Duet-Wifi Fans won't turn off
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The fan outputs on the Duet do not include a flyback diode because the usual brushless fan motors do not need one. However a brushed DC motor does.
Fortunately the fan MOSFETs are not difficult to replace, and on the Duet Wifi we added an extra resistor to protect the processor if the fan mosfet fails.
I will add a note to the documentation about adding a flyback diode if a brushed DC motor is connected.
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Thanks dc42 - so I should be OK to use the motor via the fan PWM outputs as long as I have a sufficiently rated flyback diode between the leads of the motor?
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Thanks dc42 - so I should be OK to use the motor via the fan PWM outputs as long as I have a sufficiently rated flyback diode between the leads of the motor?
The running current of 1A is well within the fan mosfet ratimg, but the startup current might be too high. Best measure the resistance with a multimeter, calculate the startup current, and compare it with the current rating on the mosfet data sheet.
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4.5 ohms, measured through the motor at rest. Not finding a calculation for startup current, though.
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Unless you're able to "zero" your meter before measuring such a low value, figure about a quarter to half an ohms of resistance is in the probe leads themselves, so the motor is likely closer to about 4-4.25 ohms.
Using Ohms law Volts = Current * Resistance, we can solve for current.
So 12V = I (current) * R 4.25 ohms
12V/4.25ohm = 2.82A
That's about 2.8 to maybe 3 amps of in-rush current depending on your probes. That's obviously not the most accurate way of calculating inrush current, but it should be pretty ballpark.
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As an aside, can I point out that this should serve as a warning to us all?
When the MOSFET failed, it failed shorted, leaving the attached device running at full. For a fan, this is annoying, but for a heater a shorted MOSFET could start a fire. The board would happily detect an overtemperature condition, but it would respond only by commanding the MOSFET to shut off, which would do nothing. If the setup used PS_ON, and if the firmware triggered an emergency stop on overtemperature, the fire would be averted.
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I would make the argument that heaters capable of auto igniting a fire shouldn't be used without direct full supervision, but that's just me.
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Unless you're able to "zero" your meter before measuring such a low value, figure about a quarter to half an ohms of resistance is in the probe leads themselves, so the motor is likely closer to about 4-4.25 ohms.
Using Ohms law Volts = Current * Resistance, we can solve for current.
So 12V = I (current) * R 4.25 ohms
12V/4.25ohm = 2.82A
That's about 2.8 to maybe 3 amps of in-rush current depending on your probes. That's obviously not the most accurate way of calculating inrush current, but it should be pretty ballpark.
The fan mosfet is rated at 4.4A for up to 5 seconds at 25C ambient temperature with 4.5V gate drive. The rating will be a little lower at 3.3V gate drive or if the board is warm, but probably still sufficient. If you want to reduce the inrush current, you could connect an inrush current limiting thermistor such as B57153S0200M0 in series with the pump.
You definitely need to use a flyback diode when driving a brushed motor.
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I've ordered a pwm/mosfet board to drive the motor, so that'll keep it off the duet. I'll look into getting the mosfet on my duet replaced, and for now, just use the next fan channel.
Is there a way to make m106 default to fan1? Or do I have to force that in gcode?
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Currently there isn't a gcode to map fan channels. I am considering adding fan mapping on a tool-by-tool basis in a future firmware release.
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Alright. I've scripted it in my slicer to control P1, so no biggie.
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Fan mapping will be supported in 1.16 beta 11 using a new F parameter on the M563 command.
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Fan mapping will be supported in 1.16 beta 11 using a new F parameter on the M563 command.
Excellent, thanks.
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Alright. I've scripted it in my slicer to control P1, so no biggie.
Can I ask which slicer you used for this? I can't figure out how to make slic3r's fan control control anything but the first fan.
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Simplify3d has the ability to run post processing scripts on your gcode.
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So I got this: https://amzn.com/B01J78FX9S
Thinking I could connect it to 12v in, and run the 12v out to the motor, and connect the fan control to the signal / ground on it.
Unfortunately, it's always on when connected that way. Anyone know if I can use a PWM controller like this to accomplish what I'm trying to do?
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Alright. I've scripted it in my slicer to control P1, so no biggie.
Can I ask which slicer you used for this? I can't figure out how to make slic3r's fan control control anything but the first fan.
You can run scripts from Slic3R but that's a bit complex for this task. What I would do is slice as normal then just use a simple search and replace using notepad++ to change the P0 commands to P1.
HTH
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In s3d the "script" is:
{REPLACE "M106" "M106 P1"}
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In s3d the "script" is:
{REPLACE "M106" "M106 P1"}
In S3D, if you go to Tools>Firmware Configuration>FFF you will be given some fields to alter what the fan on and off commands are. Presumably, you could add the P1 thing there, but I've never tried it.
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Ah, nice, that's even easier. Thanks!
