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I'm working on a project where a device I'm using has this thermistor, a Murata NCP15WF104F03RC, to measure the temperature of the device itself. This is a surface mount thermistor. The implementation of my project requires ambient temperature sensing, and comparing the two temperatures.

I figured the easiest way to do this would be to use the same thermistor for ambient temperature sensing, as they would have the same thermal resistance curves and other relevant characteristics, which would probably help with precision of comparison.

Are there any glaring flaws with this design decision?

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  • \$\begingroup\$ It sounds like you're on top of things. One thing comes to mind - air is bad for thermal coupling, so the ambient one won't respond as quickly as the internal one unless its mounted to something conductive. Depending on what's in the box, this may be non-trivial. \$\endgroup\$
    – user39962
    Mar 6, 2015 at 1:10

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I thought the following by Maxim was a good read on selecting a temperature sensor and it talks about the trouble of measuring ambient down the page some(after Table 1): http://www.maximintegrated.com/en/app-notes/index.mvp/id/3229

To summarize what it says: The two best ways to ensure the sensor measures ambient temperature and is not affected by other sources is to
a) use a thermistor or other temperature sensor with long leads and keep it extended up away from the pc board.
b) use a satellite board that you can place the thermistor on that will keep it away from other electrical components that could heat up the pcb and keep it away from other parts of your system like fans that will alter ambient readings

So it sounds like your solution of having the thermistor on a separate board away from the fans/main pcb components will work just fine. Just avoid having any components on that satellite board that will dissipate power.

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I see one potential flaw - if the device is warm it will thermally bias the board to which a second thermistor is attached and the second thermistor will read higher than ambient.

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  • \$\begingroup\$ The device is inside an enclosure with cooling fans, and the ambient temperature sensor is located outside of the enclosure, far away from the fan exhaust. \$\endgroup\$ Mar 5, 2015 at 21:58
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Sure; just mount the thermistor on a small flat heat sink which is exposed to ambient.

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    \$\begingroup\$ As long as the heat sink is either a) thermally insulated from the electronics, or b) so well-coupled to ambient that its temperature does not rise. \$\endgroup\$ Mar 6, 2015 at 1:02

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