Transducer Classification of transducers | Thermistors Application of thermistorsļæ¼

Transducer 

A transducer is a device with the technology of using instruments to measure and control the physical and chemical properties of materials called instrumentation. This device converts energy from one to another to convert signal is calledĀ a transducer.

Fig. Transducer

Classification of transducers

Transducers are broadly classified into two groups as follows.

1. Active transducers.

They are also known as self-generating type transducers. TheseĀ transducersĀ develop their own voltage or current. The energy required for the production of an output signal is obtained from the physical phenomenon being measured.

Examples. Thermocouples and thermopiles, piezoelectric pick up, photo voltaic cells.

Fig. Active transducers.

2. Passive transducers.

They are known as externally-powered transducers. These transducers device the power required for the energy conversion from an external, power source. However, they may absorb some energy from the physical phenomenon under study.

Fig. Passive transducers.

For example.Ā 
Resistance thermometers and thermistors, potentiometric devices, differential transformers, photo-emission cells, etc.

Classification based on the type of output

1. Analogue transducer. 

TheseĀ transducersĀ convert the input physical phenomenon into an analog output which is a continuous function of time.

Example. Strain gauge, a thermocouple, a thermistor, or an LVDT. Linear voltage differential transformer.
2. Digital transducers
These transducers convert the input physical phenomenon into an electrical output which may be in form of a pulse.

Classification based on electrical principal involved 

1. Variable-resistance type

  • Strain and pressure gauges
  • thermistor, resistance thermometers
  • photoconductive cell.

2. Variable-inductance type

  • Linear variable differential transformer LVDT
  • Reluctance pick-up.
  • Eddy current gauge.

3. Variable-capacitance type

  • Capacitor microphone
  • pressure gauge
  • Dielectric gauge

4. Voltage-generating type

  • Thermocouple 
  • photovoltaic cell
  • Rotational motion tachometer
  • piezoelectric pick-up

5. Voltage-generating type

  • Potentiometer position censor
  • pressure-actuated voltage divider.

Electro-Mechanical Transducers

These days electrical/electronics of measurements are being increasingly applied to measurement in many fields other than electrical engineering. These methods claim the following advantages.

 Advantage

  1. Less power consumption and less loading on the system to be measured.
  2. Friction and mass inertia effects minimum.
  3. More compact instrumentation.
  4. Possibility of non-contact measurement.
  5. Good frequency and transient response.
  6. Feasibility of remote indication and recording.
  7. Amplification greater than that produced by a mechanical contrivance.
  8. possibility of mathematical processing of signals like summation, integration, etc.

Description of Transducers

Resistance transducers

In a resistance transducer, an indication of a measured physical quantity is given by a change in the resistance. It may be classified as follows.

  1. Mechanically varied resistance                         -potentiometer
  2. Thermal resistance change                                -Resistance thermometers
  3. Resistivity change                                              -Resistance strain gauge.


Linear and angular motion potentiometer 

Such potentiometers convert the linear motion or the angular motion of a rotating shaft into changes in resistance. The device is a variable resistor whose resistance is varied by the movement of a slider over a resistance element.

  • Translatory devices have strokes from 2.5 mm to 5 mm.
  • Rotational devices have a full scale ranging from 10 degrees to 60 degrees full turn.

Advantage

  1. High output.
  2. Less expensive.
  3. Available in different sizes, shapes, and ranges.
  4. Rugged construction.
  5. Insensitivity towards vibration and temperature.

Disadvantages

  1. Limited life due to early wear of the sliding ram.
  2. The output tends to be noisy and erratic in high-speed operation or when used in a high-vibration environment.

ThermistorsĀ 

These transducers are thermally sensitive variable resistors made of certain conducting and ceramic-like semiconducting materials. They are used as a temperature-detecting element used to sense temperature for the purpose of measurement and control.

Fig.Thermistor

ThermistorsĀ are essentially semiconductors that behave as resistors with a high negative temperature coefficient of resistance. The high sensitivity to temperature changes makes the thermistors extremely useful for precision temperatures -60 degrees c. to +15 degrees c. Measurements, control, and compensation. Their resistance range from 0.5 ohms to 0.75 Mohm.

ThermistorsĀ are composed of a sintered mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron, and uranium.

Application of thermistors

  • Measurement of temperature …..major application.
  • Temperature compensation in complex electronic equipment, magnetic amplifiers, and instrumentation equipment.
  • Measurement of power at high frequencies.
  • Vacuum measurement.
  • measurement of the level, flow, and pressure of liquids.
  • Measurement of thermal conductivity.

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