Measuring voltage across piezo speaker, getting resonating noise.

[KingNothing]

Hi. I'm trying to measure the milliseconds of duration of output from a little piezo speaker that is on a circuit (a freezer control unit). I tried connecting an oscilloscope across its leads, but when I apply power, I hear this resonating noise which does not sound healthy. This is only when the device is powered.

Any suggestions on why this might be? This noise ONLY occurs when I have the oscilloscope connected. What else might I try to measure this (other than a microphone)?

 

[berkeman]

Hi. I'm trying to measure the milliseconds of duration of output from a little piezo speaker that is on a circuit (a freezer control unit). I tried connecting an oscilloscope across its leads, but when I apply power, I hear this resonating noise which does not sound healthy. This is only when the device is powered.

Any suggestions on why this might be? This noise ONLY occurs when I have the oscilloscope connected. What else might I try to measure this (other than a microphone)?

Your oscilloscope is grounded, if you are plugging into the wall outlet for power. Can it run on battery power instead? If not, you could temporarily "cheat" the power cord, by using an adapter to go from the 'scope's 3-pron power cord to a 2-prong plug (you get these adapters in the electrical department of the hardware store). You don't want to cheat a 3-prong cord in general, because it bypasses an important ground fault protection of the 3-prong cord/instrument.

 

[vk6kro]

Connect a microphone with an amplifier to the CRO.

Place the microphone in front of the piezo speaker.

Observe the output on the CRO.

No need to connect anything.

 

[berkeman]

Observe the output on the CRO.

Quick translation of Aussie to here: CRO = oscilloscope

 

[vk6kro]

:)

Yes CRO is Cathode Ray Oscilloscope. A liquid crystal type would be OK too.
Most oscilloscopes are still the Cathode Ray type, I think.