Thermocouple issues

dintid

The firmware already does some filtering.

What is a rex_c100 ?

I'll pipe in here, as I have the same problems, only with Thermocouples on hotend.

I have tried 2 different original daughterboards, so I think we can say the fault isn't here.
I've tried both channels on both boards.

Issue:
it shows temperature fine when using short wire at 10cm. - when I put by hand around it, it starts heating up just fine.
When using long wire (up to hotend) the temprature is mostly fine when thermocoupler isn't touching anything - as soon as I touch it, it jumps to 2000c
When installing in hotend it's just at 2000c constantly.

I've run a wire away from the machine, away from any potential interference, but as soon as I touch the thermocouples the temperature jumps to 2000c.

I've tried at least 5 different thermocouplers… they all work fine on other controllers/firmware combos (marlin, smoothie etc)...

dc42

Did you read the wiki page on connecting thermocouples?

dc42

I've done some more research and experiments. I can reproduce the 2000C temperature spikes (lots of them!) if I wrap the thermocouple cable around a stepper motor cable that is carrying current.

There is a mod you can do to the board to make it less susceptible to interference. The mod consists of adding a 0.01uF ceramic capacitor between each thermocouple terminal and ground. So you need four 0.01uF capacitors per thermocouple board, You can use either SMD or small wire leaded capacitors. Here is the procedure:

1. Remove the nylon pillar by squeezing the end with long-nosed pliers.

2. On the underside of the board, scrape away the solder resist to expose the copper ground plane where you want to connect the ground side of the capacitors.

3. Solder the capacitors between the terminal block spills and the ground plane you exposed.

4. Replace the nylon pillar.

Here's one I did earlier, using wire-ended ceramic disc capacitors on one channel and 0603 SMD capacitors on the other.

We will implement this modification in the next PCB revision of the thermocouple board.

Jan

Thanks!
I was facing severe issues with this problem as well.
Actually I am using the ardafruit break out board with the duet WiFi.
I applied this mod and added two 250ohm series resistors as well (up to 1000 ohm shall be ok according to the maxim faq https://www.maximintegrated.com/en/support/faqs/max31855-faq.html))
I bought some ferrite beads as well but didn't put them so far.

So far the result is very promising and I ordered my machine a half day print for test, and will report later …

dc42

I've modded some more boards and done some more tests. I tried a 100 ohm resistor in series with each terminal and no extra capacitors, but that didn't help. Adding the capacitors solved the problem, whether the resistors were included or not. So my recommendation is to add the capacitors first, and only add the resistors if the capacitors alone don't solve the problem completely.

I found it easiest to use 0805 SMD capacitors. If you have a no-clean flux pen, apply flux to the terminals spills and the bared copper. Heat one of the terminal spills with the soldering iron and use tweezers to push one end of a capacitor into the molten solder joint. Repeat for the other terminal. Then solder the other ends of the capacitors to the copper of the PCB that you bared already.

We're working on implementing this as standard in our thermocouple daughter boards.

Jan

Ok, from here I can confirm it works after the endurance run.
I finished a 14h print by now without temp faults (which was completely blocking since my thermistor hot end failed and I switched to TCs, at least as soon as I switched the motors on).

I now only have to decide if I take out the series resistors or leave then in.
But as long as it works I think I consider it as a running system

Thanks again for proposing this mod!

Jackal

David, thanks for the testing and results, I will do the mod when I got some free time.
On a side note, the thermocouples I got have crimped Y-connector, it's a bit tricky to have all 4 wires connected to the board and I ended up taping some of the prongs with insulation tape.
Is is ok to trim those connectors off and connect the bare wire to the board? Will it affect the reading?

dc42

Yes you can connect the bare thermocouple wires to the terminal block. Keep them twisted around each other right up to the terminal block, to minimise interference pickup.

dintid

@dc42:

Did you read the wiki page on connecting thermocouples?

Yes.. obviously. Since I've been trying to write "how to's for you"… Many times

But what kind of info do you especially refer to in there, as it seems you hint that there should be a solution? (apart from the soldering of small SMDs).

dintid

Just found that an old bead-type Thermocoupler works fine.

Unfortunately ALL the thermocouplers in m3 cartridges (fitting for newest E3D and UM2+) is doing the 2000c act. Yes, also if the thermocoupler board and thermocoupler is the only thing attached.