temperature controlled dc fan

Discussion in 'General Electronics Chat' started by jimma, Apr 10, 2013.

  1. jimma

    Thread Starter New Member

    Apr 10, 2013
    6
    0
    http://www.circuitstoday.com/temperature-controlled-dc-fan

    the circuit in the aboce link is nice. could you pls explain it in more detail.

    1. Also kindly tell me at what temperature will the fan start working? thank you

    2. won't there be a back emf because of the motor..? how is it being removed? im sorry im a newbie

    thank you
     
  2. Tealc

    Member

    Jun 30, 2011
    140
    10
    1. Depends on where you set R5 (at a guess, I'm not super experienced myself)

    2. Not an issue because the fan isn't starting and stopping as with a PWM circuit. Also as far as I know DC fans have a diode for back EMF anyway.
     
  3. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    R1 is an NTC sensor, meaning Negative Temperature Coefficient resistor. As temp increases, R1's resistance decreases, supplying more base voltage to the transistor. As the base voltage reaches ~ .65V, the transistory turns on, which turns on the next tansistor, which turns on the fan. The temperature threshold is determined by R1 and R2.
     
    Last edited: Apr 10, 2013
  4. jimma

    Thread Starter New Member

    Apr 10, 2013
    6
    0
    thank you.can u please tell me how i can modify this circuit for a 6V fan or lower?
     
  5. #12

    Expert

    Nov 30, 2010
    16,337
    6,821
    Change R2 and R6 by the amount: New voltage/old voltage.
    1.5k times 6/12 = 750 ohms.
    680 x 6/12 = 340...use a 330 ohm resistor.
     
  6. jimma

    Thread Starter New Member

    Apr 10, 2013
    6
    0
    hi. i bought all the things but I could not find the fan with 12V , 700mA , and 15K thermistor and 470 ohm POT

    1. can i use a 12V , 300mA fan instead of 12V , 700mA fan ?
    2. can i use a 10K thermistor instead of a 15K thermistor?
    3. can i use a 500 ohm POT instead of 470 ohm POT?


    somebody please help me..please
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    Yes you can use the fan, also the pot wont make any difference as its variable, the 15K thermistor will work,

    the temperature is set by the pot , so its a matter of trial and error as to where the pot is set for your temp setting.
     
  8. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    The temperature is set by R1 and R2.
     
  9. jimma

    Thread Starter New Member

    Apr 10, 2013
    6
    0
    thank you.. hope it works..
     
  10. jimma

    Thread Starter New Member

    Apr 10, 2013
    6
    0
    cant i use three 10K thermistors? like 2 of them are parallel and that is connected to the third one in series. when the two are parallel it makes 5K ohm and that with the series will be 15K. cus (5k + 10K)
    any comments?
     
  11. #12

    Expert

    Nov 30, 2010
    16,337
    6,821
    You are going to fumble around trying to find the right resistor value anyway, so just try it and see.
     
Loading...