The Jfet... device is available as a calibrated (measured) diode package from several sources the use of a stable voltage to drive it makes for a VERY stable reference that can supply current to your thermistor voltage divider. You then measure the voltage and apply the correction factor to it and then use it. For an Arduino there is little use for the accuracy attainable with an Wheatstone Bridge as the bridge configuration can easily measure milliohms in 10's of hundreds of ohms... .01 (10 mOhm) ohms/100 ohms = .0001%.. From a an accuracy standpoint you will get more information than you can use. The Principal reason I suggested an accurate reference/divider was to simplify your task. If it is necessary that you learn about Wheatstone, Hay or other forms of the same type of network for your education (Instructors recommended project) there are better ways of dealing with it. However if you must deal with a bridge instead of a simpler divider network then use the lowest temperature and add 10 deg to it and use that Resistance as the beginning point (Make the other resistor in the thermistor side equal to that value and the bridge can never swing negative. It will be a rising potential equal to the difference in value of the two resistors in the "variable" half of the bridge. It in effect becomes a voltage divider again... with two un needed parts whose temperature stability can be a major factor in the total accuracy of the measurement method. I have included a data sheet from one manufacturer of CC diodes. The Jfet idea I mentioned is the beginning of that technology. Basically it is a low pinchoff Jfet and a resistor in series with the source (as I remember...) and the gate is tied to the other end of the resistor when the voltage across the resistor (dropped by the diode load connected to the source, drain to V+) the Jfet cuts off limiting the current to that level.When a precise current is placed across a known value of resistance a precise voltage is developed. I discussed this on line in this thread with another person and there I explained my thoughts... Did you read my response? and my thinking?. A Wheatstone Bridge is a great tool for certain things and a VERY interesting thing to investigate, I've used them for many different things in many different ways... Used one to make an AM modulator for a transmitter I built 30 years ago. I used one to make an FM modulator several years later... these are a few uses I have Bent from the original work I did with bridges when I was in school. In short unless there is some outstanding reason why you NEED to use a bridge remember the total stability/accuracy is dependent on a good reference bias source and 3 other parts besides the measurement variable, a lot of bother for questionable returns. IMO
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CCL0035_V2.pdf (75.8 KB)