Thermistor question

Discussion in 'General Electronics Chat' started by Jacob J, Apr 29, 2011.

  1. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    0
    Hello all

    I have a question about thermistors. As I can see a NTC thermistor with the resistence of 1,5K ohm has the same temperature range as a 10K thermistor. How would I choose the right one for a project? As far as I know, the higher the resistance is, the cooler the temperature is. And if there is no resistance, the temperature is high. But why have different resistances, when the range of the thermistors are the same?

    /Jacob
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Different (25C) resistances are available to suit different applications. Isn't it pretty obvious that not all circuits will apply the same voltage or current conditions?

    Depending on the functional requirement, a bigger or smaller room-temperature resistance may be appropriate. Manufacturers supply information on the characteristics of resistances at other temperatures. Here is an example datasheet: http://www.vishay.com/docs/29048/29048.pdf
     
  3. Jacob J

    Thread Starter Active Member

    Jun 18, 2009
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    0
    Okay, so if I understand you right, then a 1.5K thermistor is used in circuits with low current and/or volts?
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    The bane of thermistors is self heating. Using P=I^2R, if you pass 1ma through a 1.5k thermistor, you will create 1.5mw of heat. To create that same amount of heat in a 10k thermistor, you'll only need about 400ua. As a general rule, any low resistance circuit will require more current to work with than a higher resistance circuit. Now that I said that, a buttload of exceptions will be coming forth;)
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    A thermistor is merely a resistive device that varies its resistance with the temperature of the environment it's in.

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

    Values depend on circuit design. I may have a circuit that I want to see 10K at room temperature and 5K at 120*F so I've got to look to see if I can find one that will do that.

    There are tons of different values (which represents the resistance at 25*C) and many different resistance response curves available.

    They can be NTC (resistance goes down as they're heated or (less often) PTC (resistance goes up when heated)
     
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