Active and Passive Transducer

Active Transducer

What is meant by Active Transducer?

         Active transducer is defined as transducer that does not requires an external energy source. In this type of transducer, the non-electrical quantity which is to be measured is converted to electrical output without any external source. As it does not require any external source this type of transducer is also known as a self-generating transducer.

Examples of an Active Transducer: Thermocouple, Piezoelectric Pick up & Photovoltaic.

Block Diagram of Active Transducer

Active and Passive Transducer 1

          The block diagram of an active transducer is shown above. Pressure, temperature, vibration, heat radiation and flow are the various input variable that can be directly converted into electrical output.

Why active transducer is called as self-generating Transducer?

         The transducer is a device that converts one form of non-electrical signal into a corresponding electrical signal. The active transducer generates electrical output directly in response to stimulation

Active transducer Classification

Active and Passive Transducer 2

Active transducer is classified based on the internal physical property of the transducer. They are listed below.

  • Thermoelectric based active transducer
  • Piezoelectric based active transducer
  • Photovoltaic based active transducer
  • Electromagnetic based active transducer
  • Chemical based active transducer

List of Active transducers with its principle.

Active TransducerParameter to measurePrinciple  
ThermocoupleTemperatureIt works in the principle of the Seeback effect. An emf is generated when the junctions of two dissimilar metals have a change in temperature.
ThermopileTemperatureWorks on the principle of the Seeback effect. Thermal energy is converted into emf.
Piezo electric pickupSound, Vibration, Acceleration & PressureAn emf is generated when an external force is applied to crystal materials such as quartz.
Photovoltaic transducerSolar cell, Light MeterAn emf is generated in a semiconductor junction device when light radiation falls on the device.

Passive Transducer

What is meant by Passive Transducer?

         The transducer which requires external energy to produce its output is called a passive transducer. This transducer requires external energy to process the output from the sensing element to produce respective electric output. The electrical parameters in a passive transducer are resistance, inductance, and capacitance which require external energy for conversion.

Examples of Passive Transducer: Potentiometric Transducer, RTD, Thermistor, Capacitor microphone, LVDT etc.,

Block Diagram of Passive Transducer

Active and Passive Transducer 3

         The block diagram of a passive transducer shown above has external energy. This passive transducer consists of a sensing element and a transduction element. The pressure, temperature, stress and displacement are the various input that can be measured.

         Sensing element like RTD, strain gauge and LVDT is used to measure temperature, applied force and displacement respectively. This sensing element is used for sensing the change in input quantity which is to be measured and convert the physical change into the electrical property such as resistance, inductance & capacitance change. This is fed to the transduction element.

         The transduction element consists of a signal conditioning circuit to measure change in electrical parameters & convert it into electrical output.

What is the key difference between active and passive transducer?

         In an active transducer, the energy required for its operation is taken from the quantity that has to be measured but in passive transducer energy needed for operation is derived from an external source.

Passive Transducer Classification

Passive transducer is classified based on the transduction medium. They are listed below.

  • Variable resistance based passive transducer
  • Variable reactance [capacitance & inductance] based passive transducer
  • Optoelectronics based passive transducer
Active and Passive Transducer 4

List of Passive transducer with its principle

Passive transducerElectrical ParameterParameter to MeasurePrinciple of operation
Potentiometric transducerResistancePressure, DisplacementWhen an external force varies the slider position of the potentiometer, then the resistance varies in the bridge circuit.
Resistance thermometer (RTD)  ResistanceTemperature, Heat radiantResistance of pure metal wire with a large positive coefficient of resistance varies with temperature.
ThermistorResistanceTemperatureResistance of certain metal oxides with a negative temperature coefficient of resistance varies with temperature.
Strain gaugeResistanceApplied Force, Pressure, Torque, DisplacementWhen stress is applied there is a change in the resistance of the wire by elongation or compression.
Hot wire meterResistanceGas flow, Gas pressureResistance of a heating element is varied by convection cooling of a stream of gas
Photoconductive cellResistance.Photosensitive RelayResistance of the cell as a circuit element varies with incident light.
Resistance hygrometerResistanceRelative humidityResistance of conductive strip changes with moisture content.
Capacitor microphoneCapacitanceSoundSound pressure varies between a fixed plate and a movable diaphragm.
Reluctance pick upInductancePressure, Vibration, Displacement & Position  When changing the position of the iron core of the coil, the reluctance of magnetic circuits is varied.
LVDTInductancePressure, Force, Displacement, PositionWhen there is a change in displacement there is a change in inductance.

Active Transducer Vs Passive Transducer

Difference between Active & Passive Transducer
ParameterActive TransducerPassive Transducer
PrincipleThe quantity being measured provides operational energy.Energy for operation is taken from an external source.
Other name Self-generating transducerExternally driven transducer
Type of outputElectrical current or VoltageChanges in quantity associated with passive elements is observed.
Conversion techniqueSimpleComplex
External EnergyNot requiredRequired
AmplificationNeededNot Required
ExampleThermocouple, Piezoelectric crystal, etc.Potentiometer, Thermistor, Differential Transformer, Photovoltaic cell, etc.

List some advantages & disadvantages of Active transducer & Passive transducer

         The below table shows advantages and disadvantages of some active and passive transducer based on transduction mechanism.

Transduction MechanismAdvantagesDisadvantages
Thermocouple1.Available in a compact dimension 
2. Rapid temperature response
3. No need for a bridge circuit
1. Nonlinear 
2. Hard to re-calibrate
3. Reference junction at cold temperature
Piezoelectric1. Excellent Spatial Resolution
2. Precise sensitive
3. Not dependent on temperature.
1. Lack of spatial resolution 
2. Charge related leaks present
3. Rely on dynamic sensing
Transduction MechanismAdvantagesDisadvantages
Capacitive type transducer1. High sensitivity
2. High Spatial resolution
3. Large dynamic range
4. Temperature independent
1. Difficult measuring circuit
2. Stray capacitances
3. Cross-talk between parts of elements
4. Vulnerable to noise
Inductive type transducer1. Output is linear 
2. Sensitivity is high
3. Power output is high
4. Dynamic range]s  high
1. Low frequency response
2. Unreliable 
3. Higher Energy consumption
RTD1. Stability is high
2. Most precise output is obtained when compared to 3. thermistor
4. Excellent repeatability
1. High cost
2. Complex circuit design
3. Current source required

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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