Heated Bed on Permanently

One of my P1Ps has an issue with its heated bed. I noticed yesterday that the bed had remained on after a print. After further testing today it seems that I am unable to turn the bed off! The printer displays a target temp of 0 - but the bed remains at 80 degrees. I’ve switched the printer off and back on - it just heats back up.
I’ve just done a factory reset of the printer - but the bed has immediately heated back up to 80 degrees.
So I’m guessing one of my control beards or mosfets is fried.
Any suggestions on what part I might need to replace?



Likely a heat bed AC board failure

Thanks @Panamon_Creel - that does look likely.
Any idea if I can buy this part? I can’t seem to find it listed on the spare parts page?

Try with a support ticket

Thanks @Panamon_Creel - I’ve submitted a ticket. Just hoping for a quick response - I need this printer up and running. If I could have bought the part I would have taken a chance.

Thanks again.

In case a DYI option if you are familiar with desoldering/soldering of PCB components: replace the SSR on the board which is what likely failed

MGR (or other brand) GJ-5-L


Thanks again @Panamon_Creel .
Yes - I’m pretty comfortable with basic soldering. That’s a good suggestion. I’ll try and order one up now as a backup plan.

If push comes to shove and you can’t find a PCB version SSR you can also use a non PCB one with 3-32VDC control and a 24-380VAC 5A (or more) output switching and run some wires.
And not to forget → everything at your own risk.

Wouldn’t the bed get much hotter than 80deg if the relay is shorted since there’s no control anymore?


I would try a factory reset as sometimes that seems to fix odd/unexplained problems.

Thanks @JonRaymond - as I said, I have tried that.

Thanks @Panamon_Creel , I’ve just ordered a compatible (hopefully) from RS online.
Hopefully I’ll also hear form Bambu and get a replacement AC Board from them.

Not exactly comforting if it FAILS ON rather than FAILS OFF.

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For a slightly deeper diagnostics simply disconnect the Heater bed control cable on the heater AC board, if bed still heats up after that then you can be relatively sure that it is an AC Board failure.

SSR’s tend to fail by short circuiting thus in the “ON” state, in critical application where a permanent ON failure could cause danger and/or critical failures then redundancy protection against that should be included.
Heck even relay/contactor contacts can weld shut in a closed state :slight_smile:
Here in this application it should not really be an issue since the Heater bed is with a PTC heating element thus self limiting in regards to over temperature.
Maybe they also included a redundancy temp limiting element in the heater bed but that I do not know.


I’ve ordered one of these from RS online. I should be compatible.
I’ll try and pull the printer out later this evening and do some more testing as you suggest @Panamon_Creel .


Slightly cheaper on aliexpress :grinning: but sure not as speedy. The alternate looks exactly the same, higher though I think but there are plenty of room in there. Aliexpress states pin spacing at 4.5mm/9.5mm and RSonline spec sheet at 5mm/10mm but I think it’s just dimensions taken on aliexpress that are off and 0.5mm difference if it exist is nothing. Rsonline one also seems to be 4.5mm wider but there also some room each side on the board. Let us know how it fits and works.


Are you sure about that? I’ve not seen a teardown on the BBL heatedbed, but on the other 3D printers that I have gotten to know, they either used either a a large PCB and heated up the traces (as in the Prusa MK3 or Ender 5 Plus) or else, IIRC, a silicon PI heater stuck to the bottom of an aluminum plate, as in the case of a creality printer CR10-S5. The Prusa approach had the virtue of keeping the heated bed lightweight and in very close proximity to the spring steel build plate. Not that I claim to be an expert on PTC heaters, but the only PTC heating elements I’ve run across are either the ceramic ones, which tend to either be sandwiched between metal heatsinks (in the case of car heaters) or else glued or otherwise stuck to the bottom of a metal plate (e.g. in the case of a heating element for a coffee maker) or else of the heat-tape variety, like you might use to keep plumbing pipes from freezing but without risking a fire because of the self-limiting nature of their construction.

That’s not to say that a non-PTC heatbed would necessarily be unsafe. There might be thermal fuses, for example, as a failsafe.

Anyhow, I just now tried a google search for examples of 3D printer PTC heatbeds, and I couldn’t find any. They might exist, but they don’t seem to be very common if they do. Or maybe I just got unlucky with the search. So, I thought I’d ask.

Just a quick update. Babmu support suggested checking all the wiring - no indues found and the problem persisted
So a soldered on the new SSR - problem solved! Not a terrible fit - just had to tweak the pins a little.

Thanks for everyone’s help. I would have liked Bambu to send me a new AC board; they might have done if I’d pursued it - but I just need this printer up and running.



Pursue it, they will send you a new board. Always nice to have one spare for later :wink:


Can’t say 100% for sure but power draw characteristics seems to indicate that there is a PTC style regulation going on.
PTC type heaters come in all type of shapes and form, even thin mylar types. A standard resistive heating element could technically still be a PTC type heater if it has an integrated PTC thermistor instead of having the PTC thermistor as a separate unit in line with that heating element.

Good to hear that