Constant Current Source, 10uA

Discussion in 'General Electronics Chat' started by cldudley, Oct 7, 2009.

  1. cldudley

    Thread Starter New Member

    Oct 7, 2009
    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? :)
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
  3. JDT

    Well-Known Member

    Feb 12, 2009
    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.