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Why does the ceramic transducer run off its reonant point

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engineer1000

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I have an impedance plot of a ceramic ultrasonic transducer, I am trying to run it at the third harmonic, the impedance plot says it should be 135khz.
I am using a half bridge switching through a transformer and then using a choke and the static capacitance of the transducer as the resonant circuit.
Calculating L is L= 1/((2��f)2) C
I am using an adjustable choke butno matter what value the choke is I can’t get the transducer to resonate at 135khz. It resonates at 131.7kHz.
I can tell this by hooking a scope across the transducer looking at the sine wave fed in to the transdcuer and finding the highest amplitude wave form .
This is at 131.7KHz.
What I want to know is why is n’t it at 135khz?
 

FvM

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I have an impedance plot of a ceramic ultrasonic transducer, I am trying to run it at the third harmonic, the impedance plot says it should be 135khz.
Are you referring to parallel or serial resonance frequency? Does the plot specify transducer mounting conditions?
 

engineer1000

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Are you referring to parallel or serial resonance frequency? Does the plot specify transducer mounting conditions?
measurements were done on the transdcuer using a Biologic VMP2 Electrochemical Impedance Analyser. I am then using a series resonance circuit to run the transducer
 

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So the analysator measurement shows series resonance (minimal impedance) at 135 kHz?
 

engineer1000

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Yes I think so, I didn't do the plot. They transducer is meant to resonate at 135khz with my series resonant circuit it resonates at 132khz
 

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I didn't do the plot. They transducer is meant to resonate at 135khz.
Yes. But the plot would show, if it's a series (minimal Z/ maximal Y) or parallel (maximal Z/minimal Y) resonance. That's the question.
 

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Yes. But the plot would show, if it's a series (minimal Z/ maximal Y) or parallel (maximal Z/minimal Y) resonance. That's the question.
Hi
I think that I have found the problem. At 135KHz the voltage across my transformer starts to collapse because of the low Z of the transducer at this frequency. I can run it just off resonance and it seems to work fine because the Z is larger.
 

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