A transducer is a device which, when actuated transforms energy from one form to another. Here input quantity is non-electrical quantity. The output quantity may or may not be electrical quantity. When the output quantity is electrical, then transducer is called as Electrical Transducer.
An electrical transducer is a device which is convert any physical quantity into a proportional electrical quantity such as voltage or electric current or resistance.
A transducer is also known as Pick Up.

Advantages of Electrical Quantities

  1. Electrical amplification and attenuation can be easily done.
  2. Since mechanical parts are less used, hence friction is minimised.
  3. The system can be controlled with a very small power level.
  4. The electrical output can be easily used, transmitted and processed for the measurement purposes.
  5. These are very small in size and can be used in small ICs.
  6. Electrical transducers plays an important role in Telemetry applications.

Well, an electrical transducer converts non-electrical quantity into an analogous electrical signal like voltage, or current. It consists of two parts
  1. Sensing or Detector Element
  2. Transduction Element

Sensing or Detector Element
It is a part of a transducer which responds to a change in physical quantity. The response or output of sensing element is also physical quantity.
Transduction Element
This part of transducer transforms the output of a sensing element to an electrical output. Hence, this transduction element acts as a Secondary transducer.

Classification of Transducers

The transducers are basically classified into 5 types as given below:
  1. On the basis of transduction form used
  2. Primary and Secondary transducer
  3. Passive and Active transducers
  4. Analog and Digital transducers
  5. Transducers and Inverse transducers

1. Classification of Transducers based on transduction Principle

This is first type of classification of transducer. The transducer are classified on the basis of principle of transduction such as resistive, inductive, capacitive, etc. depending upon hoe they convert the input quantity into resistance, inductance, and capacitance respectively. For example,
Thermistor is Resistive transducer since it changes resistance on account of change of temperature.
Magnetostriction gauge is a Inductive transducer since magnetic property (Inductance) changes on account of change of pressure and stress.
Variable Capacitance Pressure Gauge is a Capacitive transducer since it changes the capacitance on account of change in pressure.

2. Primary and Secondary Transducers

This is second type of classification of transducer. To understand the difference between primary and secondary transducer, take the example of measurement of pressure using Bourdon tube and L.V.D.T (Linear Variable Differential Transformer). The basic assembly is shown below.
Firstly, bourdon tube senses the pressure and convert into displacement of free end. This free end of bourdon tube then moves the core of L.V.D.T. and hence produces an output voltage proportional to the core displacement which in turn proportional to the pressure applied to the bourdon tube.
So, here are two transduction stages. First pressure is converted to displacement using bourdon tube and second is conversion of displacement to voltage.
Here, Bourdon tube is called as Primary Transducer and L.V.D.T. is called as Secondary Transducer.

3. Passive and Active Transducer

This is third type of classification of transducer. 
Passive Transducer
This type of transducer takes power required for generating output from an external power source. They are also called as Externally powered transducers. Capacitive, inductive and resistive transducers are examples of Passive transducers.
Active Transducer
This type of transducer do not require power for generating output from an external power source. They develop their own voltage or current output. Hence, they are also called as Self generating transducers. Piezoelectric crystals, thermocouples, photovoltaic cells are examples of Passive transducers.

4. Analog and Digital transducers

Analog transducer
This type of transducer convert the input quantity into an analog output which is a continuous function of time. L.V.D.T., thermocouple, thermistor, etc. are examples of Analog transducers.
Digital transducer
This type of transducer convert the input quantity into an electrical output which is in the form of pulses i.e. either 1 or 0.

5. Transducer and Inverse transducers

Transducer transducer
It is a device which converts a non-electrical quantity into an electrical quantity.
Inverse transducer
It is a device which converts an electrical quantity into a non-electrical quantity. It is also called as Actuator.
Transducer and Types of Transducer
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