Where can I download the thermistor footprint library & symbol library?
I could not find anywhere
Thermistors are standard parts of the “device” library.
Just type “a” for “add” in Eeschema and start typing “thermistor”
If that does not work, you may have a problem with the library setup.
https://forum.kicad.info/search?q=library%20setup%20category%3A19
1 Like
I found the symbol, I could not find the footprint
Well there is not really a special footprint for thermistors. They come in many shapes and sizes. For example thermistors can come in a normal resitor package (so for example in a 0805 imperial or one of the through hole resistor ones) but they can also come in packages like a TO-92.
And then there is the possibility that your thermistor is connected to your system via a cable as you might want to measure the temperature of something else than the environment temperature directly near your board. In such a case any connector will do (number of pins depends on measurement setup and can be 2, 3 or 4).
Note: we do not have enough information to help you out here. At least point us to the datasheet of the device you already selected. Or if you did not yet select the device then give us a short description of your system specification as we might be able to give you tips on how to get from there to selecting a fitting component.
2 Likes
I haven’t selected the thermistor yet.
I want to make a system that will close the circuit when the environment rises to 80 ° C.
I think ptc thermistor would be suitable for this.input voltage max 9v dc
Then I don’t really understand your question. Choose an appropriate thermistor from mouser or wherever you get your components from and then look for that footprint in the kicad libraries.
Or maybe you want to do it the other way. Let’s assume you have space for a 0805 smd footprint on your PCB, then look at mouser if they have appropriate types of that footprint available and if yes, just assign the 0805 resistor footprint to your symbol.
Some time ago I bought a bunch of MCP9700 “lineaized termistors” in TO92 packages. It also comes in SOT23 and MC70. I don’t know about the MC70, but the others are bog standard.
I bought them mainly because they were “cheap”, and thought the linearization was a nice extra. But it’s just a random part number. There probably are hundreds (maybe thousands) of variants.
I bought the TO92 version because it stands off from the PCB so it measures air temperature, and it can also relatively easy be soldered to a cable or mounted to a heatsink.
2 Likes
A thermistor will on its own not do what your system needs. All it does is change its resistance depending on its temperature. So you need to design some form of analog circuit with a digital output that turns off your system. This can also simply be using the analog to digital converter of a microcontroller (example arduino) and have this temperature limiting done in software.
I would however highly suggest going the route of using a something like @paulvdh suggested. Getting from the change in resistance of a thermistor to a precise temperature reading is quite tricky. To be honest measuring resistance on its own is already tricky let alone taking care of the non linear behaviour of a thermistor.
Additionally, be aware that some electronic components are only specified up to 80°C. So if your device needs to operate above that temperature then you need parts that can survive at more than that. At least the temperature control circuit needs to function above your cutoff temperature (it can not really turn itself off). The rest needs to at least survive the temperature in its off state (look at the storage requirements within the datasheet).
Or alternatively only have the sensor in the area where you expect > 80°C and the rest at a place with more reasonable temperatures. Don’t forget that even cables have a temperature range where they function (so does a solder joint). And be aware that any increase in temperature (even within the specified range) limits the lifetime of the system.
And if the 9V come from a battery please ensure it can operate at the expected MAXIMUM temperature (I assume you don’t want an explosion).
2 Likes
This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.