---
title: "LVDT with merits and demerits"
subject: "Instrumentation I"
chapter: 3
key: "semester_4/instrumentation_i:lvdt-merits-demerits"
source: https://www.bctengineer.com/q/semester_4/instrumentation_i/lvdt-merits-demerits
site: BCT Engineering
---

# LVDT with merits and demerits

Instrumentation I · Chapter 3: Transducer

- Asked in: year not given, 4+2 marks
- Video: [LVDT Construction and Working | Linear Variable Differential Transformer LVDT in Hindi](https://www.youtube.com/watch?v=ZDJ5mEKSLYg) — Electronics for You
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/instrumentation_i/lvdt-merits-demerits
- Whole chapter: https://www.bctengineer.com/md/semester_4/instrumentation_i/chapter-3

## Question

What is LVDT? Point out its merits and demerits.

## Short answer

LVDT is a passive inductive transducer that converts linear displacement into a proportional AC voltage using a movable core and differential secondaries. Merits: wide range, high sensitivity, no friction, ruggedness, direction sensing. Demerits: needs AC excitation, sensitive to stray fields, limited dynamic response, temperature effects.

## Answer

**LVDT (Linear Variable Differential Transformer)** is a passive inductive transducer that converts the linear displacement of a core into a proportional AC voltage, using a differential transformer.

#### Construction and working
```text
   |=====S1=====|===Primary===|=====S2=====|   <- tubular former
          [====== movable soft-iron core ======]  --> displacement
   output eo = e1 - e2  (S1 and S2 in series opposition)
```
- One primary winding in the centre and two identical secondary windings, one on each side, are wound on a single tubular former. A movable soft-iron core slides inside it.
- The secondaries are connected in series opposition. The primary is excited by AC (1 to 10 kHz, 3 to 15 V).
- **Core at centre (null):** e1 = e2, so the output is 0.
- **Core moves towards S1:** the flux linkage of S1 increases, so e1 > e2; the output is in phase with the primary voltage and proportional to the displacement.
- **Core moves towards S2:** e2 > e1; the output is shifted by 180°. The phase gives the direction, and the magnitude gives the displacement.

#### Merits
1. Wide range: ±1.25 mm to ±250 mm.
2. No friction or contact, so no wear; very fine (practically infinite) resolution.
3. High sensitivity (up to about 40 V/mm) and good linearity.
4. Rugged and works in harsh environments; low hysteresis and good repeatability.
5. Direct electrical output, so little amplification is needed; low power consumption.
6. Gives the direction as well as the magnitude of the displacement.

#### Demerits
1. A relatively large displacement is needed for an appreciable differential output.
2. Sensitive to stray magnetic fields, so it needs shielding.
3. Needs AC excitation, and the output needs a demodulator (phase-sensitive detector) to give the direction.
4. Dynamic response is limited by the core mass and by the carrier frequency (measurable frequency about one tenth of it).
5. Temperature affects the performance, and the residual voltage at null never becomes exactly zero (due to harmonics and stray capacitance).
6. Heavier and more expensive than some other displacement transducers.

## Check yourself

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

1. What is the LVDT output when the core is exactly at the centre (null)?
   A. Maximum output
   B. Zero, since e1 = e2 and secondaries are in series opposition
   C. Equal to the primary voltage
   D. 180° out of phase output
   Answer: B. At null the two secondary voltages are equal and opposite, so the output is zero. (Key point: LVDT working)

2. How does an LVDT indicate the direction of core displacement?
   A. By the output frequency
   B. By the primary current
   C. By the phase of the output (in phase or 180° shifted)
   D. By the number of turns
   Answer: C. Core to one side gives output in phase with primary, to the other side 180° shifted; magnitude gives displacement. (Key point: Direction sensing)

3. Which is a merit of the LVDT?
   A. Insensitive to stray magnetic fields
   B. Needs no demodulator
   C. Output exactly zero at null always
   D. No friction or contact, so no wear and very high resolution
   Answer: D. The core moves without contact, giving no wear and infinite resolution; stray fields, demodulator need and residual voltage are demerits. (Key point: Merit)

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