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Thread: Appropiate thermistor for current limiting in high-ambient temperatures

  1. #1
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    Oct 2017
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    Appropiate thermistor for current limiting in high-ambient temperatures

    We would really appreciate some help with finding a thermistor (and accompanying resistors) in order to do some current limiting in high-ambient temperatures. The circuit comes from the LT3741 datasheet (below).
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    LT3741 Datasheet Snip http://cds.linear.com/docs/en/datasheet/37411ff.pdf

    Options A, B, C and D show different methods of use, some which would allow better linearization of the R-T curve, but it's still not exactly what we want.

    The problem is that most NTC thermistors have the largest resistance differential at a temperature lower than our application. This means that we can't get a large enough range of resistance to be useful.

    Vref is 2V, and CTRL2 should be 1.5V at 80C, falling to ~0.075V at 100C.

    For example, if R2 was 120k, then the desired resistances of the thermistor would be 360k at 70 degrees C and 4.6k (or lower) at 100 degrees C. I can't seem to find any thermistors which such a great resistance differential at that range. By my calculations this is about a 28000 beta value. Most thermistors don't go above 5000.

    Do I need an op-amp to increase the voltage change? I think this blog maybe useful to your analysis. It's about Thermistor Introduction--​Temperature Sensitive Component: http://www.apogeeweb.net/article/38.html

    I would welcome any advice on how to manipulate a standard thermistor such that we get a large resistance differential at these temperatures, or are there known thermistors which operate around this range for the resistance change we require?

  2. #2
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    Jun 2015
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    Palm Springs, CA
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    Could you send the thermistor-resistor voltage to an analogRead pin on a Teensy? Then you could send analog output or capacitor-smoothed PWM back to the LT3741. The LT3741 wouldn't know the difference, but I don't know how practical that is for your application.

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