Heated bed started smoking

Hi!

I've used my reprap i3 for a few months, and although it has been a roller coaster of emotions, I am very happy with what it has become and how it prints.
After being unused for a week or two a friend wanted me to print a part for him. While heating up the bed it started smoking. I quickly pulled the plug out of the socket. Upon closer inspection I noticed that a bit of the paint had disappeared. I guess that's where the smoke came from. After doing a bit of googling I realized that my thermistor might have come in contact with the aluminium plate because I had forgotten to isolate it. I put on a piece of kapton tape to prevent it from happening again.
Could a themistor coming in contact with the bed cause a short? I measured 1.3 ohm between + and - on the heat bed, so it should still be fine, right?
The heat bed is a chinese MK2B.

Thanks for any input!

Regards,
Christian

1.3 Ohms is reasonable for a 12V heated bed, that would be about 110W, drawing about 9A of current.

A thermistor shorted to ground can do some weird things Shorted to power could do some other things, some of them very bad, like destroy your control board. Protecting from short circuits is a good thing, people should do that.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

I wouldn't expect that the thermistor contacting the heat bed would cause a short bad enough to cause smoke, except that you might get bad temperature readings from the bed, which might cause your controller to overheat the bed eventually.

Even if some other wire was contacting the bed, the thermistor wires are so fine that even if they did burn out, you probably wouldn't even notice the smoke.

So I'm thinking the problem is elsewhere; power wires touching the bed perhaps.

Try disconnecting the bed heater wires from the supply and turn on the printer. Does it show a valid bed temperature?

Is this a 12V or 24V or mains-powered heat bed?

The hot spot (partially burnt track) is very close to the +12 soldering pad. In case of a short (whisker of solder ..) the +12 V touching this track, it would create this and may even burn away.

The measured value for the heating track is OK. So most probably fine.

The Al plate is isolated from the heating tracks. Did you connect to 0V/GND.

Can you show how the thermistor is installed ?

Edited 1 time(s). Last edit at 01/18/2019 03:00AM by MKSA.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

Quote
SupraGuy
1.3 Ohms is reasonable for a 12V heated bed, that would be about 110W, drawing about 9A of current.

A thermistor shorted to ground can do some weird things Shorted to power could do some other things, some of them very bad, like destroy your control board. Protecting from short circuits is a good thing, people should do that.

I admit it, I'm an idiot. Instead of fusing the heat bed and the hot end, which I know I should have done from the beginning, I did fun stuff like making the printer look nicer.

Edited 1 time(s). Last edit at 01/18/2019 02:05PM by chrstrvs.

Quote
frankvdh
I wouldn't expect that the thermistor contacting the heat bed would cause a short bad enough to cause smoke, except that you might get bad temperature readings from the bed, which might cause your controller to overheat the bed eventually.

Even if some other wire was contacting the bed, the thermistor wires are so fine that even if they did burn out, you probably wouldn't even notice the smoke.

So I'm thinking the problem is elsewhere; power wires touching the bed perhaps.

Try disconnecting the bed heater wires from the supply and turn on the printer. Does it show a valid bed temperature?

Is this a 12V or 24V or mains-powered heat bed?

The thermistor seems to be working. When switching on the printer it shows room temperature.
It is 12 volt.

Quote
MKSA
The hot spot (partially burnt track) is very close to the +12 soldering pad. In case of a short (whisker of solder ..) the +12 V touching this track, it would create this and may even burn away.

The measured value for the heating track is OK. So most probably fine.

The Al plate is isolated from the heating tracks. Did you connect to 0V/GND.

Can you show how the thermistor is installed ?

Do you mean if the aluminium plate is grounded? If so, it is not. Should it be?
I forgot to take a picture, but it's installed like any other thermistor is installed in these heat beds. Like this.

I looked at everything today and couldn't find a reason for the smoke/short, so I put a 15 amp fuse on the between the bed and the Ramps board and started heating the bed. After a few minutes of it being stable at 60°C I started printing. After a few minutes the fuse blew and the printer halted due to "Thermal runaway".

I don't understand why the bed works fine for a while and then blows the fuse. Any ideas?

Oh, and thank you all for your help so far!

