|MadSci Network: Engineering|
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
Try the links in the MadSci Library for more information on Engineering.