A transducer is a device that is used to convert a physical quantity into its corresponding equivalent electrical signal.

In most electrical systems, the input signal will not be an electrical signal, but a non-electrical signal like voice, pressure, et cetera. These will have to be converted into its corresponding electrical signals if its value is to be measured using electrical methods. The block diagram of a transducer is given below.

A transducer will have basically two main components. They are:

• Sensing Element - the physical quantity or its rate of change is sensed and responded to by this part of the transistor.

• Transduction Element - this element is responsible for converting the non-electrical signal into its proportional electrical signal.

Some of the common methods of classifying transducers are:

• Based on their application

• Based on the method of converting the non-electric signal into electric signal

• Based on the output electrical quantity to be produced

• Based on the electrical phenomenon or parameter that may be changed due to the whole process. Some of the most commonly electrical quantities in a transducer are resistance, capacitance, voltage, current or inductance. Thus, during transduction, there may be changes in resistance, capacitance and induction, which in turn change the output voltage or current

• Based on whether the transducer is active or passive.

Following are the various types of transducers –

1. Passive Type Transducers

   a. Resistance Variation Type

   i. Resistance Strain Gauge – The change in value of resistance of metal semi-conductor due to elongation or compression

   ii. Resistance Thermometer – The change in resistance of metal wire due to the change in temperature

   iii. Hot Wire Meter – The change in resistance of a heating element due to convection cooling of a flow of gas

   iv. Photoconductive Cell – The change in resistance of a cell due to a corresponding change in light flux

   v. Thermistor – The change in resistance of a semi-conductor that has a negative co-efficient of resistance

   vi. Potentiometer Type – The change in resistance of a potentiometer reading due to the movement of the slider as a part of an external force applied.

   b. Capacitance Variation Type

   i. Variable Capacitance Pressure Gauge – The change in capacitance due to the change of distance between two parallel plates caused by an external force.

   ii. Dielectric Gauge – The change in capacitance due to a change in the dielectric.

   iii. Capacitor Microphone – The change in capacitance due to the variation in sound pressure on a movable diagram.

   c. Inductance Variation Type

   i. Eddy Current Transducer – The change in inductance of a coil due to the proximity of an eddy current plate

   ii. Variable Reluctance Type – The variation in reluctance of a magnetic circuit that occurs due to the change in position of the iron core or coil

   iii. Proximity Inductance Type – The inductance change of an alternating current excited coil due to the change in the magnetic circuit

   iv. Differential Transformer – The change in differential voltage of 2 secondary windings of a transformer because of the change in position of the magnetic core

   v. Magnetostrictive Transducer – The change in magnetic properties due to change in pressure and stress.

   d. Voltage and Current Type

   i. Photo-emissive Cell – Electron emission due to light incidence on photo-emissive surface

   ii. Hall Effect – The voltage generated due to magnetic flux across a semi-conductor plate with a movement of current through it

   iii. Ionization Chamber – The electron flow variation due to the ionization of gas caused by radio-active radiation

2. Active Type

a. Photo-voltaic Cell – The voltage change that occurs across the p-n junction due to light radiation.

b. Thermopile – The voltage change developed across a junction of two dissimilar metals

c. Piezoelectric Type – When an external force is applied on to a quartz crystal, there will be a change in the voltage generated across the surface.

d. Moving Coil Type – The change in voltage generated in a magnetic field can be measured using its corresponding value of vibration or velocity.