Piezo Transducer Testing

Introduction 
This blog shows a piezo transducer's audible response to square waves. Measurements were performed using the software Friture with an external microphone.

Piezo Transducer

Background
Following an incorrect delivery of a piezo transducer instead of a piezo sounder, it was fascinating to determine how the device would respond different frequencies.

Test Device
A piezo sounder is a device containing the piezo element and related drive circuitry. The drive circuitry generates the necessary signal to drive the piezo element at a specific frequency.
The piezo transducer (piezo element) tested in this blog was a part (ABT-414-RC) that specified the external drive at a certain frequency.

The specifications for the transducer are replicated below from the datasheet.

ABT-414-RC Transducer Specifications

Testing
To measure the response of the transducer, a function generator was connected directly to the piezo. The function generator settings were made for a square wave at 0 - 3 V pk with a 50 % duty.

Image of Test Setup

Measuring the transducer sound output was performed using the software package Friture. Friture is an open-source easy-to-navigate real-time audio analyser.

About Friture

Friture was configured with the settings displayed below, A weighting for the measurements.

Friture Settings

The microphone for measurement was a Thronmax MDrill One Pro configured for cardioid mode.

Measurements (dB) were performed at 50 cm instead of the usual distance of 100 cm. These measurements were indicative.

Frequencies between 1 kHz and 4 kHz were set on the function generator, with measured values taken from Friture. Pictured below are the plotted results taken at 50 cm spacing between the piezo and microphone.

Plot of Measurements vs Frequency for ABT-414-RC

The operation of the function generator was then changed from a single frequency to a sweep. Frequencies were swept between 1 kHz and 6 kHz over a period of 100 ms.

In the screen capture below from Friture, the first frequency peak at 2.4 kHz is seen first. There was a second resonant measurement at approximately 5 kHz.

Plot from Friture for ABT-414-RC Frequency Sweep

The sweep response above shows that the drive frequency for the piezo transducer can vary by a more than 100 Hz with minor reduction in output level.

Lastly, the function generator output frequency was set to 2.4 kHz. The measurement from Friture is displayed blow.

Piezo Measurement at 2.4 kHz

 
Limiting Piezo Voltage
To limit voltages seen by the driving source, a device such as a general-purpose diode is recommended across the terminals of the piezo. The two oscilloscope captures below show the effect on the waveform with and without a diode across the transducer. 

When measuring the sound level with the microphone, the difference in measurements for a frequency of 2.4 kHz was less than 2 dB with and without a diode fitted.

Waveform at Piezo Transducer

Waveform at Piezo Transducer with Parallel Diode

 
Summary
A piezo transducer can readily replace a piezo sounder however, implementation will depend on the hardware utilised to drive the piezo. Circuit protection should also be reviewed as part of the standard impact analysis.

Piezo Buzzer Diode Snubber

Summary

This short post illustrates, using oscilloscope captures, why a requirement exists to include a snubber, such as diode, across a driven (externally modulated) Piezo buzzer element. 

Unsuppressed Circuit
In the schematic below using a Kingstate Piezo buzzer, the snubber component usually in parallel with the Piezo buzzer was omitted.

Unsuppressed Piezo Buzzer

The capture shown in the image below was measured across MOSFET (S1) whilst being driven at a supply rail voltage of 5VDC. Voltages were measured in excess of the supply rails due to the nature of the piezoelectric element.

Component S1 Drain Voltage Unsuppressed

Suppressed Circuit - Diode
Piezo manufacturers usually state that a resistor, diode, Transorb or similar device should be placed in parallel with the buzzer to manage any energy created by the piezoelectric effect.

Component S1 Drain Voltage Suppressed

The capture above shows the reduction in peak voltage, close to the supply rails, when a general purpose diode was fitted across the Piezo buzzer. Results using a 5.1V Zener diode are identical to a general purpose diode.

Diode Suppressed Piezo Buzzer

The circuit shown above implements a diode for snubbing. For a measured peak voltage of almost 30V without suppression, a diode with a reverse breakdown voltage of double the recorded value was chosen.

Suppressed Circuit - Resistor
In some Piezo manufacturer datasheets, suppression is achieved with the use of a resistor. A resistor may be a cheaper solution to snubbing although the value of the resistor should be selected carefully.

Shown below is a capture, performed in the same manner as the previous section, using a 1K resistor in parallel with the Piezo buzzer. The resistor value was selected because it appeared to be a common value shown on posts and forums. Unfortunately the resistance is far too high and voltages in excess of the supply rails are still developed in the circuit.

Resistor 1K Suppressed Piezo Buzzer

Reducing the value of the resistor by a factor of ten yields improved suppression as seen in the capture below. The reduction in peak voltage is almost fifty percent.

Resistor 100R Suppressed Piezo Buzzer

Summary
For any device requiring electronic suppression, being prudent with the type of suppression can provide considerable differences in the result. The captures in this post were applicable to an externally modulated Piezoelectric element with suppression fitted in parallel with the Piezo.