speed control for a 12 volt fan

Discussion in 'General Electronics Chat' started by Gompers, Aug 9, 2007.

  1. Gompers

    Thread Starter Member

    Feb 21, 2007
    18
    0
    Can anyone tell me if those small 12vdc ventilation fans can be slowed down

    I added some resisters to drop the voltage, but so far no appreciable reduction of speed. Ed from Ct.
     
  2. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    They should be able to be controlled by a PWM or varying the input voltage. How much did you vary the voltage by?
     
  3. Gompers

    Thread Starter Member

    Feb 21, 2007
    18
    0
    Used a 9 vdc supply and added 7 ohm resister to the circuit. I intend to use a manual three position rotary switch to vary the speed in increments

    The motor itself will be used to wind thread using various sizes of spools The blades have been removed and replaced with an applied spindle. Thanx, Ed.
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If the motor is a DC brushed motor, addition of a resistor is going to cause a problem.

    The current drawn by a DC brushed motor increases and decreases with variations in the load. That means that when you need the motor to deliver torque to the load, the increase loading will cause the motor to draw more current and this added current will result in a greater voltage drop across the series resistor and drop the voltage driving the motor causing it to easily stall.

    You have a few choices. You can use gears to mechanically reduce the speed, you can lower the voltage supplied to the motor, or you can use the circuitry that controls the speed using "Pulse-width modulation" (PWM).

    The series resistor will only cause you grief. This is the voice of experience speaking.

    hgmjr
     
  6. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    i wud suggest gears.
    why not make a mechanical switch (double entendre :D)?
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    The advantage of gears is that as you gear the speed down you get a corresponding increase in torque.

    WIN-WIN.

    hgmjr
     
  8. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Just use an LM317, adjustable regulator..... I do that on my dimmers, using a thermistor instead of a pot/pre-set....Daniel.
     
  9. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Good suggestion, lightingman.

    Edward Schwalenberg,

    Don't forget to include a healthy heatsink. I suspect there will be a good deal of power dissipated by the LM317 under certain conditions of voltage and load.


    hgmjr
     
  10. spar59

    Active Member

    Aug 4, 2007
    51
    0
    You could try dropping the voltage using a zener diode or series string of normal diodes - you could vary the number of diodes using a switch. The output voltage will remain fairly constant at your chosen level irrespective of load with this arrangement.

    If this is a small computer type ventilation fan they often only draw around 100mA so your 7R resistor would only drop 0.7V hence you wouldn't notice much of a speed change.

    Steve.
     
  11. Gompers

    Thread Starter Member

    Feb 21, 2007
    18
    0
    Spar, thanx for the reply. I used a 4vdc supply and 6,7,and 8 ohm resisters connected through a three position rotary switch Gadget wirks fine Thanks again I'm still learning how to navigate this forum Ed from Ct.
     
  12. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Yes..I use the LM317 to supply 2 x 100mm, 12 volt fans (in parallel)..... The LM317 is just bolted to a copper pad as part of the PCB, but it is placed in the path of 1 of the fans....When the unit is on, the fan is pre-set to run nice and slow, as the temprature increases the fans speed up....This has run for about 8 years now with no prob's...Good luck...Daniel.
     
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