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I am designing a system which turns on at 45 degC. I want to place a device in series with my circuit that ideally has infinite resistance below 45 degC and then 0 resistance above 45 degC.

I guess the closest I can get to this ideal is using an NTC thermistor but I am struggling to get my head around how to select a B number.

As, to my knowledge, my ideal device doesn't exist I have worked out that an NTC thermistor with a resistance above 10K Ohm at room temp at 10 Ohm at 45 degC would work but, if my calculations are correct, this gives me a B number of 37,000 ish which seems ludicrously high.

I plotted a graph of ln(R) vs 1/T, added my two points and then worked out B as the gradient - is this the correct method? (inferred from this wikipedia article)

Does anyone have any suggestions for an alternative method or where I maybe going wrong in my calculations?

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What you really need is a thermal switch. NTC thermistors are not intended to have such sharp characteristics as you have found-out. Your calculated B value is around 10 times the typical value.

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    \$\begingroup\$ They work very very well. \$\endgroup\$ – Kortuk Aug 16 '11 at 16:37
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Get a LM335Z which is a temperature sensor with calibrated milivolt output in K. So 45 C = 318K which means the sensor output will be 3.18V. Plug that into an op amp as comparator with 3;18V on the second input and use the output straight into a 2N7000, optionally driving a small relay.

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A somewhat sledgehammer solution (though inexpensive and uncomplicated imho) would be to read whatever temperature sensor you want with the ADC of a microcontroller, control the base of a transistor with the pin of an microcontroller that is running software to "switch" at a given temperature reading. With this approach you could also do more sophisticated things in software (e.g. filtering, etc.).

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    \$\begingroup\$ A less sledgehammery solution would be to replace the microcontroller with a comparator or Schmitt trigger and potentiometer/voltage divider. Lower cost and no programming, but you don't get the software potential of your system. \$\endgroup\$ – Kevin Vermeer Aug 16 '11 at 16:49
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You can go for a fancy electronic solution with a sensor, microcontroller and a relay. But wouldn't a simple bimetal temperature switch do the trick?

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  • \$\begingroup\$ Yep, that's what I went for - I didn't know they existed until posting here though :-) \$\endgroup\$ – SimonBarker Aug 27 '11 at 16:55

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