circuit lab therm is to rs

Pat Arnott, ATMS 360 Atmospheric Instrumentation

ThermistorsTHERMal resISTORS



A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance

with changing temperature. ****** Thermistor is a combination of the words thermal and resistor. The

Thermistor was invented by Samuel Ruben in 1930, and has U.S. Patent #2,021,491.

Leads, coated Glass encased Surface mount



Thermistors are made of semiconductor materials (metallic compounds including oxides such as manganese, copper, cobalt, and nickel, as well as single-crystal

semiconductors silicon and germanium).

Contrast <<--->> Common carbon resistors, made from carbon powder mixed with a phenolic binder

glue.

Leads, coated Glass encased Surface mount



Assume a simple linear relationship between resistance and temperature for the following discussion:

ΔR = k ΔT

where

ΔR = change in resistance ΔT = change in temperature

k = first-order temperature coefficient of resistance

Source: http://en.wikipedia.org/wiki/Thermistor



Thermistors can be classified into two types depending on the sign of k.

If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, Posistor.

If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.




Resistors that are not thermistors are designed to have the smallest possible k, so that their resistance remains almost constant over a wide temperature range.




Source: http://www.science.uwaterloo.ca/~jddmarti/p352_w2007/Thermistor_50K.pdf

Thermistor-choice is based on the nominal resistance you want at the operating temperature range, on the size, and on the time constant.

Time constants are about 5 - 10 seconds. (Check this out with your thermistor).



Source: http://www.science.uwaterloo.ca/~jddmarti/p352_w2007/Thermistor_50K.pdf



Example Applications:1. Temperature measurement.

2. Time delay (self heating from large current ‘opens’ the thermistor so it can be used as a slow switch). Heating = i2 R where R is the resistance and i is the current.

3. Surge suppression when a circuit is first energized. Current needs to flow through the thermistor for awhile to heat it so that it ‘opens’, and acts again as a switch.


Demonstration

circuit lab therm is to rs

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