Controlling inexpensive BLDC for cpap part cooling - Duet 3

  • Hello,

    I've got a cheap eBay BLDC and a brushless CPAP blower that I'm planning on using to experiment with part cooling. Everything works as intended standalone (no hall, no direction/brake, just the onboard pot), but I wanted to get a sanity check before I wire it up as I don't want to blow up a board due to inexperience.

    I'm using a Duet 3, and the BLDC looks like it wants a 5v PWM/duty cycle control and presumably just otherwise the GND? It looks like the Duet 3 will only do 3v output on the I/O 0-8 pins and I assume the 4 pin OUT4-6 connections do 5v pwm? I've struggled with fan and i/o wiring on the Duet 3 because the documentation on the wiki between description and the actual image is inconsistent (for someone at my level of understanding, anyway).

    My main confusion stems from probably not fully understanding PWM. I have two wire fans connected to the duet at out4 and out5 between V_outlc1 and out4/5, and that was the only configuration that worked for me. I'm assuming that must mean that the pwm signal is switching a ground at the out pin and not +v? I don't know what exactly the PWM in on my BLDC is expecting.

    edit: Oh hey! I completely missed that there is a 5v pwm connector on the Duet 3. That uh.. simplifies things. As I'm already giving power to the BLDC, I should just need out9 and GND wired to PWM/GND, I believe?

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  • Follow up, assuming I at least mostly answered my own question with the edit above,

    It looks like the 5v PWM on the duet 3 should be just out9 and I can change my M950 to

    M106 P1 C"partcooling" S0 H-1
    M950 F2 C"out9" Q20000 

    It looks like input frequency for the inexpensive BLDC is pretty inconsequential as far as I can tell.

    At least based on my experimenting with the controller in standalone, I definitely do not need anywhere near 100% fan, or even 50%. Does maximum speed set with M106 (in config.g) actually cap the fan setting at a percentage (in DWC, or a fan setting from running a gcode job) or does it allow a more granular control between 0-100% underneath that max setting?

  • @hypnolobster Based on my experience with CPAP blowers, those things are like leaf blowers when you crank them up full blast- it would probably blow the print right off the bed, and drop the bed temperature while it's doing it! You won't need to be anywhere near 50% of the thing's maximum output for print cooling, maybe 10-20% tops.

    A couple years ago I copied a CPAP blower housing and impeller and used a HDD motor to run it. It worked well. The HDD motor is a bit underpowered compared to the CPAP blower motor, but again, you don't need that much flow/pressure to cool prints. See:

  • Hah! The man himself. Your blog is actually what led me to messing around with this in the first place. I had some RC brushless motors around and I was going to work on a printed blower and then got lucky and found an actual CPAP blower very cheap.
    You're responsible for my belt drive z axis with a Rino stepper as well, in fact.

  • Some people have been using my printable blower as a basis for an open source ventilator. They used an RC airplane motor and spun the impeller so fast it broke apart. Last I saw they were going to a two stage design to get more pressure.

  • administrators

    @hypnolobster said in Controlling inexpensive BLDC for cpap part cooling - Duet 3:

    Follow up, assuming I at least mostly answered my own question with the edit above,
    It looks like the 5v PWM on the duet 3 should be just out9...

    Specifically, Duet 3 MB6HC rev 1.0 and later have a separate 5V output labelled LASER/VFD which is controlled by the same pin as OUT9. So use that connector for your ESC, not the OUT9 connector.

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