Fan Indicator LED

Discussion in 'General Electronics Chat' started by magnet18, Feb 27, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    Hi, what I want is an indicator LED on a fan, wired so that when the fan is on max power, the LED is off, when the fan is off, the LED is on.
    Would the attached schematic work OK for this, assuming it's a 12V fan?
    Should I put in a back-flow protection diode?
    I want this to last a long time and I'm hesitant to just build it in without some input first, plus I don't really trust that it will work, considering most things I whip up in 5 min at 1am usually don't work...

    Thanks in advance,
    Your friendly neighborhood magnet man :)
  2. bertus


    Apr 5, 2008

    A 100K potmeter is very a very high value to control a fan.
    All current will go through the led and the fan will hadly move.

    Better make a simple PWM circuit for the sped control.
    SgtWookie made one a long time ago in this thread:

    The indicator led could be accross the drain and source of the fet.
    The more the fan gets , the less the leds gets and will go dimm or even off.

  3. nerdegutta


    Dec 15, 2009
  4. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    No, you're right
    I just stuck on the value I usually use for 9V:rolleyes:
    thanks :)
  5. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    I using the outputs off of a computer power supply, and I just thought of another question, will the fan/LED take too much current, or will there be any interference with whatever else I power?

    also, would the attached schematic work (also using the power supply outputs)?
  6. hgmjr


    Jan 28, 2005
    Controlling the speed of a fan or motor using a series resistance has some very real drawbacks.

    For instance, as the fan motor encounters a load, it inherently draws more current from the supply source. With a resistor in series with the fan motor the voltage across the resistor increases with this increase in fan motor current. The voltage dropped across the resistor steals voltage from the motor and the motor drive is reduced. Therefore just when you need more drive to the loaded fan motor you get less.

    It is for this reason that PWM is frequently used to control the speed of a motor.

  7. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    DC fans usually have an operating voltage range. For instance, a 12V fan may have a spec of 8V -14V (see the spec sheet for your particular fan). It will not operate well outside this range, and will most likely overheat.

    Check the power or amp rating on the fan. Let's say it is 12V @ 100mA, then the fan resistance is 120 ohms. If you were to place a 120 ohm resistor in series with the fan, the fan would see 6V @ 50mA and probably have trouble operating. Hopefully you can see from such an example, that using a 10K pot goes beyond what is realistic.

    As stated before, using a pot to control a motor is usually not very practical.

    However, rheostats/pots are used to control slot car and toy train motors.