Quote
chrstrvs

Quote
MKSA
The hot spot (partially burnt track) is very close to the +12 soldering pad. In case of a short (whisker of solder ..) the +12 V touching this track, it would create this and may even burn away.

The measured value for the heating track is OK. So most probably fine.

The Al plate is isolated from the heating tracks. Did you connect to 0V/GND.

Can you show how the thermistor is installed ?

Do you mean if the aluminium plate is grounded? If so, it is not. Should it be?
I forgot to take a picture, but it's installed like any other thermistor is installed in these heat beds. Like this.

I looked at everything today and couldn't find a reason for the smoke/short, so I put a 15 amp fuse on the between the bed and the Ramps board and started heating the bed. After a few minutes of it being stable at 60°C I started printing. After a few minutes the fuse blew and the printer halted due to "Thermal runaway".

I don't understand why the bed works fine for a while and then blows the fuse. Any ideas?

Oh, and thank you all for your help so far!

Normally all metallic parts should be grounded but it has nothing to do with your issue.

It looks like may be due to thermal expansion, the heating track of the bed gets shorted. The thermistor and wiring looks OK as it worked and detected the thermal runaway.

Disconnect the bed from the controller board, power it directly from the 12V (I suppose it is 12V as the bed is wired for 12V) via a switch, the 15A fuse and an ammeter if you have one.
If you don't have a thermometer use the printer controller as a T° probe for the bedr.
Turn ON the bed switch and look at the T°, turn off if the T° reaches 100°. If the fuse blows or smoke before, the bed is defective. Look closely where it is burnt, watch if nothing could short the 12V solder pad to the track, or at the bed mounting screws.

Note, it is an easy problem, provided one has the machine on hand.

Edited 4 time(s). Last edit at 01/19/2019 04:53AM by MKSA.

---

"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

Quote
MKSA

Quote
chrstrvs

Quote
MKSA
The hot spot (partially burnt track) is very close to the +12 soldering pad. In case of a short (whisker of solder ..) the +12 V touching this track, it would create this and may even burn away.

The measured value for the heating track is OK. So most probably fine.

The Al plate is isolated from the heating tracks. Did you connect to 0V/GND.

Can you show how the thermistor is installed ?

Do you mean if the aluminium plate is grounded? If so, it is not. Should it be?
I forgot to take a picture, but it's installed like any other thermistor is installed in these heat beds. Like this.

I looked at everything today and couldn't find a reason for the smoke/short, so I put a 15 amp fuse on the between the bed and the Ramps board and started heating the bed. After a few minutes of it being stable at 60°C I started printing. After a few minutes the fuse blew and the printer halted due to "Thermal runaway".

I don't understand why the bed works fine for a while and then blows the fuse. Any ideas?

Oh, and thank you all for your help so far!

Normally all metallic parts should be grounded but it has nothing to do with your issue.

It looks like may be due to thermal expansion, the heating track of the bed gets shorted. The thermistor and wiring looks OK as it worked and detected the thermal runaway.

Disconnect the bed from the controller board, power it directly from the 12V (I suppose it is 12V as the bed is wired for 12V) via a switch, the 15A fuse and an ammeter if you have one.
If you don't have a thermometer use the printer controller as a T° probe for the bedr.
Turn ON the bed switch and look at the T°, turn off if the T° reaches 100°. If the fuse blows or smoke before, the bed is defective. Look closely where it is burnt, watch if nothing could short the 12V solder pad to the track, or at the bed mounting screws.

Note, it is an easy problem, provided one has the machine on hand.

I did like you suggested, but I also removed the heated bed from the printer while performing the test. The fuse didn't blow and the bed had no problem reaching 100°C. I did notice something that I didn't see the last time I removed the bed, which is a small burn mark on the Y carriage.
I insulated the solder on the bed with electrical tape to avoid it touching the Y carriage. When I re-installed the bed I saw that the solder on the positive part of the bed was very close to the Y carriage, so that was probably what happened.
My theory is that my already kind of weak bed leveling springs made the solder touch the Y carriage because the springs have become weaker with time.
I have now printed a few spacers instead of springs and the fuse has not blown yet, so I'm hoping the problem is solved.

Thank you all so much for your help, it is greatly appreciated!