10points & many thanks-- Could you please explain how thermistors are used as...?

inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements?

I happen to know the existence of thermistors only today. But I've been "introduced to" them in a very scientific way,

http://upload.wikimedia.org/wikipedia/commons/thum...

what made me don't understand how exactly they are used in everyday life.

I also read Wikipedia's 7-line definition, but I still don't get.

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

Could you explain why do they differ from resistance temperature detectors? What are resistance temperature detectors?

If thermistor is this tiny metallic wrapping around this thermometer's cable?

https://encrypted-tbn0.google.com/images?q=tbn:ANd...

How do they act as current limiters?

What does wiki mean by "self-resetting overcurrent protectors"?

What are "self-regulating heating elements"?

Update:

typos--

*I'VE also read a 7-line DEFINITION OF IT ON WIKIPEDIA, but I still don't get IT.

*WHETHER A thermistor is this tiny

*How do THERMISTORS act

2 Answers

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  • Ecko
    Lv 7
    8 years ago
    Best Answer

    NTC (Negative temperature coefficient) thermistors are made from a material such as a metal oxide that varies its resistance as the inverse of temperature in a non linear way. The higher the temperature the lower the resistance. The sensitivity can be more than 5% per degree in the working range, so circuitry is simplified. These tend to require individual calibration, though there are some interchangeable types from manufacturers like YSI or Omega. These are made as a small bead of the material with the 2 wires attached. The bead might be embedded in glass or epoxy to protect it.

    Positive temperature coefficient (PTC) thermistors are made from a different material. These act more like a temperature sensitive switch. As the temperature increases beyond some point the resistance increases more rapidly.

    Thermistors can be used to limit switch on surges (inrush current) from filament lamps or charging large reservoir capacitors. This is the NTC type. It is a high resistance initially, but as it heats up from the current flowing, its resistance drops, and stays that way due to the normal load current. A PTC type might be used to switch on something when powered on, but if its temperature increases due to self heating the device switches off. This effect can behave like a fuse to limit maximum current. Once it cools down the circuit is active again. With old style CRT TVs the degaussing coil is powered this way. After switch on the current in the coil also heats the thermistor so current is gradually reduced, as required for demagnetising the screen.

    As the PTC thermistor heats, its resistance increases rapidly, so reducing the heating effect. This is a self regulating heater, though the heat may only be small.

    A resistor made of metal has a positive temperature coefficient like 0.39% per degree C for copper (platinum is similar). An RTD (also called a PRT when platinum wire is used) has this small small sensitivity, the same as copper connecting wires, but can be made in precision interchangeable types. The temperature versus resistance is highly predictable, reasonably linear, and covers a much wider range of temperature than a single thermistor. Circuitry needs to be carefully designed to reduce errors due to the low sensitivity. They are popular in industrial situations.

    The temperature sensor you show is one with a protective metal case. The thermistor bead is inside.

    The link below compares temperature vs resistance for 3 types of thermistor and an RTD.

  • 8 years ago

    tough one

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