Difference between active transducer and passive transducer
This page compares active transducer vs passive transducer and mentions difference between active transducer and passive transducer. It mentions examples of active and passive transducer types.
Introduction : The Transducer is useful to convert physiological signals produced by our body into measurable form. These physiological signals can be internal (i.e. blood pressure), emanate from body (i.e. infrared radiation) and can be derived from tissue sample (i.e. blood). All the physiological signals can be grouped into certain categories. The categories are biopotential, flow, pressure, dimensions, displacement (voz. velocity, force, acceleration), impedance, temperature, chemical composition etc. Physiological signals are produced by our body during functioning of various physiological systems. Biomedical instruments make use of various transducers for measurement different human parameters. This helps in diagnosis and treatment of various diseases. Based on type of energy, physiological signals can be classified as bioelectrical, biochemical, bioacoustic, biomagnetic, biochemical, bioimpedance or bio-optical signals.
What is a transducer
Transducer is a device which converts one form of variable or energy into another form of variable or energy. Usually, it is required to convert physiological varibles into electrical signals which are easy to be processed. The relation between input and output variable can be linear, logarithmic or square law.
Transducer is interfaced with our body in such as way that it extracts minimum amount of energy. Moreover it should not be invasive. All the transducers can be broadly classified into active or passive based on conversion of non-electrical variables into electrical signals.
Active Transducer
It directly converts input variable into electrical signals. Modern digital computers can process electrical signals and hence active transducers have become popular.
Examples of active transducers:
• Moving coil generator: Working Principle : Motion of coil in magnetic field induces output voltage. It is used in measurement of velocity and vibration.
• Photovoltaic: Working Principle : Voltage is generated in semiconductor junction of solar cell when it is simulated by radiant light energy.
• Thermocouple: Working Principle : In this transducer, one of junctions of two dissimilar metals is heated and the other is cooled, the emf is
generated across the two junctions. It is used for measurement of temperature, radiation, heat flow etc.
• Piezoelectric: Emf is generated when external force is applied to crystalline material such as quartz. It is used for measurement of
sound, acceleration, vibration and pressure variation.
Passive Transducer
It modifies either excitation voltage or modulates carrier signals. It is self generating and does not require external power.
Passive transducer consists of passive circuit element (viz. resistors, capacitors, inductors) which changes its value as a function of physical variable or based on signal developed by physiological system to be measured.
Examples of passive transducers:
• Resistive: Potentiometer, strain gauge, photo-resistive and photo-diode are common resistive passive transducers.
• Inductive: Induction of coil can be changed either by varying its physical dimension or by changing permeability of its magnetic core.
Examples include induction displacement transducer, variable reluctance transducer, LVDT (Linear Variable Differential Transducer).
• Capacitive: Capacitance of a capacitor depends upon area of parallel plates and distance between them. Example is Passive capacitance transformer.
Tabular difference between active and passive transducer
Following table mentions difference between active transducer and passive transducer.
| Parameters | Active transducer | Passive transducer |
|---|---|---|
| External power source | It does not require external battery. It creates its own electric voltage during conversion. | It is a device which requires external battery to convert one form of energy into another form. |
| Principle of working | It draws energy from measurand source. | It takes power from external source which changes the physical properties of transducer. |
| Resolution | Low | High |
| Design of signal conversion | Simple | Complex |
| Examples | Bourdon tube, Thermocouple, Photovoltaic cell etc. | Thermistor, strain gauge, Linear Variable differential transformer (LVDT) |
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