6HC 12V fans on OUT_X partially engage ground switched fans

sebkritikel

Was helping another Duet user with some issues they were having, and confirmed the behavior. I was running 3.6.0-rc.1+1 , other user was on some flavor of 3.6.0 Beta. In my case, given the following:

1.02 6HC powered by 48V
OUT 7-9 voltage jumper set to 12V
12V fans connected to out 7-9
OUT 4-6 voltage jumper set to 12V
24V fan ground connected to OUT_4 out4
24V fan +24V to 24V power supply
48V power supply, 24V power supply grounds tied together

In this case, commanding the 12V fans to spin will cause the 24V fan to spin some.

Set all three 12V fans at 0%, 24V sees ~.25V, does not spin
12V #1 at 100%, 24V fan sees ~8V and spins
Add second 12V fan at 100%, 24V fan sees ~10.5V and spins faster
Add third 12V fan at 100%, 24V fan sees ~11V and spins faster.
Set all three 12V fans at 0%, 24V sees ~.25V and stops spinning

Changing the OUT 4-6 jumper to VIN:

Set all three 12V fans at 0%, 24V sees 0V, does not spin
12V #1 at 100%, 24V sees 0V, does not spin
Add second 12V fan at 100%, 24V sees 0V, does not spin
Add third 12V fan at 100%, 24V sees 0V, does not spin
Set all three 12V fans at 0%, 24V sees 0V, does not spin

If this is a hardware limitation, I recommend documenting this, as the general understanding when it comes to Duet products is that you can used mixed-voltage components, switching them on the ground side. In the testing above, the solution is to use a dedicated OUT bank, setting it to VIN, however that may not be an option for some users.

droftarts

@sebkritikel I think I've seen the same thing once before, on a 6XD, and yes, it's a hardware limitation. See https://forum.duet3d.com/post/345691

I believe the 6HC is wired the same. It's because you're feeding 24V into OUT_4_NEG, but other fans are jumpered to 12V. I'll double check and update the docs. Thanks!

Ian

dc42

@sebkritikel this is expected, because of the flyback diodes connected in parallel with the fan output pins.

If you want to connect a fan or other load between the OUT- pin of an output connector and a positive supply other than the positive supply pin of the OUT connector, the voltage you supply should be lower or the same as the voltage supplied on the OUT connector (which on most Duet 3 boards is selected by the VOUT_LC jumper).

For example, if the VOUT_LC jumper that supplies the OUT connector is set to VIN and VIN is 24V then it is OK to connect a fan between +12V and OUT-. But if VOUT_LC is set to 12V then you should not connect a fan between +24V and OUT-

sebkritikel

@droftarts and @dc42 , thanks for explaining that!

Looks like in the "connecting fan documentation" there is a similar note (as what was explained to me above) for the Duet 2.. Perhaps indicating that is also applicable to (at least) the 6HC and 6XD would be sufficient.