capacitance of 2500pF

[dr.power]

Hi guys,

I have got several ultrasonic 40KHz sensors, each has an impedance of 500 ohms and a capacitance of 2500pF.
I want to know if the said 500 ohms impedance has the the reactance part of the capacitance of the transducer at 40KHz due to the said 2500nF capacitance?

The other question: Can i cancel or significantly reduce this capacitance at 40KHz by an inductor or something else? If so then what will happen to the output? Any change in the output power or SPL?

Thanks

 

[szzuk]

The capacitance and inductance of the sensor will change with frequency. 500 ohms will be the impedance at resonance of the entire sensor, the piezo plus metal parts, so no it isn't just the reactance.

You can cancel out much of capacitance with an inductor, this will reduce the impedance of the sensor. The sensor will use the power you input more efficiently but the sensor itself won't output anything extra.

 

[dr.power]

The capacitance and inductance of the sensor will change with frequency. 500 ohms will be the impedance at resonance of the entire sensor, the piezo plus metal parts, so no it isn't just the reactance.

Thank you for your reply.
What is the affrect of the capacitance to the driver and why the capacitance of an ultrasonic sensor is rateed at 1KHz??!

You can cancel out much of capacitance with an inductor, this will reduce the impedance of the sensor. The sensor will use the power you input more efficiently but the sensor itself won't output anything extra.

Ae you meaning that a for instance a sensor having an impedance of say 500 ohms at resonance will change its impedance by adding an inductor in series with it?

 

[szzuk]

The capacitance of piezos is measured at 1KHz, it is simply the industry standard. The capacitance isn't a big problem, it is not hard to drive, an obvious problem is that some amps don't like to drive capacitance, I can't think of anything else.

Adding an inductor will reduce the impedance from 500 ohms to for example 100 ohms.

 

[crutschow]

An inductor of the proper size can resonate with the capacitance to cancel its reactance and leave only the resistive portion of the impedance which I believe is 500 ohms. It will not reduce the value of the resistive load.