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.
Assume a simple linear relationship between resistance and temperature for the following discussion:
ΔR = k ΔT
ΔR = change in resistance ΔT = change in temperature
k = first-order temperature coefficient of resistance
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.
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).
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.
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.