Active Transducers

Table of Contents

“Active transducers are the transducers which do not use any external power for the measurement of process variable. These transducers are self generating devices which operate under the energy conversion principle. In other words, active transducer produces its own equivalent electrical output in response to the physical quantity to be measured.”

Transducer (Wikipedia link)

A block diagram for active transducer is shown below-

Active Transducer Examples-

Some examples of active transducers are given as follows-

Thermocouples
Thermopiles,
Piezo-electric crystal
Photo voltaic cell

Note- Transducers which create confusion to be active transducer but they are actually passive transducers such passive transducers example is as follows-

Potentiometer
Strain Gauge
LVDT
RTD
Thermistor
Resistive transducer
Inductive Transducer
Capacitive transducer

Thermocouples-

A thermocouple is said active transducer which is used for temperature measurement. A thermocouple sensor consists of two dissimilar metal wires jointed at both the end. Here one end joint (known as hot junction) is kept at the temperature to be measured and another end joint (known as cold junction) at ambient temperature is kept at ambient temperature.

Thermocouple is based on Seebeck Effect which states that when the junctions of the two dissimilar metals are kept at differential temperature (i.e. heated or cooled), a voltage (known as thermo EMF) is produced proportional to the temperature difference. There is no requirement of external power for temperature detection/ measurement.

A diagram showing Thermocouple is given as follows-

Thermopiles-

Thermopiles are said active transducers since the base element for measurement are thermocouples similarly based on See-beck effect. Thermopile is used to measure temperature difference. it convert thermal energy into electrical energy. It uses several thermocouples connected in series or parallel. Thermopiles are used for contact-less temperature sensing. The function of a thermopile is to transfer the heat radiation emitted from the object to a electrical voltage output. The output is in the range of tens or hundreds of milli-volts. A figure showing thermopile configuration is shown as follows-

Piezo-electric crystal-

Piezo Electric transducer are said to be active transducers which are used for force, pressure or vibration measurement. When piezo electric crystals such as quartz, tourmaline, Rochelle salt and topaz etc. are compressed or stressed resulting its deformation, a small voltage is produced across its surfaces, this property of crystal is called as piezo electric effect sometimes known as piezoelectricity. These crystals are hexagonal in shape having three axis named as optical, electrical and mechanical.

The basic principle of piezo electricity defines as when a force is applied to the crystal, it produces electricity. This is reversible phenomenon i.e. when a voltage is applied across its surface, crystal start deforming or vibrating.

A diagram of piezo electric crystal is shown below-

Piezoelectric Materials

There are various types of piezo electric material used for different applications or measurement. The first and most well known piezoelectric material used in electronic devices is the quartz crystal. Different crystal material is shown as follows-

Natural Piezo Electric Crystals-

Quartz crystal
Cane sugar
Rochelle salt
Topaz
Tourmaline etc.

Man Made Piezo Electric Crystals-

PZT (Zirconate titanate)
Barium Titanate
Lithium Niobate etc.

Photo voltaic cell-

Photo Voltaic transducer or Photo voltaic cell (sometimes known as solar cell) is also an active transducer which generates electric voltage or current when exposed to sun light or in simple words, we can say it converts solar energy into electricity. This phenomenon is known as photo voltaic effect.

Design and working principle-

Photo voltaic cell is composed of two different types of semiconductors—a p-type and an n-type. These P type and n type are sandwiched together to form a PN- Junction. A neutral zone (depletion layer) is formed in between after developing junction. There is no voltage or current flows at equilibrium state. When Light rays (composed of photons), which are simply small bundles of electromagnetic radiation or energy incident on cell, energy from the photon is transferred to cell electrons, causing it to jump to a higher energy state known as the conduction band. In conduction band, these electrons are free to move through the junction and external path, which results an electric current through it.

A diagram of photo voltaic cell is shown below-