Thermal runaway when running from UPS?
I had some really weird issue.
My MK3S is connected to UPS (APC BR900G-RS). It was printing. The lights gone. UPS switched to battery to keep printer running. And I got "thermal runaway" on MK3S shortly after.
Rebooted and tried to preheat. It looked like power was not even going to heaters. Bed's LED was off, nozzle was cold. I turned everything off.
When electricity was restored, I turned UPS and printer back on. And now everything is working normally.
If I unplug UPS from wall's socket - the issue repeats: heaters are off, UPS shows 3-4 watts of power is used by the printer (while 280 watts is a usual amount of power when both heaters are on).
The thing that there is no power going to heaters while running from UPS is really strange to me. Any ideas what it could be? Is it printer's PSU failing? Anyone else have experience with UPS?
Best Answer by Alex:
I have found this thread: https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-hardware-firmware-and-software-help/heaters-disabled-power-panic-sense-fault/
It looks like the issue is in power sensing circuitry in printer's PSU.
RE: Thermal runaway when running from UPS?
I have found this thread: https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-hardware-firmware-and-software-help/heaters-disabled-power-panic-sense-fault/
It looks like the issue is in power sensing circuitry in printer's PSU.
RE: Thermal runaway when running from UPS?
BTW, firmware is 3.8.1 (Bondtech).
RE: Thermal runaway when running from UPS?
Intereresting, I just purchased an UPS but havent’t thought of pulling the plug to see if it actually works 🤦 bummer...
Just started an 12h print but will get back with my results. I have a Apc UPS and an MK3S.
RE: Thermal runaway when running from UPS?
It seems to work fine for me, pulled the plug and let it run on batteries for about a minute and plugged it back in again.
Running my octoprint and MK3S on it at the time, FW 3.8.1
UPS: Easy-UPS BV1000I-GR Schuko
RE: Thermal runaway when running from UPS?
Well, I got unlucky...
RE: Thermal runaway when running from UPS?
I have discovered that you can unplug power panic sensor cable (black/white from PSU) to get this working. Surely, you lose power panic feature, but it should be partially restored by your UPS.
RE: Thermal runaway when running from UPS?
@sw3de
pulled the plug and let it run on batteries for about a minute and plugged it back in again.
Thermal runaway does not trigger immediately. You need to make sure heaters are still on. It is easy to check:
- Power the printer on;
- Unplug PSU;
- Use Preheat -> PLA from menu;
- Does bed's LED turn on? If no - you have a problem.
RE: Thermal runaway when running from UPS?
@aleksandr-a2
I think the issue is your UPS doesn't generate a nice sine wave. But what makes me wander why you have an issue only during heating. Usually you get just a reset in the switching phase (from Line to Battery). But you should be able to run from UPS on battery only without any issues.
Often linked posts:
Going small with MMU2
Real Multi Material
My prints on Instagram
RE: Thermal runaway when running from UPS?
@nikolai-r See linked thread. Prusa turns off heaters when nAC_FAULT line is signalled from PSU (power panic sensor cable). As I understand - this is done in hardware, not software. That is why printer continue to operate, while heaters are turned off by wrong power panic sensing.
What I really don't understand - why printer does not detect this as power panic.
PSU makes some audible hum when working from UPS battery, but voltages are around 24.25V, so it should not trigger any power panic.