Fan speed fix

I found my fans would only kick in when I turned the fan speed above 245 or so. The reason this happens is a lot of brushless DC fans do not like being fed by a PWM (pulse width modulation) signal.

I fired my fans up from a smooth variable DC power-supply and found they did indeed change speeds nicely so this led me to a simple modification.

I connected/added a 47uF capacitor across the fan connections. It now adjusts speed nicely and runs very slow with a fan speed setting of 10 or so. If you do this modification make sure you connect the + and - of the capacitor to match the + and - on the board. The capacitor smoothes the PWM into a DC voltage, which most fans do like. If you find your fan starts at too lower value you can use a smaller capacitor e.g. 22 uF.

Disclaimer: Do this mod at your own risk!

This is how all of mine are wired and how I recommend wiring them. But one thing is be sure to put a current limiting resistor before the cap or you could damage the FET on the Ramps. I use a 1ohm 1/4 or 1/2 watt resistor.

Edit: It is called an RC filter. You could also use an Inductor instead of the resistor.

Edited 1 time(s). Last edit at 10/10/2013 11:36PM by Sublime.

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@Sublime That sounds like a good safe approach. Seeing as the fan drive is open drain and not push pull and the ESR of my capacitor is not that low, I think things will be fine. If the fet blows up I will let you know. I still havnt done my first print on this Tantillus. Hopefully next week when I get my glass bed. I really like the low inertia design you have come up with.

Also be sure to unplug the power or unplug your fan before you do this, I blew the FET on my Ulitmaker while soldering on a cap to the fan while the power was on. Oops.
Luckily they are cheap and it wasn't SMT so very easy to replace.

Just to clarify. is like this? I don't want to break my RAMPS since it is SMD.

        FAN
   (+)        (-)
    | +      - |
    |----|(----|
    |         [=] inductor
    |          |
   (+)        (-)
        D9

I would have the inductor on the positive.

                   FAN
              (+)        (-)
               | +      - |
               |----|(----|
    inductor  [=]         |
               |          |
              (+)        (-)
                   D9

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FFF Settings Calculator	Gcode post processors	Geometric Object Deposition Tool Blog
Tantillus.org	Mini Printable Lathe	How NOT to install a Pololu driver