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PCRT21ASX Datasheet - Page 3

Download or read online AVX Corporation PCRT21ASX Piezoelectric Acoustic Generators pdf datasheet.



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Piezoelectric Acoustic Generators
Introduction
AVX-Kyocera manufactures a wide variety of piezoelectric
acoustic generator elements. These include external drive and
self-oscillating buzzers, beepers, ringers and receivers.
Piezoelectric acoustical transducers are non-mechanical
contact devices and feature the following advantages:
• Free from RF noise and contact sparking
• Simple, compact and light weight
• Consume little power and have long life
• Generate various timbres of pitches
Examples of applications are: telephones, watches,
calculators, appliances, automobiles, smoke detectors and a
wide variety of other electronic equipment.
The basic element in all of these is a piezoelectric ceramic
mounted on a metal diaphragm. When AC voltage is applied
across the electrodes of the piezoelectric ceramic it expands
and contracts at the frequency applied. This causes the metal
diaphragm to bend, producing sound waves (Figures 1 and 2).
The metal diaphragm is usually brass or stainless steel
and less than 0.5 mm thick. PZT ceramic material is used as
the piezoelectric element. This element is designed so that the
mechanical resonant frequency matches the frequency of the
driving signal.
VIBRATION (CERAMIC ONLY)
figure 1
VIBRATION (CERAMIC and DIAPHRAGM)
+
Vibration
Node
figure 2
2
Piezoelectric Acoustic Generators are available in two types:
1.
Two-terminal type
2.
Three-terminal type
The two-terminal type works when signal voltage is
applied on the metal diaphragm as one electrode and the
conductive-material-screened piezoelectric element as the
other electrode.
The three-terminal type has a split electrode on the
piezoelectric element. When the signal is applied between 2
and 1 in Figure 3, the phase shifted signal will be induced
between 3 and 1 so that it works as a piezoelectric trans-
former. The phase shifted signal can be used as the feedback
component in a simple oscillation circuit which operates
automatically at the natural resonant frequency of the element.
2-TERMINAL TYPE
Ceramic
+
-
-
+
Impedance Characteristics
The equivalent circuit of piezoelectric acoustic generators
(2-terminal type) can be explained by the same equivalent
circuit used for quartz crystal resonators and ceramic resona-
tors. The mechanical resonance is shown by the series
resonance circuit of R
is determined as follows:
-
f
-
r
L C
+
Ceramic
Metal Plate
In the case of the piezoelectric transducer the shunt
capacitance is larger than that of other resonators. Therefore,
the total impedance is capacitive.
1
2
1
2
3
3-TERMINAL TYPE
figure 3
, L
, C
, and its resonant frequency (f
0
0
0
EQUIVALENT CIRCUIT
R o
L o
C o
C s
figure 4
)
r

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