Thermistors

A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C

 1.)Thermistor is made up of semiconductor materials

2.)Semiconductor materials have Negative Temperature Coefficient (NTC) of resistance. Hence, the resistance of a thermistor decreases with an increase in temperature and increases with a decrease in temperature.

3.)The resistance temperature characteristics of thermistor are highly nonlinear.

4.)It has large temperature coefficient of resistance i.e. It is highly sensitive to temperature.

5.)It has low operating temperature range compared to RTD i.e., -100 to + 300°C.

6.)Thermistors are small in size.

7.)They are available at low costs.

8.)They have high self resistance. Thus, they require shielding cables to minimize interference problems.

9.)Thermistors also provide an accuracy of ±0.01°C.

10.)They are widely used for dynamic temperature measurement.

LVDT (Linear variable differential transformer)

Linear Variable Differential Transformer (LVDT) consists of one primary winding (P) and two secondary windings (51 and S2).with equal number of turns wound on a cylindrical former. The two secondary windings are connected in series opposition and are placed identically on either side of primary winding to which an AC excitation voltage is connected. A movable soft iron core is placed within the cylindrical former. When the displacement to be measured is applied to the arm of the core, the LVDT converts this displacement into an electrical signal. The construction of LVDT is illustrated in figure (5.1). The operating principle of LVDT depends on mutual inductance.

When the primary winding is supplied with A.C. supply voltage, it generates alternating
magnetic field. Due to this magnetic field an alternating voltage will be induced in the two
secondary windings. In the figure (5.2) es1 is the output voltage of secondary winding S1 and es2
is the output voltage of secondary winding S2In order to get single differential output voltage
two secondary windings are connected in series opposition. Thus the differential output voltage
is given by,       e0=es1-es2

When the core is placed symmetrically with respect to two secondary windings an equal amount of voltage will be induced in both windings. Therefore esl - es2 and the output voltage is '0'. Hence, this position is known as null position. Now if the core is moved towards up from null position, more magnetic field links with secondary winding S1, and small field links with secondary winding S2. Therefore more voltage will be induced in S1 and less in S2i.e.,es1will be larger than es2. Hence the differential output voltage is e0=es1-es2 and is in phase with primary voltage.

 

But when the core is moved towards down from null position more magnetic
field links with secondary winding S2and small field links with secondary winding S1. Therefore
more voltage will be induced in S2 and less in S1, i.e.,es2will be larger than es1. Hence, the
differential output voltage is e0= es2– es1 and is 180° out of phase with primary voltage. Thus
the output voltage e0position of the core and hence the displacement applied to the arm of the
core.

Merits

1.  LVDT has good linearity i.e.. it produces linear output voltages.
2.  It can measure displacements of very high range usually from 1.25mm to 250mm.
3.  It has high sensitivity.
4.  Since it produces high output, it does not require amplifier devices.
5.  It has low hysteresis.
6.  It consume less power (about < 1w)

Demerits

1.  It is sensitive to stray magnetic fields.
2.  Performance of LVDT is affected by variations in temperature.
3.  It has limited dynamic response.
4.  To provide high differential output, it requires large displacements