Building my snake eggs incubator(Somebody with good op amp knowledge could help)

Discussion in 'The Projects Forum' started by Cyberduke, Nov 14, 2011.

  1. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
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    1
    Hi, I am currently building an incubator for my (hopefully) soon to come snake eggs.

    For the thermostat I have purchased a velleman MK133 kit.

    But the problem it that I can only set the relay to switch on at 5-30 degrees. But it switches of at 30 degrees.

    I want the relay to switch on degrees(That could be adjusted with RV1 - done)
    I want the relay to switch off at 28 degrees. (Can be adjusted with R7)

    The problem is, I don't know to what value R7 should be changed. (I know it should be lowered)

    My uncle said that anybody with good op amp knowledge could help me calculate the correct value for R7.

    Scematic link - http://i1129.photobucket.com/albums/m505/cyberduke1/mk138.jpg
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Your statement is a bit confusing:
    "I can only set the relay to switch on at 5-30 degrees. But it switches of at 30 degrees. "

    What is your desired ON temperature, and what is your desired OFF temperature?
    - OR -
    Desired ON temp, and how many degrees hotter?

    You could specify the temps in degrees F; that would be a bit more precise.

    Like, 82°F ±2°F
    - or -
    on @ 80°F, off @ 84°F.
     
  3. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Sorry, you have probably noticed that english is not my first language.

    On 78.8F
    Of 82.4F
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    No, I did not notice that English is not your first language.
    Your English is better than many English speakers. :)

    I am not home at the moment. I will look at this problem when I return home in a little while.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    OK - can you tell me:
    1) What is the resistance value of the NTC sensor when it is exactly 78.8°F/26°C ?
    2) What is the resistance value of the NTC sensor when it is exactly 82.4°F/28°C ?

    It needs to be as accurate as possible. I suggest using water in a foam cup or similar - you can add hot or cold water and stir until it is the right temperature using an accurate thermometer; an oral thermometer works OK if you have one - and then put the sensor in the water and wait about 10 seconds for the temperature of the sensor to stabilize. Measure the resistances while the sensor is under the water.
     
  6. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Ok thanx, I will do it as soon as I get home, because today I am writing my english exams.
     
  7. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Does the cup have to be a foam cup because of its good insulation or can I use a glass?
     
  8. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Just a piece of advice for anyone attempting to do this experiment, Have patience!

    78.8°F/26°C = 7K87 ohm
    82.4°F/28°C = 7K18 ohm
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Increasing the resistance of the 6M8 feedback resistor is not desirable, as it would make the circuit more sensitive to electrical noise.

    What I would do is replace R5, the 120k resistor in the reference divider, with a 56k Ohm resistor.

    This will do three things at once:
    1) Instead of your pot being 100% up against the end, it will now be at ~64%, being closer to centered on the temp range you desire.

    2) The hysteresis will be decreased by approximately 23%, which will narrow the temp range between ON and OFF; it should now be ~1.3°C/2.34°F above the temp set by RV1. The temp range should currently be about 1.7°C/3.06°F.

    3) The sensitivity of the temperature adjustment via RV1 will increase by ~30%.

    The first two are desirable, the 3rd, not so much. It will make the temp adjustment seem more "fiddly".

    At this point, if you want to decrease the sensitivity of RV1, you could replace it with a 50K pot. The starting point would shift to ~70% of the way towards the end it is now, but the sensitivity of the adjustment would be cut in half; and ~59% as sensitive as it was originally. This will also have the effect of further narrowing the temperature range. Note that you cannot simply replace the 100k pot with a 50k pot without decreasing the resistance of R5 to 56k (you can actually go a good bit lower with R5, but not much higher).

    The above estimates are approximate, and your results may vary due to tolerances in the components used, and the non-linearity of the thermistor, as the thermistor has no compensation provided. I strongly encourage you to test the resulting circuit, monitoring performance over at least several days, taking temp measurements when the relay opens, and when it closes.
     
  10. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Hi and thank you very much

    I would only be able to get a pot in a few weeks. The only value pots that I currently have is: 47k, 10k, 220k and 50k.

    Can I use one of them? What would the difference be?
     
  11. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Sorry just ignore that question about the pots. I have accidentally red "150k" instead of 50k.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    You first need to replace R5 with a resistor close to 56k Ohms. You could actually use anywhere from 33k to 56k Ohms.

    Then you can replace RV1 with your 47k or 50k pot. If the pot is not the correct size physically, you can use insulated wires to connect it to the board.

    If you try to just replace RV1 with a lower value potentiometer without reducing the value of R5, you will no longer be able to set the temperature high enough.
     
  13. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Thank you!!! Very much, I just tested it and it seems like it is working!

    Thank you again
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    That is good news, indeed. :)

    Congratulations on a successful resistorectomy. ;)
     
  15. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    What is a resistorectomy? I am not going to say what it sound like. Lol
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    I'm joking around. ;) It should have been "resistor transplant" lol.
     
  17. Cyberduke

    Thread Starter Active Member

    Mar 5, 2011
    33
    1
    Haha you got me on that one. (I am not big on medical terms)

    LOL
     
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