Passive Transducers

Passive Transducers

“Passive Transducers are the transducers which require external power for the measurement of process variable or or processing of output from primary sensing element (Sensor). These are also known as externally powered transducers. However these transducers and circuitry may absorb some energy during measurement and detection of change in process variable.”

A block diagram for passive transducer is shown below-

Passive Transducer block diagram

Passive Transducer Examples-

RTD

– detects change in temperature and convert into change in resistance

Thermistor

– detects change in temperature and convert into change in resistance

LVDT

– converts displacement into change in inductance

Strain Gauge

–  convert force, pressure into change in resistance

Potentiometer

– converts displacement into change in resistance

Load Cells

–  convert force, weight, pressure into change in resistance

Photo Resistor or LDR

– detect light intensity and results as change in resistance

Diaphragm capsule

–  detects pressure, differential pressure and convert into change in capacitance

Passive transducers operation-

Furthermore to understand the operation and working of passive transducers, an extended block diagram is shown as follows-

As we can see from the extended block diagram, a sensing element such as RTD, strain gauge, LVDT, diaphragm capsules etc. is used for the sensing change in input process variable to be measured such as temperature, pressure, force displacement etc. and convert the physical change in to electrical property such as resistance, inductance capacitance change etc. which is fed to the transduction element or signal conditioning circuitry such as bridge circuit that uses external power to measure change in resistance, inductance capacitance etc. and convert it into voltage or current signal that can be fed to next stage like display device, recorder or controller etc.  

Passive Transducer Classification-

 A broad classification of passive transducers is showing below-

From the figure, we can say there are 3 major categories exit based on change in electrical properties as follows-

(Click on individual tab to read more)

Transducer Applications (from Wikipedia)

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