Thermostat help

Discussion in 'The Projects Forum' started by zander, Jul 26, 2008.

  1. zander

    Thread Starter New Member

    Jul 26, 2008
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    Hi
    I have one of these thermostats to control the temperature of my egg incubator.

    http://www.quasarelectronics.com/vmk138.htm

    The kit works well but i'm trying to improve the hysterisis so it tightens up from the 1 Celcius which it currently achieves. Trying to get about 0.5 C.

    I've changed one of the resistors which contol hysterisis from 6m to 10m which might has helped a bit, anyone get any other ideas of how I could try and do this?

    Appreciate any help or suggestions.

    Thanks
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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    The instructions that come with it say "decrease R7 to increase hysteresis." Instead of replacing the 6M with 10M, try a 4M and see what happens.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Decreasing R5, RV1 and R6 porportionately will have basically the same effect as increasing R7, the 6.8M resistor - except it will be less susceptible to false triggering by electrical "noise" caused by the resistors themselves.
    R5 - reduce from 120k to 62k.
    RV1 - from 100k to 50K (replace the pot).
    R6 - from 56k to 27k.
    This should significantly increase your hysterisis, because the gain of the 324 opamp will be increased.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If he replaced the 6.8M with a 4M, the hysteresis would increase. The OP wants to go the other way.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
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    Then the instructions from the website are wrong. Bad information from the internet? How could it be?:eek:

    :rolleyes:
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    Am I the only one confused? :p
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    OK, if the amplification of the opamp increases, the hysteresis will decrease.

    To increase the amplification, one needs to increase the feedback resistor, or decrease the resistance path to ground (R6) for the voltage divider, which is also part of the feedback network.

    But you can't just decrease R6, because that would throw off the remainder of the network - so you have to decrease the three of them porportionately.
     
  8. theamber

    Active Member

    Jun 13, 2008
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    What type of eggs are you incubating? I did some exotic birds eggs (senegals parrots, gouldian finches and fancy buttom quails) for quite some time. I used the foam type forced air incubator and temps variation of 1 degree are not as important as a circulated air is. A friend had a German incubator with a constant humidifier and very sophisticated controls. His hatching success on bottom quails was less than mines. Propper egg turning is also more critical as the embrio may suffer too much pressure.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    Now you've lost me. Isn't IC1D a non-inverting comparator? Don't R6 and R7 set thresholds? Won't the thresholds move farther apart if R7 is decreased?
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
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    6
    GAAAK!

    Never mind!

    I mis-read the opening post, they want to "improve" hysteresis, not increase it. It needs to decrease, which means a larger R7 or a smaller R6 to bring the thresholds closer together.

    I really should just go to bed now...
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    I've done much worse - as a matter of fact, I do it all the time! :rolleyes: ;)

    At 10M, I think our OP is quite at the edge of what increasing R7 will do for them; so reducing R6 is the only option - but along with that, the rest of the network has to be reduced, or his temp range adjustment will be terribly skewed.

    The mod I suggested should decrease the OP's hysteresis by about 1/2, which is what they are looking for.
     
  12. zander

    Thread Starter New Member

    Jul 26, 2008
    6
    0
    Hey thanks,
    Very Generous advice, especially to SgtWookie.
    I'm just looking to hatch chicken eggs.
    Did it the first time and got 6 out of 12. Three were Malformed so not a good good experience for the kids.
    Not sure if it was due to other problems but gradually trying to remove every possibility of messing up the hatch and making the best thermostat possible.

    Will get to work and report back on results.

    Once again, thanks very much.
     
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