Thermistor | Working and Construction | Semiconductor

Thermistor

What is Thermistor ? Explain working.

Working of Thermistor :

Basically Thermistor is a short form of a word thermal resistor,  Thermistors are a temperature measurement device that has a negative temperature coefficient.


A$ the temperature increases the resistance goes down  and as the temperature decreases the resistance goes  up.  This is just 0pposite to the effect of temperature changes on metals.


Thermistors are made from semiconducting materials mixtures of metallic oxides such as chromium, nickel, cobalt, copper, iron etc. These metallic oxides are mild, mixed in appropriate proportions, and are passed into the desired shape with appropriate binders and finally sintered given below.

Thermistors may be shaped in the form of small piece of following shapes.


Fig. Thermistors

Positive temperature coefficient thermistor :

In positive temperature coefficient thermistor then resistance increases with temperature.


Fig. Temperature resistance relationship

Advantages :

1) Thermistors have a relatively large but negative resistance change.
2) Metals have an approximately linear temperature resistance relationship.
3) The range for resistance  thetmometers greater, being from - 160 to 600°C.
4) It is more stable.

Disadvantages of thermistor : 

1) It requires external energy source.

2) Unsuitable for a wide range.
3) Need of shielded power lines.

Semi-conductor or pizoresistive gauge.

~ These gauges are produced from silicon or
germanium crystals in which exact amount of
special impurities such as boron have been added to impart certain desirable characteristics.
~ The process is called doping and the crystals are known as doped crystals. he gemi-conductor gauges are classified into :
resistance whose n-type
1. Negative or decreases in response tensile strain.
2. Positive or p-type whose resistance increases in response to tensile strain.
~ The breaking stress of the material rises as the cross sectional a.ca decreases. Accordingly these gauges are constructed in the thinnest possible cross-sections and as such can be bent to a much smaller radius of curvature without fracture.
The gauge is in the form of a single rectangular ilament about 0.05 mm thick by 0.25 wide and 15 to 12 mm is length.
a) A single film semi-conductor gauge with leads on either side.
b) A combination of n-type and p-type provide
temperature compensation because both have positive coefficients of resistance. The semi-conductor gauges are usually provided with a plastic or stainless steel backing and are bonded to the test-surface by the same
methods as wire and foil gauges.

Advantages :

1. Semi-conductor gauges have a very high sensitivity in comparisons to the metal gauges.
2. They have a high gauge factor in the range of 100 to 200.
3. Semi-conductor gauges have low hysteresis.

Disadvantages:

1. They are suitable for large strain measurement.
2. Their linearity is poor ± 0.75 % over 1/10 full scale and ±20 % over full scale.