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Transducers - II
In the last chapter we discussed classification of transducers, characteristics of
transducers and selection factors of transducers. In the last chapter some of the important
types of the passive transducers are discussed. In this chapter, we will discuss, some of theimport,lI1ttypes of the active transducers.
The transducers which generate an electrical signal directly in response to the physical
pdrameter whithout requiring external power for the operation are called active
transducers. Active transducers are also known as self generating transducers. Active
transducers are further classified as thermoelectric transducers, piezoelectric transducers
and photoelectric transducers.
Thermoelectric transducer is a temperature transducer which converts thermal energy
into an electrical energy. The most commonly used thermoelectic transducer is
thermocouple. Thermocouple is generally used as a primary transducer for temperature
mec1surement in which changes in temperature are directly converted into an electrical
signal. The thermocouple behaviour can be explained on the basis of thermoelectric
phenomena namely seebeck eff ect, Peltier effect and Thompson effect. Let us study the
thermoelectric phenomena in brief.
9.3.1 Thermoelectric Phenomena
In 1821, the great scientist Prof. Seebeck discovered that if the two wires of different
metals are joined together forming closed circuit and if the two junctions formed are at
different temperatures, an electric current flows around a closed circuit. This is called
Seedbeck effect. He also observed that if the two metals used are copper and iron, then
the current flows from copper to iron at hot junction and from iron to copper at cold
junction as shown in the Fig. 9.1.
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Cold Junction
(b) Practical thermocouple circuit
Fig. 9.3
1 \ lIsed to measure e.m.f. as shown in the Fig. 9.3(b). The hot junction is sometimes called
measuring junction while the cold junction is called reference junction.
The two wires of the thermocouple are generally twisted and welded together. ln
generala junction may be formed by two methods; namely twisted weld and Blltt weld.
Intwisted weld, two wires are t\"'isted together in several turns and welded together. ThisIlpe of welding is used for larger sized wires which gives mechanical strength. Tn Butt
weld,two wires of comparatively smaller sizes are fused into a round bend.
To measure higher temperature, the wire used should be heavier. But if the size of the
wireincreases, the response time of the thermocouple increases. So size of the wire is
selectedsuch that above mentioned two conditions arc compromised. Usually for noble
m e ta ls , the wire of diameter 0.5.mm is selected; while for the base metals, the diameter of
thewire ranges from 1.5 to 3 mm.
9 . 3 . 2 . 1 Materials used for Thermocouples
The thermocouples are made from a number of dif ferent metals including
copper-constant, iron-constantan, chromel - constantan, Platinum-platinum-rhodium, etc.
T h e y cover wide range of temperature. Say from -200C to 2800 C
Following is the table illustrating the ranges of temperature measurements in the
thermocouples of different materials.
Material used Types of Temperature range
Thermocouple
Copper-Constantan T - 250C to 400 C
lron-Constantan J -200C to 850C
Chromel-Alumel k -200 cC to 110C
Chramel-Constantan E -200C to 850C
Platinu m-Platinu m-Rhodiu m S o C to 1400 C
Tungston-Molybdenum - o C to 2700 C
Tungston-Rhenium - o C to 2600 C
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advantages of higher output, high sensitivity and capability to withstand high mechanIcal
stresses.
A piezoelectric quartz crystal is hexagonal prism shaped crystal, which has pyramIds
Jt both ends. This is shown in the Fig. 9.5 (a). The marking of co-ordinate axes are fixedfor such crystals. The axis passing through the end points of pyramids is called optic axis
or z axis. The axis passing through corners is called electrical axis or x axis while the a X Is
passing through midpoints of opposite sides is called mechanical axis or y axis. The axes
are shown in the Fig. 9.5 (b).
z axisI
I
I
Diamond
shaped
Pressure
~
Force summing
member
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converted to electrical energy is called photovoltaic effect. Photovoltaic cell is the common
eXilmple of this type.
Fig. 9.14 shows structure of photovoltaic cell. It shows that cell is actually a
PN-junction diode with appropriately doped semiconductors. When photons strike on the
thin p-doped upper layer, they are absorbed by the electrons in the n-layer; which causes
formation of conduction electrons and holes. These conduction electrons and holes are
separated by depletion region potential of the pn junction. When il load is connected
across the cell, the depletion region potential causes the photocurrent to flow through the
load
N-dopedsemiconductor
The photo transistor has a light sensitive collector to base junction. A lens is used in a
transistor package to expose base to an incident light. When no light is incident, a smallleakage current flows from collector to emitter called IeEO, due to small thermal
generation. This is very small current, of the order of nA. This is called a dark current.
When the base is exposed to the light, the base current is produced which is
proportional to the light intensity. Such photoinduced base current is denoted as I)...The
resulting collector current is given by,
. Ie "" hfeI)..
The structure of a phototransistor is shown in the Fig. 9.15 (a) while the symbol is
shown in the Fig. 9.15 (b).
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To generate more base current proportional to the light, larger physical area of the
base is exposed to the light.
'6 (~lA)20
H=Radiation
C flux density
75~16
:;C < . >
~212:J
< . > 50 'EQlC/)
Ql
C 1 l 5 8c ot)
25 ~(5u4
0
2 4 6 8 10 0
Radiation flux density 20 40mW/cm'
2
H=7 mW/cm2
H=6 mW/cm2
H=5 mW/cm2
H=4 mW/cm2
H=3 mW/cm
Vccvolts
(a) (b)
Fig. 9.16 Phototransistor characteristics
The Fig 9.16 (a) shows the graph of base current against the radiation flux density
measured in mW/cm2. The Fig. 9.16 (b) shows the collector characteristics of a
phototransistor. As light intensity increases, the base current increases exponentially.
Similarly the collector current also increases corresponding to the increase in the light
intensity.
A phototransistor can be either a two lead or a three lead device. In a three lead
device, the base lead is brought out so that it can be used as a conventional BJT with or
without the light sensitivity feature.
In a two lead device, the base is not electrically available and the device use is totally
light dependent. The use of phototransistor as a two lead device is shown in the
Fig. 9.17 (a) while the Fig. 9.17 (b) shows the typical collector characteristic curves.
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'c(mA)
10 240 mW/cm
8 2
30 mW/cm
6 2
20 mW/cm4
210 mW/cm
2
Dark current
0 5 10 15 20 25 30VCE(V)
Each curve on the
characteristic graph is
related to specific light
intensity. The collector
current level increases
corresponding to increase
in the light intensity. In
most of the applications the
phototransistor is used as a
two lead device.
The phototransistor is not sensitive to all the light but sensitive to light within a
certain range. The graph of response against wavelength is called spectral response. A
typical spectral response is shown in the Fig. 9.18.
Relaycoil
~Relay......~ contacts
The various photo transistor
applications are punch-card
readers, computer logic circuitry,
lighting control, level indicators
and relays .
The light operated relay circuit
using phototransistor is shown in
the Fig. 9.19.