Working with 24v but having a noctua as heatsink fan in 12v

  • Hi,

    Currently thinking of using a 40mm noctua fan in 12v as the heatsink fan, but everything works at 24v. I know from the documentation that light is linked to heatsink fan port on the smart effector but I'm not sure about what is used in wiring and for what.

    I mean Is it possible to put a DCDC stepdown at the duet side but exclusively for the heatsink fan port on the smart effector (so I can keep 24v for what's left on the smart effector) ?

    Side note : noctua fan draw max 0.05Amp @12v, maybe just a resistor in serie would do the job since it's 0.6W, like (24-12)/0.05=240ohm ? My question here is for practical solution not state of the art of course 🙂 )

  • administrators

    The Schematic for the smart effector is here:

    what this shows:
    is that the input on the smart effector has separate pins for the hotend fan and that the circuit for the hotend fan also powers the LEDs. So if you supply 12V for the hotend fan into the wiring for the smart effector at the duet side then that should work. You will need to setup the LEDs with the jumper as if they were 12V.

  • @t3p3tony Thanks for your quick answer, so basically :

    • #1 : I keep what's wired on my duet, except I unplug the wire I use for my heatsink fan +
    • #2 : I put a DCDC stepdown (24->12) between V_FAN pin (I'll pick the one I used for the heatsink fan on FAN1) and the wire ending on the 2 x 3 pin black Molex on HF+pin
    • #3 : I'll set the jumper on the smarteffector to go back to 12v mode for leds.

    2nd choice, due to the low max current involved with the fan (0.05 @12v max) , 240ohm resistor in serie in the fan wire may do the trick?

  • The way the Duet electronics is setup, any PWM switching (on the controllable fans) is handled on the Ground side of the fan. This allows you to supply the power of your choosing (granted that it shares the same ground) to the fan. For non-controlled fans, you can do the same or even have a separate board (like strip board with headers), to which you supply the different voltage.

    On my setup, I have 24V Vin, but I only have 1 fan that uses 24V (extruder heatsink fan, so since it is an always on, it gets its 24V more directly from the power supply) - the rest are 12V fans as well as a 12V LED strip, these are run from a DC-DC Buck converter (input directly from the PSU), set to 12V and have this power fed into the Fan Voltage pin near the fan outputs - this ensures all 12V fans get their 12V and I have full control over them all.

    A Buck converter is much more efficient at reducing voltage, compared to a regular regulator IC and cost only a few bucks (on Amazon I have seen a 8x converter kit for ~$12).

    As to your suggestion of the resistor: it does not work like that. A fan is an inductive load, and not a resitive load: for a resistive load the Ohm's Law is perfect, but the inductive load works a lot different. Very basic: while the fan is spinning up, or below its ideal speed, it draws a lot of current (usually the rated amps), and as it is reaching its ideal speed, this current drops. This is where you will run into issues: the current is no longer enough to keep the voltage over the fan within its limits, and it blows. Also, don't add 2x fans in series, same issue will happen.

    Running a single fan from an LM7812 (12V IC voltage regulator) is fine (though give it a heatsink),

    I have previously tried a few other methods, including a reverse bias Zener diode (of sufficiently rated amps and voltage) - while it worked initially, the diode got extremely hot. Another option, not ideal, would be to build a simple voltage regulator (similar idea to what you have, except that you need a 12V zener diode reverse biased parallel to your fan):
    Basically it will "short" the excess voltage to ground, through your resistor; this will keep the current through the resistor enough to create a large enough voltage drop to have stable volts.

    Just an extra note, on newer boards there is a 1A fuse on the fan outputs, so just remain below that. Older boards does not have this protection, so remain well below that.

    My recommendation is simply the DC-DC Buck converter.

  • administrators

    @hergonoway said in Working with 24v but having a noctua as heatsink fan in 12v:

    #2 : I put a DCDC stepdown (24->12) between V_FAN pin (I'll pick the one I used for the heatsink fan on FAN1) and the wire ending on the 2 x 3 pin black Molex on HF+pin

    Yes, but you don't need to use the V_FAN pin out for that fan ig you don't want to. power the DC-DC converter from the Duet 24V supply or an always on FAN pin if that is more convenient, just ensure the ground is common.

    0.05A @12V = 240R so, yes, in theory, if theon board controller on the fan was happy approximating a simple resistance this should work. I have not tried this with fans with on board controllers however they may not like it.

  • @jacotheron & @t3p3tony thanks for your replies, I'll keep up with a DCDC step down at the Duet side then, I've got a bunch of these littles guys :


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