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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.