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A thermistor is an analogue, passive, resistive, input transducer used to measure temperature.
Negative temperature coefficient resistors (NTC Resistors)
The thermistor resistance changes over such a large range that normal graphs are not useful. Log graphs solve the problem. Here is an example based on the data sheet for a thermistor with 10K at 25 C.
If possible, three design aims need to be met.
For example in a refrigerator, the cooling pump might come on if the temperature rises above 5 Celsius. From the graph above, the thermistor resistance is about 25kΩ. A 27kΩ series resistor could be used. In a 12 volt circuit, at 50, the current flowing would be ...
I = V / R
I = 12 / ( 25 + 27 )x103
I = 0.23 mA
This current would not have much of a heating effect.
The series resistor is similar in value to the thermistor at 50C.
At much higher temperatures the current would still be less than 12 / 27k = 0.444mA.
If the thermistor has a maximum heat dissipation of 0.5 Watts and the power supply voltage is 12V, at what temperature does it become impossible to meet all three design requirements? You can assume that the circuit is turned on for a very short time so the heating effect of the current flowing can be ignored.
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