Definition of transducer - Understanding Transducers

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Definition of Transducer

What is a transducer?

Transducer is a device that converts energy form one form to other, to make it readable for measurement. Thus it converts energy to a readable form e.g. thermometer that converts heat energy into height of mercury column. In transducer output is controlled by the input.

Role of transducer

They play a vital role in field of measurements. As we discuss earlier transducer covert physical quantity into electrical signal. So without transducer, it would be very difficult to measure the continues physical quantity e.g. light intensity,speed,flow,temperature,radiation,electric flux etc.The quantities are first converted into electrical signal ,then they are monitored by dedicated equipments. Some one could not imagine the measurement of these continuous physical quantities without transducers.

Types of transducers

They are broadly divided into two categories;

  1. Active transducer
  2. Passive transducer

Active transducer: Such type of transducers required external source of energy to operate. Energy is supplied through a separate voltage source. An example is potentiometer, which senses the resistance by flow of a minute current through itself. Most of the transducers now a day are active.

Passive transducer: They convert one form of energy to other form without any use of energy. Passive transducers convert physical quantities like;temperature,pressure,speed etc.

Transducers are further subdivided as;

  • Resistive transducer
  • RTD
  • Thermistors
  • Inductive changing transducer
  • Capacitive changing transducer
  • Displacement transducers
  • Velocity transducers
  • Pressure transducers

Resistive transducer

These transducers operate on the principle of, variation in resistance. Resistance is changed by several ways including;

  • sliding viper along a rheostat,
  • applying physical stress,
  • Light variation on photosensitive element
  • temperature variation.

RTD abbreviated as Resistance temperature detector. The resistance of RTD changes with the variation in temperature and this change in resistance in monitored in term of variation in current/voltage. Normally RTD are constructed from materials like Platinum. Ni and Germanium are used for manufacturing of RTDs for special applications. When it comes to performance Platinum RTD (PRDS) are best. RTDs are used in thermometer, having range between B.P of O2 to milting point of Antimony.


  • Widely used to measure high temperature.

Types of Transducers


They are temperature sensitive semiconductors. Like RTS thermistor their resistance changes with variation in temperature. However they are manufactured from material having negative temperature coefficient (i.e. resistance decrease with temperature rise), in contrast to the RTSs, which have positive temperature coefficient. Thermistors are encapsulated in, with material like transition metal oxide. These oxide exhibit a high change in resistance with a small variation in temperature. Thus they are more sensitive, almost 400 times greater than an IC thermocouple. They are ideal to measure temperature of ICs, animal body.

Major advantages/Applications

  • Sensitive enough to sense temperature up to 0.01C.
  • Chemically stable.
  • Fast response time.
  • Small in size Major limitation


  • Limited temperature range -50C to 300C

L-Changing TDs

Inductive transduction takes place when measurand changes the inductance (Self or mutual) of a coil. A simple method for changing-L is to move a sensing element in magnetic field. This motion causes an induce emf.

Major advantages

  • No wear and tear because of no sliding contact, as in case of potentiometer.


  • Linear variable differential transformers (LDVT)
  • Tachometer use inductive transducer to convert speed into electrical signal for monitoring speed.

Capacitance-Changing transducers:

In this type of transducers the measurand (quantity to be measured) changes the capacitance of circuit. This change is monitored in term of some other physical quantity.


  • Automatic LCD based touch system.
  • Capacitive microphone that uses acoustic pressure to change plate position. This change is monitored in term of audio signal.

Displacement transducers

This type of transducers are used to sense the position of an object .The physical variable (i.e. motion), being measured, is made to change the resistance. This change in resistance is measured in term of voltage.


  • Quite sensitive to monitor cracks in wall and buildings.

Velocity transducer

They operate on the basic principle of generator, which state that when ever there is a relative motion between conductors and magnetic an emf is generated. The voltage generated is monitored in term of speed. Thus faster the relative motion, greater will be the emf produced .


  • They are widely used in speed monitoring devices e.g. car speed meter.