Cabinet Temperature Controller

Discussion in 'The Development Lab' started by kool-lites, Oct 25, 2014.

  1. kool-lites

    kool-lites Full Time Elf

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    With the changes to the layout this year, I have several new 350W SMPS out in the garden. Most have been in fairly large cabinets with some form of passive cooling. This year things have got smaller so I am using some fan forced cooling.​
    But I don't want the fan running all the time.
    So I have set out to make a simple fan controller.​
    Design Requirement:​
    • Operate of 12 Volts
    • Keep fan OFF until needed
    • At 40degree ( this is my first guess) turn the fan ON slowly)
    • At 60degree and above make the fan go flat tack.
    • Between these two set point, the fan speed is to be set proportionally
    • Capable of driving 2x 80mm or 120mm fans
    The Temperature measurement uses the Dallas DS18S20. I have a number left over from a previous project.
    [​IMG]

    [​IMG]
    So the Prototype is built, the code is finished and the PCB design complete. I hope to get them out to the fab house this week.​
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer

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    Looks good.
    I'd be highly tempted to run with lower temps like 30C and 50C. Ideally you don't want the electronics to be climbing too much above ambient temperature and not turning on the the fan until the air temp inside the enclosure hits 40C means that some of the stuff inside the boxes is likely to be well above 40C. Simply moving the air around when the temp is above 30 should prevent too much of a temperature rise.
     
  3. BradsXmasLights

    BradsXmasLights WiFi Interactive

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    Yeah I was just about to say 40+ sounds a little too hot.

    Perhaps you could make it adjustable with a trimpot?
     
  4. multicast

    multicast Senior Elf Generous Elf

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    I'd be putting a diode in reverse polarity across the FET to protect it from the back emf of that fan. Its eventually going to spike.
     
  5. OP
    OP
    kool-lites

    kool-lites Full Time Elf

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    QFK,
    Good find. Diode added.

    AAH,
    That is an issue I wondered about. The inside temp will always for above ambient. During the day when the ambient is 30 and a above when the load is supper low also (I don't run a sequence during the day), there is no need for the fan to on as convection should assist. I also place my controllers out of direct after noon sunlight. Tropical lids are also another solution to play with, but that is another story for another time.

    I may wind it back to 35 and see how it pans out.
    In hindsight 60degree is probably too high. 50 it is.

    Thanks for your input guys. Much appreciated.
     
  6. David_AVD

    David_AVD Good news, everyone!

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    Have you tried to PWM the fan yet?
     
  7. OP
    OP
    kool-lites

    kool-lites Full Time Elf

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    Yep. Only works for duty cycle of 25% and above
     
  8. David_AVD

    David_AVD Good news, everyone!

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    Does the PWM make the fan whine much? What frequency did you use?
     
  9. OP
    OP
    kool-lites

    kool-lites Full Time Elf

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    No whine.~1MHz
     
  10. David_AVD

    David_AVD Good news, everyone!

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    Wow, that's a super high PWM frequency! I'm surprised the FET does it.
     
  11. OP
    OP
    kool-lites

    kool-lites Full Time Elf

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    I originally had the start duty cycle 20% and the motor whined.
     
  12. OP
    OP
    kool-lites

    kool-lites Full Time Elf

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    Ok I'll check when I get home. I am pretty sure I have clk/8 set with the system clock set to 9.6MHz.
     
  13. OP
    OP
    kool-lites

    kool-lites Full Time Elf

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    Following on from what David implied these shouldn't be working, I got a couple of beefier server fans out and tried them. And yes they do whine on start up.

    The PWM clock is 9.6/8 MHz.
    Lifting the start duty cycle to 50%, lifting the lower threshold to 35 and lowering the upper threshold to 50 degrees makes them turn on faster appears to be a good compromise.
     
  14. davrus

    davrus Silent Elf

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    Are you going to offer these boards for sale ?
     
  15. multicast

    multicast Senior Elf Generous Elf

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    Also, Do you really need such a chunky FET? That FET is super sized for this task?


    The energy required to drive that FET at 1Mhz, is quite high, and the associated losses will be high, To make it cooler, i'd drop the PWM frequency to 1kHz or so, theres no point in it being so high


    Whats the purpose of the 470R Resistor on the gate of the FET?
     

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