Thermistors are temperature sensing instruments that have a semiconductor material in them. The property of this material is to exhibit a large and precise change in its electrical resistance when subjected to small change in its body temperature.
The History
The word thermistor comes from the combination of words “thermal” and “resistor”. In the year 1833, an English scientist, Michael Faraday first reported having encountered a change of resistance with change in temperature. He noted this in the material silver sulfide which is a semiconductor. This later led to the production of semiconductors for commercial use in the 1930s. Since then the semiconductor technology has come a long way and have played a great role in changing the world we see.
Types of Thermistors
Thermistors can be classified into two basic types, Positive Temperature Coefficient (PTC) thermistor and Negative Temperature Coefficient (NTC) thermistor. As their name suggests, in PTC type an increase in temperature results in an increase in its electrical resistance, and in NTC increase in temperature results in decrease in electrical resistance.
Of the two, NTC thermistor is more reliable since it can handle extreme environment conditions and noise. There are many benefits of using an them over a PTC thermistor, following is a list:
1. They have a fast response time (around 4 seconds)
2. They provide better resistance accuracy (around 1 percent)
3. NTC thermistors are customizable and can be packed in any form to meet the design requirements.
4. Because they can efficiently work even if they’re in small size, the cost of their production is low and are economical to use.
Manufacturing of NTC thermistors
NTC thermistors are used for temperature measurement, compensation and used as resistance thermometers. They are made from pressed, disc, rod, plate, bead or cast chip of semiconductor materials which are usually sintered metal oxides. By raising the temperature, number of active charge carriers also increases. This makes the material more electrically conductive.
The nature of flow of electrical current in this type of thermistor is described by the formula:
I = n . A . v . e
Where,
I = electric current in Amperes
n = density of charge carriers in count per meters cube
A = cross-sectional area of the material in meter squared
v = drift velocity of electrons in meters per second
e = charge of an electron in coulombs
NTC thermistors come in following two types of common configurations:
1. Epoxy: They are made by dipping the thermistor into epoxy and then soldering it with jacketed PVC/Teflon wires.
2. Glass Encapsulated: These are made by dipping the thermistor in molten glass rather than epoxy. They are capable of handling extreme environment. Glass coated thermistors have almost zero short and long term drifts.
NTC Thermistors are used in various industries. And for all your electronic components online need you can browse through Analog Technologies store.
