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Ultrasonic Transdcers

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spuffock

Member
I am trying to wring the best possible performance out of a pair of cheap ultrasonic transducers. The drive signal comes from a bridge output stage running on 3.7 volts.
I have measured the capacitance of the sensor and tried to tune it with an inductor, but find that this path leads nowhere, I get less signal than before. A transformer seems to be the only answer.
I was wondering if anyone knows much about the failure mode of these transducers if I exceed the maximum ratings. In trying to produce a short burst, I have found it possible to "unring" the transducer by applying several cycles of drive in antiphase at the end of the burst, but the start of the burst could well be speeded up. I thought that maybe 100 volts, but only for a few cycles until the sound amplitude built up to the continuous level at 30 volts. If the failure is caused by overheating or mechanical failure I should be able to get away with it, but if it's dielectric breakdown then I'm sunk.
Any comments?
 
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Hero999

Banned
I would have thought that dielectric breakdown would be the limiting factor.

Yes, you're correct that a transformer is the only really sane way of driving a transducer from a 3.7V supply. Use a switch mode power supply style ferrite cored transformer a mains or audio transformer will be no good for ultrasonic frequencies. The primary current might be unreasonably high if the transducer you're using is really powerful.
 

spuffock

Member
Well, we have an argument, I think it'll be a heating problem. An experiment is called for.
I'll try a couple of hundred volts dc and see if it breaks down. If not, I'll try a couple of hundred peak to peak, 1ms pulse 10 pps. I'll post the results, since it doesn't appear to be commom knowledge.
Get back in a day or 2
 

mneary

New Member
The transducer is already a resonant device at the operating frequency. Measuring the capacitance, using some other frequency, gives misleading results.

As Hero says, step up transformer is your best bet. I'd say the piezo would shatter before the dielectric breaks down, but only because you asked us to guess. :D
 

spuffock

Member
Experiments have shown the following. The devices will withstand 300 volts dc for at least 10 minutes, so it wont be dielectric breakdown on its own.
Using 10 cycle pulses at 10 pulses per second:
Increasing the drive level showed an increase in output up to 120v pk-pk, above which the output levelled off.
At about 160v pk-pk there was an abrupt discontinuity, possibly caused by physical damage taking place. Further operation at lower levels showed a permanent reduction in output of about 5%.
Examination of the residue under the microscope revealed no visible damage.
So there we are. It looks reasonable to use them up to about twice the manufacturers ratings for short pulses and expect them to carry on.
As for myself, I'll use them for proof of concept, but for building any saleable kit I'll do the obvious, and buy some that will do the job within their ratings!
Hope the audience have gained someting useful.
 

mehlo

New Member
Hello Friends
I am using four ultrasonic transducers to measure wind speed and direction. I want to measure the time of flight back and forth for each pair. I am using a PIC18F4520 to calculate the wind speed and direction. I need to send four PWM signals to the transducers but my PIC has only two CCP modules for the PWMs.
Where can I get the other signals?
Please help!
Sharp!!!!
Sprite
 

dknguyen

Well-Known Member
Most Helpful Member
Mehlo, have you ever seen this button before?

 

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