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Ceramic caps can have a piezoelectric effect??? — Parallax Forums

Ceramic caps can have a piezoelectric effect???

groggorygroggory Posts: 205
edited 2010-10-08 12:20 in Propeller 1
I was looking into using the LT1964 negative voltage regulator and came across this in the manual:
Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress,
similar to the way a piezoelectric accelerometer or microphone
works. For a ceramic capacitor the stress can be
induced by vibrations in the system or thermal transients.
The resulting voltages produced can cause appreciable amounts of noise, especially when a ceramic capacitor is
used for noise bypassing. A ceramic capacitor produced
Figure 4’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
Page 12-13 in http://cds.linear.com/docs/Datasheet/1964fb.pdf

By the engineer tapping on a ceramic bypass cap with a pencil he was getting about a 1.2mV spike in the output voltage. Ugh.

I was designing a circuit that would be used on a pretty vibration prone device. I was using a bunch of ceramic caps in my butterworth filters in my amplifier stage before my ADCs. Maybe I'll go through and replace them all with polyester film caps. :-/

Comments

  • $WMc%$WMc% Posts: 1,884
    edited 2010-10-07 17:41
    groggory wrote: »
    I was looking into using the LT1964 negative voltage regulator and came across this in the manual:


    Page 12-13 in http://cds.linear.com/docs/Datasheet/1964fb.pdf

    By the engineer tapping on a ceramic bypass cap with a pencil he was getting about a 1.2mV spike in the output voltage. Ugh.

    I was designing a circuit that would be used on a pretty vibration prone device. I was using a bunch of ceramic caps in my butterworth filters in my amplifier stage before my ADCs. Maybe I'll go through and replace them all with polyester film caps. :-/
    '
    Thanks for the info. I always thought of ceramic as being very hard and not flexing much.This hint will allow me to revisit some older projects that were plagued with noise,And that used ceramic caps!
  • prof_brainoprof_braino Posts: 4,313
    edited 2010-10-07 18:07
    Could this be used a sensor?
  • RaymanRayman Posts: 14,889
    edited 2010-10-07 18:12
    prof_braino wrote: »
    Could this be used a sensor?

    My first thought exactly! Could be a poor man's accelerometer...
    Or perhaps better put, a high G range accelerometer...

    I'm trying to guess what kind would be best... I'd guess my 0805 4.7 uF multilayer would be best, just because I'm sure they have the highest amout of "multilayering" possible...
  • Luis DigitalLuis Digital Posts: 371
    edited 2010-10-07 20:20
    Hello,

    Long ago (when I was a child :) ), I discovered that some capacitors for tube radios and TV could make a sound. Were constructed in exactly the material is now used as high-frequency speaker.

    Shape: circular.
    Exterior Material: soft plastic, easy to remove.
    Color: usually pink or Light Blue.

    I kept one for the future, the future arrived, but do not know where I kept it or if it exists. :P
  • LeonLeon Posts: 7,620
    edited 2010-10-07 22:07
    Disk ceramics are prone to that problem, rather than multi-layer surface-mount devices.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-10-07 23:44
    If you look hard enough, virtually all dialectrics will give microphonic effects, including plastic covered wire. Ceramics are just particularly good at it.
  • BigFootBigFoot Posts: 259
    edited 2010-10-08 08:37
    There are some new ceramic caps available with flexible terminations that might help your problem. Check out

    http://www.avx.com/docs/Catalogs/flexisafe.pdf

    Russ
  • Graham StablerGraham Stabler Posts: 2,510
    edited 2010-10-08 08:39
    The cure is to avoid tapping your circuits with a pencil :)
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2010-10-08 11:47
    This is a real issue in industrial applications. It is good to know and keep in mind.

    I've a friend that services computers on train cars that grind the rails periodically in order to maintain high speed rail track. He does so all over the world and the computers tend to fail quite frequently as the whole rail car with the track grinders is a very heavily vibrating platform.

    Mostly he has had to deal with premature failure of hard drives, but he also has a rather large collection of retired hardware that just doesn't work and the reasons are unknown.

    I did suggest that he look into a SBC that had been vibration certified, but his employer was committed to standard PC motherboards due to their availability and nominal costs. Considering how many by-pass capacitors are in the average motherboard, it is quite likely some are behaving badly.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-10-08 12:20
    That could be due to metals work-hardening. Joints and tracks will become more and more britle as they get constantly bent back and forth. The boards would have to be suspended independantly so that the high G vibrations didn't get through.
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