i3Mk2S - Thermal Runaway...and Smoke  

RE: i3Mk2S - Thermal Runaway...and Smoke

If the heater is heating, however the electronics is not showing this, usually the termistor is wrong. And because the electronics does not know what is the current temperature, it just continues heating until something starts to smell. If you have a multimeter, you can disconnect the terminstor from the main board and measure the resistance. It should have roughly 100k Ohm. 

Here is some guide: https://help.prusa3d.com/en/article/multimeter-usage_2117

Or you can simply try to replace it. Do not run the printer again without having the temperature measurement working.

RE: i3Mk2S - Thermal Runaway...and Smoke

@milos-v

Thanks very much Milos.

Do you happen to know where to get a thermistor, and whether there is a "how to" replace it? I built the printer from a kit, so presumably I must have fitted it at that time, although can't remember the specifics.

Cheers!

RE: i3Mk2S - Thermal Runaway...and Smoke

The strange thing is, according to the Prusa info:

"Thermal Runaway is a safety feature designed to prevent the printer from accidentally catching fire."

So how come with mine, it said "THERMAL RUNAWAY" on the LCD, yet the head continued to get hot and would no doubt have caught fire if that was going to happen?

It stopped printing, but it clearly didn't switch off the power to the nozzle heater.

RE: i3Mk2S - Thermal Runaway...and Smoke

This is would be what you need: https://shop.prusa3d.com/en/mk3mk3s/205-hotend-thermistor-e3d.html?search_query=thermistor&results=2

But it is a standard original thermistor for E3D hotends. So you can buy it almost everywhere. Look for e3d thermistor cartridge, original should be fine.

RE: i3Mk2S - Thermal Runaway...and Smoke

Thanks Milos.

RE: i3Mk2S - Thermal Runaway...and Smoke

It will require the hotend disassembly and cables dissasembly, which is extremely ugly work. So I would first try to measure, if the thermistor is really wrong. 

RE: i3Mk2S - Thermal Runaway...and Smoke

I just checked both thermistors (bed and nozzle, and they both read 121 K Ohms.

So what's next?

Thanks.

RE: i3Mk2S - Thermal Runaway...and Smoke

The thermistor came out of the head quite easily - the grubscrew wasn't very tight.

I noticed that the resistance varied between about 75 and 120 K Ohms when it was removed and replaced. May be something to do with the heat difference between the cold head block and warm hands?

RE: i3Mk2S - Thermal Runaway...and Smoke

I believe it should be more 100 k Ohms. But damaged would show much different value. So you unpluged the thermistor from the main board and measured? I mean you measured the connector pins? 

Does the printer when powered on show the ambient temperature? Both bed and hotend should in cool state show the actual ambient temperature on the display. Does it fit?

RE: i3Mk2S - Thermal Runaway...and Smoke

@milos-v

Thanks,

The bed nominal temperature is high (28 deg.C)

The nozzle nominal temperature is normal (20 deg.C)

When I select preheat for PLA:

The nozzle LED temperature doesn't change from nominal, but the nozzle gets very hot.

The bed connector LED comes on, and the bed heats up as normal. This is reflected by the LCD temperature display (which was initially reading above nominal). I haven't let the bed get to target temperature, because the nozzle was getting very hot again.

Cheers!

RE: i3Mk2S - Thermal Runaway...and Smoke

@milos-v

And yes, I disconnected and measured from the pins.

RE: i3Mk2S - Thermal Runaway...and Smoke

SORRY! I made a mistake with a plug!

The ambients (nominals) are 27 deg.C and 29 deg.C

The ambient in the room is 22 deg.C

Same as before on preheat: The bed and nozzle get hot, but only the bed nominal increases on the LCD.

RE: i3Mk2S - Thermal Runaway...and Smoke

OK, if I will not comment, why printer shows ambients 27/29 and you have 22 in the room. It can be sure some problem with the main board. Still it would be good check the thermistors. I did some measurement with the spare thermistor I have here. I have now some 23/24 °C ambient temperature in room and thermistor shows 97 kOhms. When I just heat up in my hand, impedance drops down to 68 kOhms. When puting out of the window (2°C currently), it raises over 200 kOhms. So try to measure yours 🙂

RE: i3Mk2S - Thermal Runaway...and Smoke

And do not use preheat for the tests. Because of wrong measurements printer will not run the cooling fan. And this will finally cause clog of your extruder or a damage to PTFE tube. For the test, put the heating on for couple of seconds only. The impadance has to go down.

RE: i3Mk2S - Thermal Runaway...and Smoke

Here is an overview of the Semitec 104GT thermistor. The values may vary slightly, but should roughly correspond to the values in the table.

Temperaturtabelle für Thermistor Semitec 104GT

RE: i3Mk2S - Thermal Runaway...and Smoke

Ok so I measured with the sensor in my hands - about 70 K Ohms. With an ice cube next to it, it goes up over 270 K Ohms.

Ambient in the room is about 20 deg. C and it's reading about 29 deg C. 

I can't believe the main board has gone. It's not been used much. That's a big disappointment, and £120 + postage. Pfffffff.

Any other diagnostics before I have to spend money? 

Thanks for your help - much appreciated.

RE: i3Mk2S - Thermal Runaway...and Smoke

@karl-herbert

Thank you.

Based on that, and Milos's tests, I would say my thermistor is working perfectly.

RE: i3Mk2S - Thermal Runaway...and Smoke

I do not remember, if both bed and heater thermistors have the same board connector. But you could switch those two to see, if really main board  channel is wrong. 

I do not know how to check the board itself and will not dissasemble it because of you 😉 May somebody else can help how to crosscheck main board issue. 

Have you done any changes to firmware recently? Or just try to reload the firmware once again, to eliminate software issue.

RE: i3Mk2S - Thermal Runaway...and Smoke

@milos-v

100 k Ohms at 25C... so 121Kohms is probably  about room temperature, and 70k Ohms may be near body temperature...

If the heater wouldn't switch off while heating and now won't switch on, you may have a problem with the Mosfet  that drives the power, and now  perhaps the fuse has gone! or maybe  the mosfet failed short circuit and then burned out, or perhaps the heater has now gone open circuit! due to overheating. time for some more investigation... 

regards Joan