Duet Maestro Bed heating uncontrolled

NTX9

There is no visible short at the back of the board and the red LED lights as soon as power supplied.

NTX9

Prior to this fault, I had an issue with the hotend thermister. It was throwing inaccurate readings and may have been shorting. Could this have damaged the mosfet?

If the mosfet has failed, this seems like a extremely problematic issue. The amount of power being applied to the bed seems like a tremendous fire hazard.

elmoret

@NTX9 said in Duet Maestro Bed heating uncontrolled:

Prior to this fault, I had an issue with the hotend thermister. It was throwing inaccurate readings and may have been shorting. Could this have damaged the mosfet?

If the mosfet has failed, this seems like a extremely problematic issue. The amount of power being applied to the bed seems like a tremendous fire hazard.

It’s the same amount of power as is applied when normally heating up...?

Most people size their bed heater so the bed cannot exceed safe limits in the event of controller or FET failure. If one chooses not to do this, sound engineering practice calls for a thermal fuse.

NTX9

@elmoret
In regard to your comment about sound engineering, the engineering was done by M3D for whatever that is worth.

You may be right that the bed may be sized to prevent exceeding safe limits. However, with the mosfet fail, the temps hit greater than 100c and were still climbing. I don't know if I would call that safe...

infiniteloop

@NTX9 said in Duet Maestro Bed heating uncontrolled:

I don't know if I would call that safe…

As we deal with lots of electrons and a lot of heat in our hot ends, the print bed and maybe a heated chamber, safety is definitely a thing we should consider thoroughly. As every single component (such as a mosFET) can fail, a second line of defence will help.

However, there exist already several threads about this topic, so it might be best to have a look at these - they offer a lot of proposals and, to put it in @Danal’s words: „strong opinions“

dc42

@NTX9 said in Duet Maestro Bed heating uncontrolled:

Prior to this fault, I had an issue with the hotend thermister. It was throwing inaccurate readings and may have been shorting. Could this have damaged the mosfet?

No.

If the mosfet has failed, this seems like a extremely problematic issue. The amount of power being applied to the bed seems like a tremendous fire hazard.

That's why I always advise users to either select a bed heater that can't get to a dangerously high temperature even when run at full power for an extended period of time, or to fit a thermal cutout to the bed in series with the bed heater. I have a Crane Quad too, and the bed heats up slowly enough that I doubt that it can reach a temperature high enough to be a fire risk - but I haven't actually tested this.

But I repeat, it is extremely unusual for a bed heater mosfet to fail unless there is a short circuit across the bed heater. In printers with moving beds, there is always the risk of the bed heater wires chafing unless suitable precautions are taken.

NTX9

@dc42
I will examine the bed wires to see if that is the case.

NTX9

@dc42
I submitted a tech support request to M3D but have not seen any response yet. Frankly, I am not confident that M3D will warranty this board.

Is it possible to use an external Mosfet or SSR with the Maestro? Perhaps by connecting to the E1 heater header and re-configuring the firmware? Or by connecting to the Expansion header?

NTX9

On further investigation it is fairly obvious that the wires shorted against the aluminum bed. Looking at the way this connection was set up by the manufacturer, this short seems like it was inevitable.

Phaedrux

yikes

droftarts

@NTX9 Yikes indeed! Best check yours, @dc42 !

Ian

dc42

@droftarts said in Duet Maestro Bed heating uncontrolled:

@NTX9 Yikes indeed! Best check yours, @dc42 !

Ian

I will!

NTX9

For what it is worth, I have two and the newer one is in better shape.

Is the chip next to the LED on the board, the Mosfet that has been damaged? Or is there a diagram that shows the layout of the board.

The leads on that appear substantial enough that I might brave replacing the chip.

M3d customer service is slow at best and I don't have much faith that they will warranty the board.

dc42

@NTX9 said in Duet Maestro Bed heating uncontrolled:

Is the chip next to the LED on the board, the Mosfet that has been damaged?

Yes, the one marked 036N04L. To replace it you will need a hotplate as well as hot air, because of the amount of heatsinking that the PCB provides.

My Quad is in better shape than your photo, but there is some bare wire very close to the edge of the aluminium. I'm going to put some Kapton tape in the gap and over the edge of the aluminum.

droftarts

@dc42 @NTX9 Those bare wires on the heatbed do look crazily close to the edge. Could you desolder them and resolder the other way around, so they looped back, with insulation over the edge of the bed? Or would you need to heat the bed to 200C first, as it's a massive heatsink?

Ian

NTX9

@droftarts
Ian, That is exactly what I plan to do. I may also add some kind of printed cover if I can find or design something that seems suitable. I am not sure how difficult re soldering will be but the alternative is to replace the bed.

NTX9

@dc42 said in Duet Maestro Bed heating uncontrolled:

Yes, the one marked 036N04L. To replace it you will need a hotplate as well as hot air, because of the amount of heatsinking that the PCB provides.

Yikes, that sounds like a golden opportunity to totally bogger up the board.

Maybe I will go with an external mosfet afterall.

Are there pins on the expansion board that would bypass the onboard mosfet? Or would I need to attach to the gate pin on the onboard mosfet?

NTX9

While this issue is clearly the result of a short at the heated bed, it would seem like the Duet board would have a fuse to protect against this kind of fault.

dc42

@NTX9 said in Duet Maestro Bed heating uncontrolled:

While this issue is clearly the result of a short at the heated bed, it would seem like the Duet board would have a fuse to protect against this kind of fault.

Sadly, fuses are usually too slow to protect mosfets. OTOH, in many cases the short-circuit protection of the PSU does provide sufficient protection - but evidently not in this case.

A Former User

Unfortunately fuses aren't fool proof; if you're looking for a warranty claim I'd look to the printer manufacturer instead.