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### Topic: 200 Ohm thermistor: calculations (Read 1 time)previous topic - next topic

#### AlxDroidDev

##### Oct 09, 2012, 02:57 pm
I've read several tips and info on how to work with a 10k Ohm resistor, and I've made one work very well, being just a bit off from a DS1624 digital temp sensor.

However, I now have to make Arduino work with a 200 Ohm thermistor. Any help on how to make it work with Arduino?
Some of my projects:
Shield for DS1337+, DS1624 and AT24C1024B (RTC, temp & mem): http://forum.arduino.cc/index.php/topic,126197.0.html
CHDK Camera remote shutter (BT, IR, USB): http://forum.arduino.cc/index.php?topic=295377.0

#### MarkT

#1
##### Oct 10, 2012, 12:09 am
The usual simple circuit for a thermistor is a potential divider with a fixed resistor.  So another resistor around 200 ohms would be suitable - having similar values means the divider voltage is most sensitive to changes in the thermistor's resistance.

However, that means about 12mA flowing, leading to a dissipation of 30mW or so.  This means the thermistor will self-heat by many degrees, completely ruining it as a temperature sensor!

Two approaches in order of complexity:

1) use a transistor or FET to switch on the current to the divider just before taking a reading, then switch it off.  A reading takes about 110us by default, so the self-heating will be insignificant so long as don't read too often (10 times a second is reasonable).

2) Use a larger fixed resistor, such as 2k2, then amplify the voltage across the thermistor with a rail-rail opamp (gain around 5 to 10).  This keeps the self-heating down to a very low level (1mW dissipation in this case).  It also means the thermistor is getting an almost constant current so that the voltage reading is a nearly-linear function of resistance (not the case at all for 1)
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