Heat bed keep heating even when no print

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Heat bed keep heating even when no print

Postby gmoille » Wed Sep 12, 2018 1:42 pm


I have an issue with the N1 printer we recently got. The heat bed started to heat continuously, even when no print job is performed. Even when temperature of 0 is set, the heat bed heats as much as it can, reaching temperature higher than 110 degrees C. When printing, it doesn't care about the temperature settings (however the nozzle is fine). On the screen though, I can see the temperature of the bed along with its set temperature, so I guess it is not a regulation issue. Do you have any idea what could cause this and how to fix this?


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Re: Heat bed keep heating even when no print

Postby Vicky@Raise3D » Wed Sep 12, 2018 7:21 pm

Sounds like either power cable of the heated bed, or motion controller board or heated bed itself has something wrong.
Please contact our support to figure out how we can help at http://help.raise3d.com.
Please do not use the printer until fix the issue in case over heating.

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Re: Heat bed keep heating even when no print

Postby Jetguy » Thu Sep 13, 2018 3:34 pm

Just an educated guess knowing this printer very well.
If you place the bed glass clips in the wrong spot and they short the bed heater traces to ground of the frame that would cause the heater to heat uncontrolled
the more unlikely thing is the FET shorted on the mainboard and is damaged.

Follow the logic on this. #1 The bed is ALWAYS powered!!!! This is important to know and understand about 3D printers in general, for a dozen reasons, we use N channel MOSFET transistors that switch on the low side (aka GROUND) NOT the positive wire feeding the bed. Again, this is a fundamental concept to "get" or else typical troubleshooting and meter reads will give you false assumptions and you make further mistakes.
Again, the bed is ALWAYS powered whenever the power supply is on.
The return lead or both wires of the bed heater in the OFF state of the heater are 24V when compared to DC power supply ground.
The reason the heater doesn't heat in this state is power (voltage times current) is not flowing. You have to go from a high source (+24V) to DC ground or negative and then there is 24V across the bed heater and then it heats.

So, with a fair level of certainty, again, a bed clip shorting through a trace on the bottom side of the bed will not blow the FET and the reason is, that short is to ground the same thing the FET is doing in parallel, shunting to ground. The short causes less current and voltage to flow through the FET not more- thus never overloading or damaging it- you are literally bypassing it with a short to ground turning the heater on.
The FET is rated for something like 70 Amps. The 24V source to the bed is fused with a automotive blade style fuse just for the heated bed circuit.

OK, so testing:
Step 1 is remove the bed clips, power on the printer, if it does not continue heating- right there is your answer.

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