Download
therm istors therm al res istors n.
Skip this Video
Loading SlideShow in 5 Seconds..
Therm istors THERM al res ISTORS PowerPoint Presentation
Download Presentation
Therm istors THERM al res ISTORS

Therm istors THERM al res ISTORS

268 Vues Download Presentation
Télécharger la présentation

Therm istors THERM al res ISTORS

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  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 <<--->> 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: 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

  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.