What is a Transducer

A transducer is a device that is used to convert one type of physical quantity in to another. The most prevalent of these and ones which are discussed here are electrical transducers that are used to convert quantities like pressure, temperature, displacement etc. in to an equivalent current or voltage form. Notice that changes in both the current and voltage will be produced in the output electrical circuit of the transducer but at least one will vary in proportion with the input physical quantity of the transducer. The magnitude of the input physical quantity can thus be measured by measuring the magnitude of the corresponding current or voltage produced and correlating it to the input quantity by taking in to account the conversion factor of the transducer.

A few examples of some transducers are:-

Potentiometers used for measurement of displacement by converting it in to a corresponding voltage drop change. LVDT or linear Variable Differential Transformer for conversion of displacement in to a corresponding voltage. Strain Gauges for conversion of pressure in to a corresponding change in voltage drop. Thermistors and Thermo couples for conversion of temperature in to corresponding electrical forms. Piezoelectric Crystals that convert pressure in to an induced voltage.

A transducer can be classified as either an active or a passive transducer. A passive transducer is one in which an electric source is needed to power the output circuitry of the transducer and the quantities are usually measured in terms of changes in current and voltage. These transducers can’t function without this output electrical supply.

An active transducer meanwhile requires no such electrical supply for its functioning. These are ones that produce an induced output voltage or current in relation with the input physical quantity. Piezoelectric crystals mentioned above are prime examples of these kinds of transducers. These crystals convert pressure applied on their faces in to a corresponding electrical voltage due to the stress developed on the crystalline structure.

The characteristics of a transducer can be defined by the following points:-

Nature of the input physical quantity and the conversion principle used. Input characteristics of the transducer namely operating range, sensitivity and loading effects. Transfer characteristics of the transducer namely transfer function and various errors present. Output Characteristics of the transducer namely type of electrical output and output impedance.

Thus there are a variety of transducers that respond to different physical quantities using different conversion principles. They have their own characteristics along with their own advantages and disadvantages that make their suitability depend on the nature of the application.

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