D.S.Vidyasagar

Understanding

TRANSDUCERS

What is a transducer…?

Transducer is an

electrical device which

converts one form of

energy into another

form: Microphone

which converts sound

waves into electrical

signals.

Classification of Transducers

Active Transducer Passive Transducer

It is a device which works on external battery supply during conversion of one form of energy into another form.

It is a device which creates its own electric voltage during conversion. It does not require external battery supply.

Active Transducer

Solar cell, piezo electric

crystal, thermocouple,

etc.

Passive Transducer

LDR, speaker,

microphone, thermister,

etc.

Instrumentation System

An instrumentation system consists of three

main blocks.

Input circuit

Processing circuit

Output circuit

Along with the transducer at the input and the

output device like measuring instrument at the

output.

input circuit

processing circuit

output circuit

transducer input

finaloutput

How thermistor works?

Thermistor (Rx) is used

in a Wheatstone bridge

circuit. It can convert

change in temperature

into its corresponding

change in potential

difference across points

D-B.

What is LDR…?

It is a photo sensitive

device which changes its

resistance with the

change in intensity of

light.

Thus in dark its

resistance is high and in

light its resistance is low.

Details of LDR…

It is made up of photo sensitive material

like…

Cadmium Sulphide (CdS)

Selenium (Se)

Cadmium Selenide (CdSe)

Lead Sulphide (PbS)

How photo relay circuit works?

As shown in the circuit,

when LDR is dark, T1 & T2

both are OFF. So the relay is

deactivated.

When light falls on LDR, its

resistance drops and both

transistors turn ON. So relay

is activated to glow the LED.

Capacitive Transducer

It is a passive transducer

known as pressure

transducer. It is used to

measure external

pressure as well as

displacement also. It

works on the principle of

a capacitor.

Farad ..

d

oAKC

Where –

K = dielectric constant

A = area of each plate

d = distance between plates

How it works…?

ExternalP R

E

S

S

U R

E plate (A)

changed positionof diaphragm (A)

insulated support

diaelectric medium

static plate (B)

heat isolation cavity

enclosure

d

Linear Variable Differential Transformer (LVDT)

It is a passive transducer.

It measures force in

terms of displacement of

ferromagnetic core of a

transformer. It is based

on the principle of electro-

magnetic induction.

How it works…?

ac input

ac output(V -V )sec1 sec2

sec-2sec-1

primary coil

Ferrite Rod

A B

magneticallysaturable core

handle

Piezo Electric Crystal

It is a passive transducer. It measures force in terms of displacement of ferromagnetic core of a transformer. It is based on the principle of electro-magnetic induction.

This crystal is found in Rochelle salt and Tourmaline materials.

The 3 axes of crystal

There are three imaginary

axes of piezo electric

crystal:

x-axis or electrical axis

y-axis or mechanical axis

z-axis or optical axis

Specialty of piezo crystal

The piezo electric crystal is a dual type of transducer.

When its slice is subjected to pressure, it produces proportional potential difference.

And when it is connected to a high frequency ac voltage, it vibrates to produce US waves.

Applications of piezo crystal

This circuit is designed around

the common emitter amplifier

stage of a Colpitts Oscillator. The

input signal to the base of the

transistor is inverted at the

transistor output. The output

signal at the collector is then

taken through a 180o phase

shifting network which includes

the crystal operating in a series

resonant mode.

Continued…

In the circuit, the output is also fed back to input which is IN-

PHASE with the input providing the necessary positive

feedback. Resistors, R1 and R2 bias the resistor in a Class A

type operation while resistor Re is chosen so that the loop

gain is slightly greater than unity.

Capacitors, C1 and C2 are made as large as possible in order

that the frequency of oscillations can approximate to the

series resonant mode of the crystal and is not dependant

upon the values of these capacitors.

What is an Opto Coupler?

An Opto coupler, also known as an

Opto-isolator or Photo-coupler, is

an electronic components that

interconnects two separate

electrical circuits by means of a

light sensitive optical interface.

It consists of one LED and a photo

transistor or photo diode for

coupling the light signals.

Inside the Opto Coupler…

The LED is connected

to the input signal

through a resistor. As

the LED glows, it

conducts the photo

transistor and its

collector potential

changes in accordance

with the input signal.

Thus the two circuits

are coupled through

Types of Opto Coupler

Simple Transistor type is used

for general applications.

Darlington pair is used for

large current output.

SCR is used for toggling

action for DC loads.

Triac is particularly used for

AC loads.

What is gas sensor?

A gas sensor is used to measure

the traces of unwanted gas present

in required gas.

It works on the principle of chemical

reaction. The gas under test is

passed

through the sensor and if it

contains

any unwanted gas it will react with

the electrolyte and will produce

proportional PD. The presence of

Application of gas sensor

A typical gas sensor setup

A pressurized gas under test is shown on

the left. It is passed through channel which

contains particular type of electrolyte that

will react with the unwanted gas. If the

traces of such gas are present then due to

chemical reaction, the PD isproduced which is

measured with help

of a simple

voltmeter. The

scale of voltmeter is

calibrated in terms

of percentage (%).

Industrial transducers

Liquid pressure transducer PT-100 NTC Thermistor JR20-TC pH level sensors

www.yashplus.com

Designed by: D.S.Vidyasagar