Duet 3 6HC Out3 Loud Buzzing

TLAS

I've encountered an odd issue - my newest Duet board has a loud buzzing when connected to a fan. The buzzing is about 2X the loudness of the fan itself at ~20% PWM.

Anyone know if this is an indicator of a faulty board? Other than the buzzing, it seems to be working just fine.

moth4017

not sure what fan you have but try changing the PWM frequency
e.g M950 F3 C"fan3" Q20000 B2

TLAS

@moth4017
Yep that was my first thought too. Changing it changes the pitch of the noise but not the intensity / loudness.

moth4017

@TLAS what type of fan? and does it run correctly from 30% to 100%?

TLAS

@moth4017 it’s a somewhat larger (200mm?) fan with high airflow. At 20% or so it does a burst sequence (on 0.5 seconds, off 5 seconds) about three times before arriving at a constant speed.

In the fan side everything seems to work perfectly fine. The issue is the buzzing that appears to be coming from the motherboard.

moth4017

@TLAS i run a 120mm fan 1.6A but i use the heated bed output to control it and had no issues can you use that output ?

TLAS

@moth4017 i’ll give that output a shot next. I’ll
Be down an output for another fan but it shouldn’t matter in the short term.

droftarts

@TLAS

it’s a somewhat larger (200mm?) fan with high airflow.

That sounds like a big fan. What current does it draw, and what voltage does it run on? Fans connected to the fan headers are current limited by the fan header and voltage select connectors to 2A, ie each bank of fans is limited to 2A total for the 3 fans connected.

Ian

TLAS

@droftarts Exactly why I had it connected to one of the heater output lines.

TLAS

@moth4017
So I tried on the bed heater OUT0 line - and I got the same response with buzzing!

As I was poking around thinking the buzzing was a weird wiring short, I was able to change the pitch when I touched the power inlet ferrite inductor beads. I removed them and the problem was solved.

But WHY????

Ferrite inductors should smooth the voltage at the power inlet, although also probably changing the impedance. Anyone have an idea why that would cause buzzing from the board (mosfets?)?