How did I break the 5V rail?

Beun · 2 Mar 2019, 16:43

When I was installing the Z axis on my printer (a work in progress) I noticed flickering and horizontals moving bars (like filming a CRT monitor) on my LCD. I lost internet connectivity and after a reboot the board seemed unable to load the printer settings from the SD. I plugged it into my PC via USB and everything worked fine. I immediately suspected the buck converter to be faulty.

I measured 2,7V on the 5V rail when the board was connected to a 6S lipo (used for troubleshooting, I use a 24V Meanwell in my printer) and the 3V rail showed ~1,9V. I was surprised to measure 4,93V on the top side side of the small inductor (L3) and 2,7V on the bottom side. Is it safe to assume this is the culprit? Because I also measured 4,93V on C98 and 2,7V on C99.

How did I break my board? The only thing I can think of is back EMF created by spinning the Z motor (doesn't the TMC2660 protect the board against these voltages?). I remember pulling on a belt en seeing some LEDs flash on the board. At the time, I only had 3 steppers and 3 end stops plus the PanelDue connected to my Duet 2 Wifi (v1.03). Also, what size is L3?

Thanks in advance

Wyvern · 3 Feb 2019, 17:42

Did you by chance hook up the stepper with the duet powered on? even though the duet is protected from such things, it's still advisable not to plug or unplug steppers while powered on.

I've moved axes before powered off and that caused the monitor to blink- I try to refrain from doing this, but it has not harmed my duet, it has never harmed a ramps before either.

I'm curious to see what happened.

dc42 · 3 Feb 2019, 19:03

I suggest you check the resistance of L3 using a multimeter. It should be lower than your multimeter can measure (0.04 ohms according to the datasheet). If it reads open circuit, than it has failed.

The part number we use for L3 is MPZ2012S221AT000. But any ferrite bead in 0805 size rated at 3A or more will do if you are not fussy about meeting CE EMI limits. Or even a wire link.

A continuous short circuit from 5V to ground might fuse L3.

Beun · 2 Mar 2019, 20:14

@wyvern said in How did I break the 5V rail?:

Did you by chance hook up the stepper with the duet powered on?

I didn't (un)plug any steppers, I'm aware of the fact that steppers shouldn't be hot-swapped.

@dc42 said in How did I break the 5V rail?:

I suggest you check the resistance of L3 using a multimeter. It should be lower than your multimeter can measure (0.04 ohms according to the datasheet). If it reads open circuit, than it has failed.

Lol how on earth didn't I think of this right away. It has been a long day for me.

It reads 1120k ohms, but when I reverse the polarity of the multimeter leads it somehow reads out of range (>2000k ohm). I don't know why this gives me a different reading but it's definitely dead. I'll order the inductor and bridge the pads in the meantime. Or I might use one of those tiny MP1584 boards that I must have laying around somewhere, they always come in handy.

I still wonder what happened. It could have been something simple like a bad inductor. Thanks to both of you. Especially to you dc42. I'm new to the 3D printing scene and I really enjoy working with the Duet WiFi.