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Choosing R1 needs to be done carefully to achieve a proper reading. If chosen too small, the flowing current inside the NTC will lead to losses that in turn heat up the device thereby falsifying the measured results. If, on the other hand, R1 is chosen too large, the measured voltage gets too small and in turn the measurement looses accuracy again.
To minimize the influence of the current, a thermal view is helpful. The thermal conductivity for the NTC is 145K/W. If a 1K influence is tolerable, the power dissipation inside the NTC may not exceed Pmax=6.9mW.
Assuming that a measurement up to 100°C needs to be done, the NTC will reach a value of R100=493. From this, the maximum current can be calculated to be
With a supply voltage U1=5V and a current limit of 3mA, the resistor R1 becomes
As there is no such resistor, 910can be chosen, leading to Imax=3.56mA; any value that limits the current to I<4mA can be considered as long as 1K difference is tolerable.
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Figure 7 Basic schematic to get a digital temperature information
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