MK2S preheat Bed Thermal Runaway error - while temperature is increasing - Hardware, firmware and software help

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

JoanTabb — Fri, 11 Apr 2025 22:13:34 +0000

did the 12 AWG, give you issues when soldering?

regards Joan

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

Leo P — Fri, 11 Apr 2025 18:32:29 +0000

12AWG did the trick - bed now not only doesn't pull a error, but actually heats up within an appropriate amount of time

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

JoanTabb — Tue, 25 Mar 2025 23:29:51 +0000

Hi Leo,
I don't think there is a safety hazard using 12 gauge wire, the fuse on the motherboard is much lower rating than the cable rating.

BUT it might be difficult actually soldering 12AWG wire, onto the heatbed.

regards Joan

RE:

Leo P — Tue, 25 Mar 2025 19:16:37 +0000

Yeah, I definitely did. Using 14AWG, it was still slow - but much faster than before. I might try 12AWG, but do you think there's a limit (safety wise)? Im assuming not

Thank you, regardless

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

JoanTabb — Sat, 22 Mar 2025 10:08:38 +0000

18AWG has a lower current capacity than 16AWG or 14 AWG,
using 18AWG wire will cause a greater loss in the wiring and lower power delivery to the Heatbed.
LARGER AWG numbers relate to SMALLER WIRE...

I think you have used the wrong wire

regards Joan

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

Leo P — Sat, 22 Mar 2025 06:13:27 +0000

So the 18awg wire finally arrived and I immediately soldered it.... only to run into the same issue. 12.7V measured across my beds solder joints

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

JoanTabb — Fri, 28 Feb 2025 09:41:27 +0000

Hi Leo,
you should be measuring at the solder pads. on the heatbed.

strandcount is important, the Prusa wire probably has about 22 strands,
Flexible silicone wire has a lot more finer strands, which makes it less likely to break, in use

at this site on ebay
eBay item number:176373249402
the 14AWG wire has 400 strands
regards Joan

RE:

Leo P — Fri, 28 Feb 2025 04:54:12 +0000

The voltage across it is 12.5 Volts. I screwed the POT in a bit more to ensure it was over 12, as it was 11.5 before. I'm measuring the Terminals in the Rambo, not the soldered pads on the heated. Should I be checking that as well?

14AWG is a good idea, thank you. Should I be concerned about strand count or anything else besides AWG? Ill likely buy from Ali if it is not an issue.

RE: MK2S preheat Bed Thermal Runaway error - while temperature is increasing

JoanTabb — Thu, 27 Feb 2025 21:27:24 +0000

It sounds like the heat bed is taking too long to heat up,
What is the voltage across the heatbed terminals, whilst the heatbed is heating?

the Mk2 is a 12 VOLT so any stray resistance will have a significant effect, often the cable starts to deteriorate and go high resistance as strands break in the conductors
the original cable is 16AWG. if the terminal voltage is low, you could replace the wires with 14AWG flexible silicone wire, which has a lower resistance
regards Joan

MK2S preheat Bed Thermal Runaway error - while temperature is increasing

Leo P — Thu, 27 Feb 2025 18:38:37 +0000

I have a MK2S connected to octoprint. This allows me to monitor the exact temp, on a graph. Whenever I try and preheat my printer to ABS temps (255/100), while the bed temperature is increasing, the firmware throws a thermal runaway error and shuts down the print. The issue is, the bed temperature is regularly climbing and not crashing (which I assume is the point of the thermal runaway). If I sit there and repeatedly restart the connection and preheating, it can eventually reach up to ~90 degrees, after around an hour (lol). Steps taken so far include:
- Screwing the Pot in PSU to give the bed 12.5V (measured with multimeter)

- Checked Resistance (~1.5 ohms)

- Unplugged and replugged bed thermistor and heater in RAMBO

- Checked Fuses

- Turned off fan cooling (even though it happens during preheat, not during a print)

Additionally, the Bed's LED stays on, and there are no melted cables.