Thermistors THERMal resISTORS

1
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
2
ThermistorsTHERMal resISTORS

• 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 ltlt---gtgt Common carbon resistors, made
  from carbon powder mixed with a phenolic binder
  glue.

Leads, coated
Glass encased
Surface mount
3
ThermistorsTHERMal resISTORS

• Assume a simple linear relationship between
  resistance and temperature for the following
  discussion
• ?R k ?T
• where
• ?R change in resistance
• ?T change in temperature
• k first-order temperature coefficient of
  resistance
• Source http//en.wikipedia.org/wiki/Thermistor

4
ThermistorsTHERMal resISTORS

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

5
ThermistorsTHERMal resISTORS

• 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//en.wikipedia.org/wiki/Thermistor

6
ThermistorsTHERMal resISTORS
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
7
ThermistorsTHERMal resISTORS
Source http//www.science.uwaterloo.ca/jddmarti/
p352_w2007/Thermistor_50K.pdf
8
ThermistorsTHERMal resISTORS

• Example Applications
• Temperature measurement.
• 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.