Solved Homemade PWM smoothing harmful for the hardware?
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Hi,
I have a T3DP3D Kossel XL+ and use two 50x50x15 radial fans for part cooling and a 40 mm axial fan for hotend cooling.
With this setup I have an amazing part cooling power and I would like to reduce the fan speed to have a more quiet setup. But unfortunately due to the PWM noise the fans do not really become much quieter if I slow them down.
Hence I designed a small piece of circuit which I want to install in the wire going to the fans to smooth out the PWM signal.
My plan is to build this circuit (the resistance is a fan) (Sorry for the long link, it contains the full circuit https://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000001+10.20027730826997+50+5+43 R+96+160+48+160+0+2+10000+24+0+0+0.99 d+176+384+96+384+2+default d+176+384+176+160+2+default w+176+160+288+160+0 209+288+160+288+384+0+0.00009999999999999999+0.001+1 w+288+160+400+160+0 r+400+160+400+384+0+342.86 l+288+384+176+384+0+0.01+0 g+96+384+48+384+0 w+288+384+400+384+0 d+96+160+176+160+2+default o+6+64+0+4098+80+0.1+0+2+6+3 o+7+64+0+20481+5+0.4+1+2+7+3 o+0+64+0+20737+40+0.4+2+2+0+3 ) on a small board, solder it into the middle of the wire connecting the fans and - to avoid too heavy EMI with the smart effector, - fold a thin sheet (I think 0.5 mm could be foldable without a machine) of galvanized steel around it and connect both sides of it by soldering them together so that only the ends with the wires are still open.
What is more I have to set the PWM frequency for the fans to 10 kHz in order to let the 10 mH inductor work properly.
As you can see in the link above, I build in two diode at the power source and the ground to avoid voltage peaks (like the >40 V when switching the fan from zero or low power to full power) reaching the smart effector and the Duet board. My capacitor is rated 50 V and the diode 40 V. I think the fan should also have no problem with a millisecond peak of high voltage and current.After all this description follows my actual question . Can I harm my Duet3D hardware with this circuit? And has maybe somebody with a better understanding of electronics (I needed some help to get explained which type of diodes (Schottky) and capacitor (electrolytic) I need) some idea of a possible mistake I have done in this circuit which will make it not working, destroying the fan, the printer, burning or something else? I would really like to hear your opinion before I test this circuit on my printer.
P.S. I worked some evenings on the burning aspect because with 500 Hz I would need a 100 mH inductor and they have a really low current rating before they heat up. The 10 mH inductor I have ordered now has a current rating of 90 mA and the equilibrium mA I have at 100% fan speed are only 66 mA. So I think the heat development should be ok(?).
Thank you for reading this long post
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A smoothing circuit like that is good and will not harm the Duet, with the following provisos:
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You don't need the first two diodes on the left hand side (the ones in series with the Duet fan outputs).
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You only need the third diode (the flyback diode) if you are connecting it to a fan output on a Duet WiFi/Ethernet with PCB revision 1.0 or 1.01 or a Duet 06 or 085, or to a heater output on any Duet. Duet WiFi/Ethernet 1.02 and later, and all Duet Maestros, have built-in flyback diodes on the fan outputs.
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You can increase the PWM frequency to 50kHz, then you can use a smaller inductor e.g. 2 or 3mH.
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If you are concerned about EMI, locate the circuit close to the Duet so that the wires to the fan carry the smoothed PWM.
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Thank you for the info.
With 50 kHz an 1 mH inductor is also working in the online tool. Since I can get this with a 130 mA rating I will try it to avoid too much up-heating over time.
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Oh, I just found one problem. If I leave out the two extra diodes in the simulation tool I get a huge oscillation into the circuit which is not damped out even after 100 ms and has amplitudes of up to 40 V and 6 A. Current is running backwards through the fan outputs of the Duet.
Is that not a very bad thing?
EDIT: Reseted the circuit in the link
(The changed circuit: https://www.falstad.com/circuit/circuitjs.html?cct=$+1+1.0000000000000001e-7+25.510281670702206+38+5+43 R+176+160+48+160+0+2+50000+24+0+0+0.99 d+176+384+176+160+2+1N5711 w+176+160+288+160+0 209+288+160+288+384+0+0.00009999999999999999+0.001+1 w+288+160+400+160+0 r+400+160+400+384+0+342.86 l+288+384+176+384+0+0.001+0 g+176+384+48+384+0 w+288+384+400+384+0 o+5+64+0+4098+80+0.1+0+2+5+3 o+6+64+0+20481+5+6.4+1+2+6+3 o+0+64+0+20737+40+25.6+2+2+0+3 ) -
The circuit you are simulating needs to include the mosfet, which is effectively a switch. That's very different from applying a square wave from a low-impedance source.