Flyback diode for air pump control

Hi there, I guess this one goes directly to DC42...

I have been experimenting with air pumps for cooling, and forgot to include a flyback Diode to one of them ... now the PWM FAN control doesn't work anymore, both Duet 0.8.5 PWM FAN outputs seem to run with more or less 100% power.
Is there anything I can do to repair the board? I have soldering skills and access to decent soldering station at work... I am hoping to be able to replace a component or even IC chips if necessary.

Thanks in advance!

If it's only the PWM fan that isn't working, you can probably fix it by replacing the fan mosfets. It's TR5 for the FAN0 mosfet and TR9 for the FAN1 mosfet. The part number is PMV40UN2. It's best to use a hot air desoldering station to remove the old ones. Take anti static precautions when fitting the new ones, because until they are connected, they are quite sensitive to static.

On the Duet 085 there is the possibility that the processor is damaged too. The Duet WiFi has additional protection for the processor in the event of a fan mosfet failing short circuit.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Ok, lets hope for the best...

What happens was, that the pump was working fine until I tried to reduce the PWM setting... well, exactly why there should be a flyback diode anyways... after that BOTH PWM outputs are dead (well, running close to 100% PWM actually). Any idea why both were damaged?

How can I check if the CPU is fried? It seems like DWC didn't notice anything wrong... even after a restart I was able to home the delta. Does it mean that I was lucky and CPU is working fine, seems to be a silly question, but I rather ask
Is there something else I should check?

Thanks for all your help!

Edited 1 time(s). Last edit at 02/27/2017 08:52AM by Teilchen.

You could solder a small load between MOSFet gate and GND to see if the CPU is still sending a PWM signal. Maybe a LED with series resistor? It would dim according the PWM duty cycle.

Well, actually with PWM set to zero both FANs are speeding quite fast, but if I set the PWM to 100%, I can hear the FANs going faster... so there is a change there, it is just that the zero point is somewhere at 90% or something...
Do I understand it correctly, that this part of CPU is sort of not important for normal operation? I mean if I have fried the PWM control inside the CPU I still can use Duet as 'normal'?

I'm puzzled that both PWM outputs have stopped working properly, but I still suggest that you try replacing both MOSFETs.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I don't know Duet schematics, but maybe you just killed a gate- resistor or something else outside the CPU.
Or the MOSFET is partially shot and with the right PWM at the gate it runs at 100%

Hm... if DC42 is puzzled then I have to start worrying .
Ok, my PWM FAN1 is set to Thermoplastic mode and is cooling the Hotend.
The PWM FAN 0 was used as a part cooling FAN, and this one was killed by me (airpump without Flyback diode).
Now both FANs run quite fast permanently, but if I change the PWM set point in DWC, I can hear that the Part cooling fan starts to spin faster. Does it make more sense now?

I have measured TR9 and TR5 Mosfets with a multimeter (FETs are still on the Duet)
TR9: Gate-Drain shows 18Ohm, Source-Drain has no continuity, Gate-Source also no continuity
TR5: Gate-Drain 150Ohm, Source-Darin - 130Ohm, Gate-Source - 30Ohm

So these two show different results, not sure what they should have though... one would be definitely damaged...
Is there something else I should check?

So I have exchanged both MOSFETs for a new ones. The FAN1 PWM (the one that was killed actually) works well again. However, the FAN0 PWM still runs at full power, with no control available. The corresponding LED is always on.
If I run M106 I1 or I0, I can see the FAN1 ist changing its state, but FAN 0 - does not.
Lucky enough I can leave the Hotend FAN always on, but I still would prefer to fix things properly...

Any help will be appreciated...

Assuming you didn't static-damage the new FAN0 mosfet when you fitted it, that suggests to me that the microcontroller pin driving it has failed.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Yes, I have measured a constant 3.3V at the MOSFET Gate input. I as I can see it has a direct connection to the corresponding ARM CPU pin...
Would you say it is safe to use the Duet board as it is? Or is there a chance that there is more damage?
Also, is there a way to remap the fan control to something else, like Hotend PWM, etc (I have a DueX4 as well)?

I will most probably order the CPU separately and resolder it using the re-work station at work... we have some guys doing this for a living, they have assured me this should be a now problem job for them...

Thanks for the support!

Yes it's safe to use as it is now that you have replaced the MOSFETs, and assuming you add a flyback diode before you connect a brushed motor to the fan output again.

I suggest you run M106 P0 with no additional parameters to make sure that the fan settings are correct, in case you have somehow set it to be thermostatic with a low temperature threshold or something like that.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

The CPU has been changed thanks to my colleague, and after reflashing it with Bossac my Duet ist working fine, and my printer is complete again.

Thanks for all the help!

I have been printing for a while after the CPU change, but the quality seems to be not so great as before... this might be a pacebo effect, or the effect of the changed cooling, but still...
David, is there anything special I have to do after changing the complete new processor? I cannot think on anything but to flash the correct FW version with Bossac? I went straight to 1.18 latest...
Is it possible, that the Temperture reading are somehow off now? The CPU temp is reading different, but that was expected...
Anything else I should think about?

There is nothing extra you need to do after changing the processor, other than flash firmware to it using SAM-BA and recalibrate the CPU temperature. The temperature readings from thermistors may change a little due to different offset error in the ADC, especially at higher thermistor temperatures.

Edited 1 time(s). Last edit at 03/13/2017 08:32AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks David, first print looked like the extrusion temperature would be way too hot.
How much deviation would you expect based on your experience? Are we talking about 1-5C, or 10-30C range?
I had to drop from 250C to 230C to get visually comparable results, does it make any sense?

No, such a large drop doesn't make sense to me. But I haven't swapped different Duet WiFi boards into my printers, and in any case I use a PT100 to measure hot end temperature, so I don't have any data.

You can check the temperature calibration by putting a fixed resistor of around 220 ohms in place of the thermistor, and seeing what temperature it reads, and how that compares with the expected reading for the S, B and C values configured in your M305 command. Then you can adjust the M305 L parameter if necessary to get the correct reading.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].