EEVblog #855 - Ceramic Capacitor Piezoelectric Effect

[modrobert]

Feature: * Subtle high pitched indicator sound when the backlight is on, as a reminder, to preserve battery life.

[hopski]

It does turn itself off after 10 minutes to save battery life. seems a bit long, maybe that could be reduced.

[tszaboo]

So another industry story from the same effect:
We had an instrument, measuring voltages with some 0.003% accuracy, with some kHz speed. So obviously, it had a precision reference voltage, which was filtered quite a lot. The issue was, that it was picking up any vibration of the PCB, and there were fans in the system (loud ones). So I traced down the issue to the ceramic capacitors filtering the reference voltage. There was a PI filter,  CRC, 10 uF, 10Kohm, 10uF (from the top of my head) on the output of the reference voltage, X7R capacitors. That is an 1 Hz filter, going to an opamp for buffering. So when I tapped the capacitors with a pen, they were creating up to a few mV  (!) of noise on the output. That is how much noise these can generate.
The caps were replaced with SMD film capacitors. Spare no expense. All the noise was gone, I could hammer those all day long. I guess that was lesson learned.

[artag]

One can also get test microphones (usually electret condensors) with extremely wide frequency ranges (1Hz-200kHz) that I would guess you could use to zero in on a sound source by sweeping it in close range across the board.
ref:
http://www.aco-japan.co.jp/eng/Meas-Mic/meas_mic.html
http://bertrik.sikken.nl/bat/mics.htm

There are also a number of web articles and even a thread in here somewhere about how to use a mic with an oscilloscope.

Or you could use a small ceramic multilayer cap as a microphone. It's not made to pick up sound from the air so it should be more selective for mechanical contact. Don't forget that the actual guilty device may not be the 'loudest' place - that might be the circuit board node or other component that's moving as a result of the vibration.

[wraper]

you could use a small ceramic multilayer cap as a microphone.

Nope you couldn't. That would be an awful microphone, which wouldn't be able to sense anything but a huge vibration.

[elCap]

The big inductor on the back side of the PCB is for the piezo buzzer.
In my Brymen 257s the buzzer did not work when I first got it. I didn't bother to return it for warranty repair.
After I had the meter resting on top of my power supply, heating it a little, it suddenly started to work. Took the meter apart, and found that there was a small crack in the inductor. After replacing it the buzzer started to work. I didn't have the big value needed so the volume is a bit on the low side now.
So if one wants a higher volume from the buzzer, install a bigger value inductor.

[Muttley Snickers]

I think that Dave mentioned that the inductor on the rear of the BM-235 board was for the Non Contact EF Detection which according to both the BM-235 and the BM-257 manuals is located on the top right hand side of both meters as shown in the picture below.

[elCap]

I didn't completely trace out the circuit but I'm quite sure the inductor (L4 in BM-257) is part of the buzzer circuit as replacing it fixed my non-working buzzer.
First image that came up from a google search for a piezo buzzer circuit:
http://2.bp.blogspot.com/-9Db008ehTHA/Tul-pu7SF8I/AAAAAAAAATU/r7wYX7CdL5U/s1600/12+Volt+Piezo+Buzzer+Circuit+Diagram.png

[Barny]

Could it be, that the inductor gets used for both purposes?

Makeing noise and detect ?

[Godzil]

I've checked with my BM257s, with my ear stuck on it without the backlight, I can hear a really really fait buzzing that slightly change with the backlight is on, but as the backlight is not white, the voltage is probably lower, the design is different so not really an interesting test after all, but mine is faintly buzzing even without the backlight

[iXod]

If room permits, replace with different composition cap? Electro SMD or radial (laid on side)? I've seen some pretty small radials...

Or poly-something?

That should eliminate the issue, no?

[cte7ds]

Checked my Brymen BM869 and BM189. Couldn't hear anything in a quiet room with the backlights turned on and my ear pressed against the backs. 

However, I've gotten (and keep on getting) my fair share of this problem, thanks to Logitech... (https://www.eevblog.com/forum/repair/ceramic-capacitor-whine-in-logitech-g-series-mice/) 

[Georgitsu]

This was great! I noticed a similar high-pitched squeal on a instrument I was designing, but I didn't know what it was from. First I thought it was from the speaker output, but later discovered this piezo-electric effect of certain capacitors and realized that was surely the cause.

Cool video! 

[Synthetase]

I've checked with my BM257s, with my ear stuck on it without the backlight, I can hear a really really fait buzzing that slightly change with the backlight is on, but as the backlight is not white, the voltage is probably lower, the design is different so not really an interesting test after all, but mine is faintly buzzing even without the backlight

Yep, I tried exactly the same thing, same result.

[EEVblog]

Have you shown this video to Brymen? If so, what did they say...?   

Got word back from Brymen R&D. Cap has been changed from 10uF to 10nF, problem solved. Guess they didn't really need that 10uF after all.
All new units I get will be fixed.

[WackyGerman]

Got word back from Brymen R&D. Cap has been changed from 10uF to 10nF, problem solved. Guess they didn't really need that 10uF after all.
All new units I get will be fixed.
[/quote]

 

[Godzil]

Have you shown this video to Brymen? If so, what did they say...?   

Got word back from Brymen R&D. Cap has been changed from 10uF to 10nF, problem solved. Guess they didn't really need that 10uF after all.
All new units I get will be fixed.

They are a good brand, and I'm really happy to have discovered them from your video, I really love my 257S (and thanks to Frankie for selling them!)

[jancelot]

Would it be possible to replace the ceramic capacitor to totally eliminate the sound?
EDITED: yes it's possible watch until the end of the video.

By the way, the Brymen Bm257s makes a very low noise in the audible range, if you put the ear in the right side of the screen (as you look at it), with either the screen backlight on or off.