What type of fan control circuit is this?

Discussion in 'The Projects Forum' started by GRNDPNDR, Feb 12, 2013.


    Thread Starter Well-Known Member

    Mar 1, 2012
    I don't know if this is the right section for this type of post, but this one is for those who truly know their electronics.

    This is the entire circuit I've been working with, reverse engineered into a diagram.

    All I would like to know is HOW this circuit works to control fan speed based on R4, which increases fan speed as resistance increases.

    I can kind of follow the path of electricity through the circuit, but could someone actually give a description of what it does, how it works, and if this is a specific circuit type.

  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    You will often find the TL431 in feedback circuits.

    On C9 you have the main DC output voltage. This is the one you want to regulate via R4.
    R4,R3 and R2 form a voltage divider that determines the voltage at the reference input of the TL431.
    The TL conducts if the reference voltage is higher than 2.5V.

    That means, if you increase R4 the reference voltage would decrease and the TL would cease to conduct.
    If it does not conduct the optocoupler U4 is off and there is no current into the FB pin of the VIPer22. This leads to an increase in PWM pulse width on the primary and consequently the DC voltage on the secondary.

    See the datasheet for the VIPer (page8)
  3. JMac3108

    Active Member

    Aug 16, 2010
    Its an offline switching power supply. It converts AC mains power into a DC voltage to run the fan. R4 is in the PS feedback circuit which is used to set the DC output voltage of the power supply. Adjusting R4 changes the DC voltage to the fan and therefore its speed.