Bed Heater Output for Air Pump?

GoremanX

I'm hooking up a Berd-Air part cooling setup on my delta printer. Am I safe hooking up the air pump directly to the bed heater output on my Duet 3? The Berd-Air comes with a PWM relay to prevent a fan header from being damaged, but my bed heater output happens to be free anyways and I'd rather use that if I can rather than add yet another relay to my setup.

The air pump is rated at 1200ma@24v, which is well within the 18A limit for the bed heater output. I'm just not sure if I need to do anything special to prevent damage. Is this a brushed motor? I have no idea! The people who sold me the kit can't even tell me if the PWM relay they provided can handle a 24v signal...

edit: actually, if I'm not mistaken, I could connect the air pump directly to output 7, 8 or 9, right? Those can each handle up to 2.5 amps and have a built-in flyback diode to protect the headers from inductive loads? I happen to have 2 of those outputs free.

dc42

There are two potential issues with connecting brushed DC motors to output on the Duet:

Brushed DC motors are inductive, so a flyback diode is needed.
Brushed DC motors draw a much larger current when they start, or if they stall. This may exceed the peak rated current of the mosfet.

I recommend that you use out1, out2 or out3 on the Duet 3 if any of those are free. Those all have large mosfets (rated at more than 50A) and flyback diodes built in.

Outputs out4-out9 on the Duet 3 have a higher rated current (more than 11A) than on other Duets so they would probably be safe to use too, but I without knowing the stall current of the Berd Air, I can' t be sure.

GoremanX

@dc42 Now you have me very confused, because the documentation here:
https://duet3d.dozuki.com/Wiki/Duet_3_Mainboard_6HC_Wiring_Diagram
states that outputs 1 to 3 do not have flyback diodes built in and require an external one for inductive loads. Are you saying the documentation has it backwards?

hypnolobster

@dc42
I'm replying to this rather than make a new post, just so future searching is easier for some people.

I've got essentially the same issue as @GoremanX
I've got a Duet 3 and I'm not sure if it'd be better to connect it to out_1-3 or out_0.

The documentation does say that out_1-3 require external flyback diodes but you say here that they're built in. Is the latter true? Can I directly connect an inductive load to those outputs?

For that matter, are inductive loads safe on out0? It'd actually be easier in my case to run the pump off of out_0 because I won't have to move my outputs around.

GoremanX

@hypnolobster I ended up running an inductive load (magnetic relay) on out7, since the documentation specifically states that out7 through out9 have built-in flyback diodes. I'm hoping that the documentation is correct since @dc42 's statement here seems to contradict the documentation. I haven't had any issues so far. I ended up running different pumps on out8 and out9 that have brushless motors and only draw 0.5a each, so the issue became moot. I'm currently only running resistive loads on out0 through 3.

out0 may or may not have a flyback diode built-in, but it has its own replaceable fuse with a very high rating, so I'd feel safe running an inductive load on that output. But then I'm no electrical engineer, and would love to have definitive answers as to which outputs have flyback diodes built-in

dc42

To clarify: on the Duet 3 MB6HC and Duet 3 Mini, all outputs except OUT0 have flyback diodes build in. So a BerdAir or similar can be connected to OUT1, 2 or 3 on the 6HC, and OUT1 or OUT2 on the Mini. If you connect it to OUT0 then you must add a flyback diode.

GoremanX

@dc42 This is great news! However could the official documentation be changed to reflect that? Because right now, it explicitly states that out1 through out3 do not have flyback diodes built-in:

"2-pin JST VH OUT_1 thru OUT_3: these are intended for extruder heaters or fans. Maximum recommended current 6A each. If you connect inductive loads to these outputs, you must use external flyback diodes."
https://duet3d.dozuki.com/Wiki/Duet_3_Mainboard_6HC_Wiring_Diagram