Discussion in 'General Electronics Chat' started by mxcud, Apr 2, 2010.

  1. mxcud

    Thread Starter New Member

    Jun 30, 2009
    I have recently modified a fan controller circuit to include an LED to tell me when the fan is on. I have also modified the circuit to add extra Lanes to allow more fans connected.

    A fan controller adjusts the speed of a fan.
    A fan controller is able to adjust the speed of a fan simply by changing the voltage to the fan.

    The input voltage is 12.2v.
    Each fan's operating voltage ranges from 2.8v to 12v and draws 0.23A.
    The input voltage is connected in parallel to a voltage regulator(LM317), this is so that each lane will recieve 12v.
    The voltage regulator controls the voltage to the fan.
    The voltage regulator is able to vary the voltage by adjusting the value on the 10k Potentiometer.
    The voltage regulator allows the current range from 1.25v to 11v.

    The LED is connected in the circuit at the voltage divider(at R3 and R2), the resistors divide the voltages at R3(300ohm)=2v and R2(1500ohm)=10.2v.
    The LED is only there to tell me when the switch to the fan is on.

    The original circuit is taken from:

    My modified version is:

    I have run simulations in National Instruments Multisim and everything went smoothly.
    I have just one concern, the Amps draw cannot exceed 25A.

    I am wondering if I have done anything wrong.

    Last edited: Apr 2, 2010
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Your image did not make it into the post.

    You can attach your schematic here using the forums features. You don't need to use a second party storage location.

  3. SgtWookie


    Jul 17, 2007
    The LM317 has a minimum dropout of 1.7v.

    That means if your input is 12.2v, the most you'll see at the output of the regulator is 12.2v-1.7v=10.5v.

    Many computer-type fans won't start running unless they have around 7v across their terminals. They might KEEP running on lower voltage if they were started at >7v.

    So, this gives you a usable adjustment range of about 3.5v.

    The 300 Ohm resistors (eg:R3, R5, etc) in parallel with the LEDs serve no useful purpose.

    The 1.5k resistors (eg:R2, R4, etc) used for LED current limiting will do just that - if you're using red LEDs with a Vf of 2.2v, they'll get about 6.8mA current through them (12.2-2.2)/1.5k=6.8mA. You'll be able to see them just fine. Did you want them to be brighter?

    Why don't you build one and see how close I am to what you observe?
    Last edited: Apr 2, 2010
  4. mxcud

    Thread Starter New Member

    Jun 30, 2009
    I just connected one of the fans to 2xAA rechargeable batteries(in series). The fan is able to run at a constant speed and to blow a light breeze.