How can I test ultrasonic transducers

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julio_w

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Hi All,

I am trying to build a large ultrasonic cleaning bath, I already bought 50 PZT transducers 60 Watts and 40 Khz each from a supplier on china, and I am going to buy a 3 Kw ultrasonic generator to connect all transducers in parallel.

So before buying the large generator I wanted to test the transducers one by one just to be sure that what i get is what i order, so I decided to buy from ebay one small ultrasonic cleaner of 50 Watts 40 Khz and then I was thinking in replacing the transducer with the new ones one by one and test.

So my question is how do I test? I know you can do the foil test but that will just tell me that there is ultrasound going on but I want to know the actual frequency of the transducer. So can I just buy a transducer receiver that cost around 2$ and connect at the terminals an oscilloscope? not even sure if that is possible.

Thanks for reading and I would appreciate your help.
 


I can describe how I test ultrasonic transducers:
As a receiver I connect a 40 kHz or 24 kHz device to an oscilloscope.
I use a 555 as a tunable ultrasonic generator and connect a tested device to its output. As I tune the generator around the expected frequency, I can see on the scope the received voltage when the both devices are located close to each other. By tuning the voltage to a maximum I can check the resonance frequency. By rotating one device against the other I can see how directional it is.

For high-power devices I used a 3W transistor amplifier with an output transformer tuned to 24 or 40 kHz as needed. The voltage on the transducer was ~100V p-p and the receiver can be separated several meters to see the signal.
With high-power ultrasonic devices be aware that they should be immersed in water (for cleaning), or attached to a resonant plate. Also a good heat-sinking may be important.
For other frequency than 24 or 40 kHz I can use a small electret microphone from a cell phone or a tape recorder. It is not resonant and only operates with 3 to 12 V positive power supply connected through a 10 kOhm resistor, to supply the internal FET preamplifier. Old-style crystal microphone or phono pickup can also be used.
Do not expose yourself or other persons to high ultrasonic power, the hearing can be damaged. Use tested devices as above to check transducer operation. Pets like dogs and cats may be disturbed by ultrasound, too.
 


Thanks for answering I still have a couple of questions....

1. What do you mean with a 40 kHz device? just the transducer receiver? or something more complex than that? can you give me an example of a device you had in mind?
2. If i used the receiver connected to the oscilloscope would I get voltage only if there is a 40 Khz signal? or other frequencies will trigger some voltage in the receiver?

In my case the transducers will be glued with epoxy in the bottom of a stainless steel sink which will function as a resonant plate and as a security measure i will be using ear protection while testing and in the future using the machine (thanks for the recomendation).

Will the microphone idea give me the frequency if connected to an oscilloscope?

Thanks in advance and sorry if there are some very basic questions...
 

I found this schematic for a bat detector, they claim they can detect up to 100 khz and basically divide the frequency by 16 to get an audible signal...

**broken link removed**

So I was thinking that if that is true I could do something similar eliminating the frequency divider, ear plug, etc and if I attach an oscilloscope at the exit of IC2 i should be able to measure the frequency, am i right?
 

The frequency of the generator can just be measured with your CRO. You need to vary the frequency of the generator and see where the peak mechanical energy output is when its operating in water. I think that using multiple transducers which are not selected for having the same resonant frequency, will lead to some not actually delivering much power. You should be able to "pull" the resonant frequency slightly by putting an inductance in series with each one.
years ago I tested ultrasonic generators and we used small brass letter scales with the weight pan sitting over the transducer in water and it used to exert about 50 gmms of pressure on the underside of the pan. So I would try something like this out. Carefully note the frequency and the bandwidth of the power output.I doubt if a CRO can be used for this as the frequency will have to be read to .1%. Once you have the results for all your transducers, see if using a certain frequency will result in all the transducers generating power. Any transducers that are "low power" at this frequency must be replaced or fiddled with to get them to work, Try putting a small inductor in series with them individually to see how it effects the resonant frequency.
Frank
 

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