DC Fan control

Discussion in 'The Projects Forum' started by nathomas, Apr 22, 2011.

  1. nathomas

    Thread Starter Member

    Mar 3, 2011
    82
    0
    I have a 3W fan(12V) that I need to control using a potentiometer. I necessarily don't have to use the pot directly but the user has to turn a knob to control the fan speed. I looked at various pots with the ratings I need but they are too big and they will reduce the torque on the fan motor. Is there a very simple PWM circuit I could use with the pot that doesnt need a uC or high standard stuff?

    Another part of the question is that I need to vary an output of mine from 0V to 12V efficiently using a pot. what resistance value pot should I use? It will work as a voltage divider.

    Thanks.
     
  2. russpatterson

    Member

    Feb 1, 2010
    351
    16
    I. If you want to make a motor controller for your 3W 12V fan you can build a PWM circuit with a 555 timer. I haven't done that but I think you'd find a lot of examples via Google. You could also buy a motor controller if you don't want to build one.

    II. You need to make sure the pot is rated to handle the power that will be dissipated by the resistance in the pot (so it won't be efficient at all, it will just burn away the unwanted power as heat). How much current do you want to pull through your pot. With voltage and current you can get power and know the rating you need. P=VI where P is power in watts, V is voltage and I is current.
     
  3. electronis whiz

    Well-Known Member

    Jul 29, 2010
    519
    27
    It depends on the type of dc fan you want controled if it is just a brushed dc motor a pwm controler will probly work. however if your using a brushless (muffin fan) i think a pwm will probly not work at all. becase tecnicly these fans are like stepper moters. they are controled by a transistor and i beleve a hall efect sensor. the only way i can see to control them is to get fans with a builtin thermistor sensor. remove the thermistor solder a pair of wires in then run it out to a potentiometer. the potomiter will act fairly similar to a thermistor. highter temperiture= higher resistance= higher speed. this should be the same with your potomitor.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,123
    3,048
    Bad idea, stick to PWM. You CAN'T efficiently use a pot in a power circuit; the only thing it can do is drop voltage across itself and generate heat. That's not efficient.

    Also,a voltage divider works nicely when there is more (wasted) current flowing through the divider than through the load. Works great when you measure voltage with a multimeter. However a varying load will disrupt the voltage setting if the current through the load is more than, say, 10% of the divider current.

    It's better to use pots to control information, not power. In other words use the pot to control the PWM generator. This allows using small cheap and plentiful pots instead of expensive, power rated pots.
     
  5. RogerTango

    New Member

    Apr 22, 2011
    13
    0
    I have not done PWM with a 555, I use PicAxe chips myself, but you should be able to vary the duty cycle of the PWM with a pot, and using the output to pulse a cheap N channel MOSFET, maybe a TIP120 in the supply line to vary the speed of the fan.

    This is one of the projects I am working on, vary the speed of a DC motor. I have not executed this yet, but from the research I have done this is the work plan I will be using.

    HTH,
    Andrew
     
  6. nathomas

    Thread Starter Member

    Mar 3, 2011
    82
    0
    Well heres the tricky part. I want the potentiometer to turn only from start to stop and have a higher resolution. I got a 5W 5k pot and the problem is that the pot needs to turn only 10-20 deg before the fan switches on/off. Is there a way I could make the fan go from on to off throughout the whole rotation of the pot. I was thinking of using an MIC502. Would that help?
     
  7. russpatterson

    Member

    Feb 1, 2010
    351
    16
    What I usually do is hook the pot wiper to an analog input pin on the microcontroller. Then in code you do whatever you want to the pwm based on whatever input. The sky's the limit.
     
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