Go Down

Topic: 200 Ohm thermistor: calculations (Read 788 times) previous topic - next topic

AlxDroidDev

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?
Learn to live: Live to learn.
Showing off my work: http://arduino.cc/forum/index.php/topic,126197.0.html

MarkT

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)
[ I won't respond to messages, use the forum please ]

Go Up
 


Please enter a valid email to subscribe

Confirm your email address

We need to confirm your email address.
To complete the subscription, please click the link in the email we just sent you.

Thank you for subscribing!

Arduino
via Egeo 16
Torino, 10131
Italy