Run airpump from bed heater output

Exerqtor

So now that I've finaly taken the jump over on RRF3, i thought it was about time to finally swap over to berd-air on my CoreXY, and my thought was to avoid potentially letting the magic smoke out of the fan mosfet.

My printer is 12V but my bed is 24v, so i already run the bed via it's own big external mosfet. The external mostfet's signal input is for the time being hooked up straight to the Duets(2 Wifi)'s "bedheat". And the part cooling (5015 12v) hooked up to "fan0".

Now going over to berd-air my thought was to both free up a fan port or at least not blow one with the way larger motor on the airpump. So my thoughts is to run the airpump from "bedheat", and then hook the signal inputs for the external mosfet up to one of the expansionheader heater outputs. Let's say "exp.heater4" just for reference.

Since i'm all green on RRF3 and the use of the "M950" i wanted to make sure i get my shit straight before i deploy the new config.g.

My config.g "proposal" looks the following:

;  Heaters
M950 H0 C"exp.heater4" T0        ; create bed heater output on exp.heater4 and map it to sensor 0
M307 H0 B0 S1.00                 ; disable bang-bang mode for the bed heater and set PWM limit
M140 H0                          ; map heated bed to heater 0
M143 H0 S120                     ; set temperature limit for heater 0 to 120C

; Fans
M950 F0 C"bedheat" Q100          ; create fan 0 on bedheat and set its frequency
M106 P0 S0 H-1 C"Part Cooling"   ; set fan 0 value. Thermostatic control is turned off

And when it comes to wiring it's pretty much just to hook up the +/- from the airpump to the "bedheat" +/- screw terminals, and the external mosfets signal + to "exp.heater4" and the mosfet signal - to some GND pin? Or did i get that wiring back asswards in reguard to what should be +/- ?

(Didn't quite know what category this fits best under lol)

dc42

The exp.heaterN signals on the expansion connector are active low with 3.3V signal level. If your external mosfet has an optically isolated input and will accept a 3.3v signal, then you can connect its input terminals between +3.3V and the exp.heaterN output.
The bed heater output is not protected by a flyback diode, therefore when connecting a brushed DC motor such as BerdAir, you must add an external flyback diode in parallel with the motor, with the correct polarity.

HTH David

Exerqtor

@dc42
Ahaa, well then i have to rethink this some more, i believe the mosfet need 5-24v on the control side.

Alternatively i can reasign the bed to e1heat, and run the airpump on the bedheat (with diodes), and throw my led lights on fan0.