Thermocouple Daughter-Board Faults
I've been having interesting problems with the thermocouple daughter-boards (purchased from Filastruder). The first card believe blew out my original Duet Wifi. The card got super hot, causing something catastrophic to happen somewhere causing a loss of recognition over USB and Wifi.
So fast forward a month to my 2nd daughter-board. I had both channels working well the first week, but kept getting temperature spikes to 2000C when my heaters were on (I use PWM as my heaters are overpowered). So I reflowed my electronics layout and shielded both thermocouples. Voila - problem solved. However, when I put everything back together, only one of the thermocouple channels would work. I could switch the thermocouples, both wires worked fine, but only one channel would actually read anything. So this week, the thermocouple channels wouldn't work at all. After about 6 different re-wirings, the board started producing VSSA faults and getting hot - exactly like what was happening before. I've disconnected it so I don't lose my Wifi card again.
Does anyone have any idea what could be happening? None of my heaters were on during this testing. I've got a 3rd daughter-board coming soon, but I'd rather not blow it up like the ones I did before.
I think you must have an intermittent short between the thermocouple and the hot end metalwork, and also a short between the positive supply to the hot end heater and the positive supply. Such shorts are surprisingly (to me at least) common when thermistors are used, so I expect they are equally common with thermocouples.
What i don't understand is why you are getting VSSA faults. The VSSA signal is not fed to the daughter board. Could it be that you have a short between the bed thermistor and some metalwork too? Are you using any other thermistors?
We have a new version of the thermocouple board in preparation that is tolerant of shorts between the thermocouple and the heater supply voltage - just like the PT100 board has always been. But of course even with this version you will get a temperature spike whenever a short occurs.
I think you were right on most every point. The 1 thermistor I was using had randomly shorted to the heater block (since replaced with cartridge-style). And the heater had somehow internally shorted to the heater block as well.
One of the thermocouples was shorted internally to its shielding cable, apparently not well made. So the combination of a heater shorting to ground, coupled with the shorted thermocouple shorted to ground is probably what was blowing my boards.
By the way, can I request that you add those capacitors to future thermocouple daughter boards? Even if it's just a slot where you can plug in an optional capacitor, it would still be substantially better.
We've already added the capacitors to all our stock of thermocouple daughter boards.
Awesome. I'll probably be ordering one of those in the somewhat near future.