# Alternative to NTC Thermistor

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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They work very very well. –  Kortuk Aug 16 '11 at 16:37

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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