after.

The real question for John, of course, is why did the caps fail?

Old age, dried out, etc?

Or under spec'ed for the application?

JC

Digikey doesn't stock the 5100pF but Mouser does.

Mouser Part #:              667-ECW-H20512HV

Manufacturer Part #:    ECW-H20512HV

Manufacturer:                Panasonic

Description:                   Film Capacitors 0.0051uF 2000VDC 3%

Pricing (USD)                  1:  $1.01