4 pin (PWM) to 3 pin (DC) fan adapter

Discussion in 'The Projects Forum' started by obones, Jan 23, 2008.

  1. obones

    Thread Starter New Member

    Jan 23, 2008
    1
    0
    Hi all,

    I have a PC here with a motherboard for which the CPU fan port has 4 pins, which assignments are as follow:
    +12V
    GND
    Tach (speed sense)
    PWM (speed control)

    However, the fan that I'm using only has 3 wires and as a result cannot react to the PWM signal sent by the motherboard to regulate the speed according to temperature.
    So my project here would be to create a very simple circuit to take the PWM signal and regulate the +12V, with a minimum voltage applied to the fan in case the PWM time off is too high.
    Note that I have found a product that claims to do this "out of the box", but I'd really like to be able to do such a circuit myself, as I find it a nice way to learn the basics here. The product datasheet is visible here:
    http://www.paqt.co.uk/docs/PaQ_PWM.pdf
    Looking around, I found this post in the forum:
    http://forum.allaboutcircuits.com/showthread.php?t=6266
    The TI application note attached to it helped me figure out that the PWM signal is of the form +12V = On and GND = Off
    So my initial thought was to simply control a MOSFET with the PWM signal, with protection on the gate, obviously. Something like that:

    Code ( (Unknown Language)):
    1.  
    2.                   +12
    3.                    o
    4.                    |
    5.                   .-.    +12
    6.                   | |     o
    7.                   | |     |
    8.                   '-'  ||-+
    9.                    |   ||->
    10.                    +---||-+
    11.                    |      |
    12.        ___       |/       +---o To Fan +
    13. PWM o-|___|--+---|        |
    14.              |   |>      ---
    15.             .-.    |     ---
    16.             | |    o      |
    17.             | |   GND     o
    18.             '-'          GND
    19.              |
    20.              o
    21.             GND
    But this does not give me any control on the minimum voltage applied to the fan. So my next idea is to use two bipolar transistors with a minimal gain on the first one. Something like this:

    Code ( (Unknown Language)):
    1.  
    2.                     +12
    3.                      o
    4.                      |
    5.                     .-.
    6.                 .-->| |
    7.                 |   | |
    8.                 |   '-'
    9.                 |    |
    10.                 '----+                +12
    11.                      |                 o
    12.                     .-.                |
    13.                     | |              |<
    14.                     | |         .----|
    15.                     '-'         |    |\
    16.        ___           |        |/       |
    17. PWM o-|___|---->|----+--------|        +--o To Fan +
    18.                      |        |>       |
    19.                     .-.         |     ---
    20.                     | |         o     ---
    21.                     | |        GND     |
    22.                     '-'                o
    23.                      |                GND
    24.                      o
    25.                     GND
    What do guys think of these ideas? What values could you suggest for the resistors and capacitors?

    Thanks in advance for your help

    Edit: Apparently, I was wrong assuming that the PWM is +12V as I found the following document :
    http://www.formfactors.org/developer\specs\REV1_2_Public.pdf
    It is specified at +5V, a TTL level then.
    But I don't believe it changes my proposed designs
     
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