Re: How do I measure Electrical Resistance (Lineary)

Greetings, Professor Calvo:

If I read you correctly, you have a humidity sensor that requires at least 50 
mA of current and changes resistance with relative humidity.  I will have to 
answer your question in general terms because I don't know the resistance 
values versus humidity of your sensor.

You can make a constant current source from a transistor in common-base 
configuration.  Choose a transistor that can handle 50 mADC with ease and the 
supply voltage you will choose.  Make the humidity sensor the collector 
resistor.  Design the emitter circuit to have an emitter current of 50 mADC.  
Then, the collector current must be 50 mA as long as the transistor is not 
saturated.  The voltage across the sensor will then be proportional to its 
resistance, and that relates to the relative humidity.  The design problem is 
to make sure the transistor does not saturate over the sensor resistance 
range.  This will affect choice of supply voltage.

I hope this helps.

Larry Skarin