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The Thermistor Circuit is Complete. Finally!

The Final Thermistor Circuit.

The Final Thermistor Circuit.

Because of the loose tolerances used in electronic components designing circuits is not as easy as it sounds if you need close tolerances on the final circuit. Adjustable components and some sort of calibration procedure helps and are necessary, but making sure those adjustments can cover the complete range of the the possibilities is not as easy as it looks.

To put this in context, imagine a piston going into a cylinder.  The nominal bore diameter is 4″. (The size of a 350 small block V8 cylinder.)  If this was designed around +/- 5% tolerances you could end up with situations of trying to cram a 4.20″ piston in a 3.80″ cylinder bore.  It just ain’t gonna get it!

Only some of the components were really a concern.  Those were Z1, R2, R13, R14, R16,  R18 and of course the thermistor itself.  I wrote and ran a computer program with these in all combinations of each one at the nominal value, the lowest tolerance value, and the highest tolerance value.   I had two purposes in doing this.  First I wanted to make sure I had the necessary range of adjustment in R6. Second, since I have had to modify some assumptions from the first runs when I linearized the thermistor I wanted to make sure I still had the desired accuracy.

R16 and R18 guarantees I can get no where near above 5 V feeding the Arduino A/D converter.  The voltage output will run from 0V at 80 deg C to 1.1 V at -30 deg C and the Arduino will be set with the internal reference of 1.1 V.   It should work well.  (Famous last words!)
I do have the python program and will e-mail it to anyone asking for it, just beware that the documentation within the program fell apart.   There were a couple of reasons for that, but mostly because I just kept adding to the same program instead of copying it to newer versions and deleting the older parts that had already had been used to answer questions.   I also waited too long to get back to the program and got lost within it.  So it was a combination of 3 of my bad habits that ended up costing me a a lot of time.

So what is the next step?   The next step is to lay the circuit out physically so it can be assembled.  I have already done much of that and will present it in a future post probably sometime early next week.

The next experiment will be to come up with a insulating coating for the thermistor connections to allow them to be immersed in water.   To do that I will use 1 MΩ resistors soldered to short “pig tails” of the wire I intend to use for the thermistors and then insulate the junction with various insulator compounds.  Measurements will be taken:

  1. Before insulating and after soldering.
  2. After the insulation cures
  3. Immersed in Ice water.
  4. Immersed in warm water
  5. Immersed in warm salt water to increase the conductivity of the water.
  6. A final measurement at room temp. with the resistor dry.

I will probably do 5 of each type of insulation as well as 5 non-insulated control set to make sure my ohmmeter has not drifted.   I will also do some bare leads to make sure the water is actually conductive.

Resistors are being used because the coating on the resistor is similar to the epoxy coating on the thermistors.    The insulating material I will try is epoxy glue and possibly some other glues such as RTV silicon rubber.

The maximum variation in temperature on all of the runs from ideal temp to the calibrated temp was less than 0.5 deg C.   In real life it will depend upon how accurate our reference thermometer is that we will use to set the 70 deg C water for calibration of our circuits.

Anyhow it is done…… Celebration time.

Some previous posts in this series are:

  1. Learning the Arduino The Thermistor Circuit
  2. The Thermistor Circuit – Zener diodes
  3. One More Step Closer to completing the Thermistor Circuit
  4. Modeling the Thermistor
  5. Goals of Calibration of the Thermistor Circuit
  6. “Linearizing our Thermistor”.


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