Duet 2: using heater7 for fan PWM

Marius Breuer

can i use pin 31 "HEATER7" of the Duet 2 Expansion header for the PWM input of a Noctua fan?
Normally the PWM signal requires 5V signal levels, however Noctua's whitepaper on the matter mentions an internal pullup to "3,3V/5V".
https://noctua.at/pub/media/wysiwyg/Noctua_PWM_specifications_white_paper.pdf
"HEATER7" is conveniently directly next to the PB6 rpm input and I have already used all 3 pwm fan connectors.
Using "HEATER7" for PWM control already works when using the Duet Expansion Breakout Board, but I'm unsure if that expansion board performs any levelshifting.

I'd be connecting directly to the expansion header and the expansion board would obstruct the PB6 rpm pin, so using that board is no option.

Thanks!

T3P3Tony

@marius-breuer The EBOB does level shift the heater signals and the buffer on the EBOB also provides some level of protection to the main processor on teh Duet from a faulty fan (although that is not guaranteed in all cases.) Note that the buffer/level shifter on the EBOB also inverts the logic of the PWM signal so working without it may require using a "!" infront of the pin name when the heater is created.

With all that said, if the fan is pulled up to 3.3V than it will probably work fine, if its pulled up to 5V then it will probably damage the micro controller on the Duet.

Marius Breuer

@t3p3tony then i can't risk plugging the fan PWM directly into the expansion header. The remaining 3 PWM fans (configured to run at 5V) are intended to drive small (5mm) LEDs with around 20mA at 1,2V per LED. (one LED per fan header).

An alternative seems to be, to drive these LEDs from the expansion header at 3,3V (with an adjusted resistor in series of course) to then have available PWM fan outputs for the Noctua fan.

Can the expansion header (or rather the Microcontroller) drive two heater pins (ie heater 3 and 4) with each pin drawing 20mA (40mA in total)?

I could also use the extruder heaters H0 and H1, although I'd need a larger resistor to step down the 24V to 1,2V.
At 20mA that's nearly 0,5W heat disspation per LED, not great, not terrible.

Are there other downsides to either method, and which would you pick?

T3P3Tony

@marius-breuer said in Duet 2: using heater7 for fan PWM:

Can the expansion header (or rather the Microcontroller) drive two heater pins (ie heater 3 and 4) with each pin drawing 20mA (40mA in total)?

no the maximum is 8mA on some pins and 4mA on others from memory - would need to check the datasheet to be sure.

The best plan is to use a small mosfet to switch either than fan or an LED. or if the LEDS signal things that are driven for another output (like heater on) then connect them to that output in parallel with appropriate resistors.

Marius Breuer

@t3p3tony The LED's are controlled individually and not connected to other outputs. Mounting options for a diy'ed PCB with mosfets is very limited, hence I'll probably use the 24V heater outputs with larger resistors in series to the LEDs. The Duet is mounted directly next to a larger fan, so cooling these resistors shouldn't be a problem. Thanks for your advice!