---
title: "Classification of transducers with examples"
subject: "Instrumentation and Measurement"
chapter: 5
key: "semester_4/instrumentation_and_measurement:classification-of-transducers"
source: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/classification-of-transducers
site: BCT Engineering
---

# Classification of transducers with examples

Instrumentation and Measurement · Chapter 5: Sensors

- Asked in: year not given, 5 marks
- Video: [CLASSIFICATION OF TRASNDUCERS|TYPES OF TRANSDUCERS|TAXONOMY OF TRANSDUCERS|SENSORS & TRANSDUCERS|S&T](https://www.youtube.com/watch?v=vGlBlsTwCfA) — Simplified EEE Studies
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/instrumentation_and_measurement/classification-of-transducers
- Whole chapter: https://www.bctengineer.com/md/semester_4/instrumentation_and_measurement/chapter-5

## Question

Classify transducers. Differentiate between active and passive transducers, and between primary and secondary transducers, with examples.

## Short answer

Transducers are classified by energy source (active/passive), function (primary/secondary), output (analog/digital), principle, etc. Active transducers generate output directly from the measurand (e.g., thermocouple); passive need external excitation and change R, L or C (e.g., strain gauge). Primary converts measurand into mechanical signal (e.g., Bourdon tube); secondary converts that into electrical (e.g., LVDT).

## Answer

A transducer converts one form of energy or physical quantity into another, usually into an electrical signal. A sensor is the sensing element of a transducer.

#### Classification
1. **By energy source:** active (self-generating) and passive.
2. **By function:** primary (sensing) and secondary.
3. **By output signal:** analog and digital.
4. **By principle:** resistive, capacitive, inductive, piezoelectric, thermoelectric, photoelectric, Hall effect.
5. **By quantity measured:** displacement, force, pressure, temperature, flow and so on.
6. **By operating mode:** transducers (input-to-output converters) and inverse transducers.

#### Active and passive

| Point | Active transducer | Passive transducer |
|---|---|---|
| Power | Generates output emf or charge from the measurand | Needs external excitation |
| Also called | Self-generating | Externally powered |
| Output | Voltage, charge, current | Change in R, L or C |
| Examples | Thermocouple, piezoelectric crystal, photovoltaic cell | Strain gauge, thermistor, LVDT, potentiometer |

#### Primary and secondary
- **Primary transducer:** first element in contact with the measured quantity; it converts it into a mechanical or other signal. Example: a bourdon tube turns pressure into displacement.
- **Secondary transducer:** converts the output of the primary into an electrical signal. Example: a potentiometer or LVDT attached to the bourdon tube gives voltage.

A load cell is a combination: an elastic member (primary) with strain gauges (secondary).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. An active transducer
   A. Needs external excitation
   B. Generates output emf or charge from the measurand itself
   C. Changes only R, L or C
   D. Is always digital
   Answer: B. Active transducers are self-generating, e.g. thermocouple and piezoelectric crystal. (Key point: Active transducer)

2. Which is a passive transducer?
   A. Thermocouple
   B. Photovoltaic cell
   C. Strain gauge
   D. Piezoelectric crystal
   Answer: C. A strain gauge needs excitation and changes resistance. (Key point: Passive example)

3. In a bourdon tube with an LVDT, which is primary and secondary?
   A. Bourdon tube primary, LVDT secondary
   B. LVDT primary, bourdon tube secondary
   C. Both primary
   D. Both secondary
   Answer: A. The primary senses pressure as displacement; the secondary converts it to an electrical signal. (Key point: Primary vs secondary)

4. A load cell is a combination of
   A. Two thermocouples
   B. An elastic member (primary) with strain gauges (secondary)
   C. A bourdon tube and a manometer
   D. A capacitor and an inductor
   Answer: B. The elastic member converts force to strain, which gauges convert to resistance change. (Key point: Load cell)

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