Hotend Thermistor Error on Toolboard 1LC

lilmike

I've been using a Duet 3 Mini 5+ and a toolboard (V1.1) combination on my SK-Go2 printer since August of last year. It's been a pretty solid solution, the printer has "just worked". I'm running RRF 3.3. Yesterday, while I was preparing the printer for a print, (preheating and cleaning the nozzle prior to setting Z zero), I got an error on my hotend thermistor. The reported temp was 2000C, which suggests something is short-circuited. Preheat was in the normal operating range, approximately 250 C. Cleaning was with the exact same brass brush I have used to clean hotends on the last three printers I have owned. Nothing was out of the ordinary. I restarted the printer and the error remained.

I removed the sock and found that one of the thermistor wires looked a little frayed. Additionally, the thermistor was loose in the block. When I checked it, it tested OK (about 100 KOhm). Still, given the condition, I elected to change it out since I have a spare.

I swapped the thermistor (E3D cartridge style, semitec) for a new one, the error persisted. Reset the tool board, the error persisted. Re-flashed the tool board, the error persisted. Unplugged the thermistor, then the wire to the tool board, the error persisted. Both the old and new thermistors checked fine (approximately 100 KOhm, and lower when I held it in my hand).

This suggests the problem is with the board, so I swapped out the board (fortunately, I have a spare).

Once I swapped the board, properly set the address, then restarted the printer, behavior is back to normal.

Any thoughts as to what could have gone wrong? Anything I can do to mitigate this in the future?

jay_s_uk

@lilmike there have been a couple of reports of something similar in the past. It has been down to either a short in the wiring or static build up, both of which damaged one of the resistors on the board.
You can still use the second temperature input if you aren't aleady.
DC42 recommends making sure the hotend is grounded to make sure theres no static build up. Other thing is making sure your wiring is ok

dc42

@lilmike I am fairly sure that this type of fault is caused by either electrostatic discharge to the thermistor wire, or a short between the thermistor wire and a heater wire (typically via the hot end metalwork). Duet3D products usually survive these incidents, however the small size of the thermistor series resistors used on the tool board makes them less robust when these errors occur.

lilmike

@dc42 Static discharge is a possibility, it is winter and I have noticed a zap or two when I touch the printer, but nothing really out of the ordinary. I don't think the hotend wires could have shorted to the thermistor, but I never checked the heating element for a short to the case.

Is this something that can be repaired? Is this something that the warranty would cover, or is this on me? Have later version boards been changed at all to mitigate this?

More importantly, how can I ground things more effectively to mitigate this risk? I have the metal printer chassis tied to earth already, but the carriage and toolboard may be isolated due to the printer's construction. Should the toolboard be grounded at one of the mounting points, or is there a better practice than that? My hotend is in a metal mount, on a metal carriage, but the toolboard is isolated on a printed carrier board to adapt it to a standard NEMA-17, so it may be isolated from the carriage. I will check that. The toolboard has power wires heading back to the chassis though. Is it acceptable to tie the DC (-) to earth as well? I seem to recall reading that's not a good idea. I'll search for some guidelines on grounding this evening after work.

@jay_s_uk So I can just not use that particular thermistor input and map the hotend to the other input? That might work for me if the board can't be repaired/replaced under warranty. So far I only need a single thermistor input on the toolboard.

Thank you both for the replies.

jay_s_uk

@lilmike yes. You can just map temp1 to the hotend.

dc42

@lilmike to avoid static build up I suggest you connect the hot end metalwork to VIN- or one of the grounds on the tool board through a resistor with value around 100K. Direct to VIN- is OK too.

lilmike

@dc42 Thanks for the tips. I will work on getting things properly grounded this weekend.

The meter indicates that the toolboard is electrically isolated from the carriage metalwork, and the carriage is isolated from the chassis of the printer, so I will tie both to a common grounding point. It's a belt-driven CoreXY, I think I need to ground my drive motors as well, as they do not have continuity to the machine chassis either.

The cartridge did not short to the case, so there's no chance it was a Vin to thermistor short. Must have been static discharge, and there have been a few zaps along the way.

Thanks again for all the help.

I have other questions about setting up multiple toolboards, but that is for another thread.