Hello group,

I am new here, and have a question. I am looking to measure a temperature through a thermistor. The process is very cold, so I don't want to introduce any more heat than absolutely required.

There is an existing instrument that reads the thermistors, built in the dark ages (original 80C86), and I am building an upgraded test unit using a modern microcontroller (Atmel AVR).

The current instrument sends out a constant 10uA through the thermistor, then reads the voltage drop to determine the resitance, then looks in it's interpolation table to determine temperature.

I have all of the digital and software pieces figure out, but I do not know how to build a constant current source that supplies such a small current.

Any pointers?

 

LM334 current source should do you - there are more sophisticated ics

http://www.national.com/mpf/LM/LM334.html

 

Not too difficult to do. Circuit attached.

Another method would be to pulse the current through the thermistor, only when you need to take a reading. Keep the duty cycle very low. eg. On for 100us every 100ms.