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Testing Testing Testing … Smoke ’em if you got ’em.

The component side of the Thermistor Board.

The component side of the Thermistor Board.

Sooner or later there comes a time… a time when you just got to plug it in and find out if it is going to work. Hence my title “Smoke ’em if you got ’em”. Success, no smoke came out. However,there has been more than one problem. This post will be about several things and much of it will be about troubleshooting. Also, like always, there are new questions and things to be learned.

The back side of the board.

The back side of the board.

The board was constructed using an experimenters board with perforations on 0.1″ centers.   This saved me drilling all the holes, but it meant I had to do point to point wiring.  I still have mixed feelings about doing it that way, but it was probably a good way to go with creating just one board and that is what I intend to do.   However, as soon as I get this project completed, I will design some very simple circuits and learn a CAD for PC boards and try my luck at etching a real PC board.

Actually testing the board.

Actually testing the board.

The actual testing of your own creation is much harder than testing a kit that you have assembled and testing a kit is harder than troubleshooting some electronics that once worked.   The kit adds the unknowns of possible bad parts and possible bad assembly techniques.  Your own creation adds the additional unknown of messed up calculations and assumptions.  I seem to have had all of those!

The final version of the circuit.

The final version of the circuit.

When I tested the circuit I decided to attempt to adjust the potentiometers as close as possible to the final calibration values. I will have enough problems during final calibration adjusting the temperature of my standard temperature and I will want to get calibration done as quickly as possible.  This also gives me the luxury of testing the circuit completely before I add the extra unknowns.

Step 1 in the calibration is to adjust R4 of each circuit so the parallel resistor is the same as the 25 C resistance of the thermistor.   I used 10K resistors to simulate the thermistor during the test.  The calibration method was to use the voltage from the zener diode, Z1 through the 10 K resistor and measure the current.  I then put it in parallel with the R3, R4 combination and then measured the current again and adjusted R4 until the current was exactly double the initial reading.   I also measured the voltage across the zener in both cases.  There was a change of about 0.5% drop with the increased load of the parallel resistors, but at this time I am not going to worry about that.  (I may concern myself with it during the actual calibration — then again I may not.)

Step 2 is when things started getting rough.  I looked at my previous data produced  when I designed the circuit and found that at 70 deg C an ideal thermistor will have approximately 1.68K resistance and the produce an output of 0.0484 Volts after calibration.  At 0 deg C the resistance will be about 33.5 K and will produce an output after calibration of 0.866 V.  I built a couple of resistor combinations “close enough” for testing at these two values and proceeded to test.

Problem no. 1:  The power supply from the computer was not good enough to get the job done.  It only has a +10 V positive voltage and the working circuits reached saturation when I tried to get 0.866 volts out.  (I installed a divide by 11 voltage divider to use this board with the arduino AD converter. This means the op-amp was maxing out at about 8.6 V with the 10 V power supply.)  I used another power supply I have available which can put out + and – 15V.  Problem no. 1 fixed.

Problem no. 2:  Only 2 of the six circuits worked at all.   When the board was designed the left and right side were slightly different.  The two working circuits were both on the same side of the board.  I checked the voltage on all the pins of the IC’s and found I only had one of the power supplies connected on the left side of the board.  I quickly determined I had wired this pin to the ground connection on that side of the board.  Problem 2 Fixed!  I now 3 of the six circuits rough calibrated.

Problem no. 3 was a wire was not soldered and the loose connection caused me to not get a reference voltage to one of the amps.   Now I have 4 of the six circuits rough calibrated.  And that is where I am at the moment.

If you notice on the pictures showing the component side of the board, the IC for No. 1 is missing.  I am in the process of replacing it.   A trick I learned back in my younger days is once one circuit operated correctly it could become a reference for troubleshooting the others.  This works great when troubleshooting a stereo where one channel works and the other does not.  Voltages can be compared and potentiometers adjusted to get close to the initial calibration.  In the case of the No. 1 circuit the output of the final amplifier was maxed out.  I then measured both the inverting and non-inverting input to the amplifier.  Since this circuit is an summing inverter amplifier both the inverting and non-inverting inputs should be at zero.  However, the inverting input was the same as the output voltage of the previous amplifier.  My initial assumption was my feedback resistor, R14, was not connected correctly or was faulty. (Remember, my design and construction…so unknown no. 1…. all new untested parts so unknown no. 2.)  After checking those assumptions and ruling those out as the problem, it looks as if the op-amp circuit itself is the problem.

The easiest way to replace an IC circuit in this type of construction as well as in actual printed circuit, PC, boards is to cut it out.  Cut all of the pins on the chip so it will only be necessary to remove one pin at a time.  This saves over heating the board and possibly causing traces to become de-laminated from the base board in the case of PC boards.

For those of you just joining this blog, the post: “The Thermistor Circuit is Complete, Finally” has links to most of the previous posts in this series.  Tinkering has a price… it takes a lot of time.   (But it beats the heck out of watching TV.)

Other Testing:
I did a second test of water proofing the resistors.  I added a second coating to the ones coated with the sealing glue.  I also coated the ones with fingernail polish with a coating of the sealing glue and added a coat of fingernail polish followed by sealing glue to the uncoated “control group” and 5 new resistors I added to replace those in the insulation spray group.   I only had problems with a leaky covering in these final two groups when I tested them in near boiling salt water.  Refer to the post “Encapsulating the Problem – Waterproofing the solder joints” for details on the initial test.

However, I did discover two new things.   First, the sealing glue becomes very soft and sticky at these high temperatures.  It will work for the temperature range we are working in, but in the future I will look into some other techniques.   The second thing is the resistance of the 1M resistors changed at the high temps.  I expected that would be the case and all of them changed approximately by 25K Ohms.  The interesting thing is that all of them decreased by this amount.  Theory says they should increase.   I will experiment with some that are not coated in warm air when I do the actual calibration of the thermistors.

What is next?
After I get past the current problems I will need to come up with an “environmental  chamber” to calibrate the thermistors.  I have some ideas.  Stay tuned.  Finally some rubber is meeting the road and we are doing something besides theory!


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2 comments to Testing Testing Testing … Smoke ’em if you got ’em.

  • Gary

    Actually Tom I am familiar with Glyptal although the only use we used it for when I was a technician was we put a little dab n pots once they were calibrated to act like a seal. When I started this process I looked around for some of that and even stopped in an electronics shop. They said that had not seen it in years.
    I stopped looking locally after that and tried the alternatives. I do think I saw it in on of the on-line sources, but it was relatively expensive and of course the minimum order hassle.

    Thanks for the feed back.

  • Tom

    I understand the problems you are having with the protective coating. Many years ago I went through a similar process and wound up usuing a product called “Glyptal”. I purchased it through my local electronics outlet but I think even Radio Shack had some in stock at the time.
    Essentially it was a conformal coating product, it came in a small bottle with an integral brush, (like nail polish), and was very easy to apply. I used it on a circuit that was tested in an environmental chamber in several different fluids from silicinoe oil to salt water and from -40 to +120 degrees C. The failures were the ones that were incompletely covered with the product or that the product chipped off. (It’s somewhat brittle once cured.)
    I wound up giving each one a double coating and to the best of my knowledge we only had one fail in the field, that due to a failed IC.
    I don’t know if it’s still on the market but it cured the problems I was having and was thin enough that it didn’t affect the temperature sensing too adversely.
    I hope this has been helpful, good luck with the rest of the project! (I have to go back through your notes to find out what you’re doing to see if I have any more thoughts for you.)

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