[Closed] Melted Terminal Block for Heater Bed  

Re: Melted Terminal Block for Heater Bed

I was talking to Johnny from Ultimachine and he told me once the got burnt once, they are highly likely to burn again if you use the new male part. So what I suggest, if you have a soldering iron, just strip the cable form heatbed at the end, and solder it onto the pcb directly from the bed. Then you will never ever have to worry about it.

iqzan.j I can see from the picture, that you dind't tighten the screw in the plug at all, no wonder it burnt.

Anyways guys, when I get back from Makerfaire Rome, I will make some sort of a guide for new builders, so this wont happen and guide how to repair this.

Re: Melted Terminal Block for Heater Bed

josefprusa, actually, I loosened the screw to show the melted insulation. It was tightly screwed before the picture was taken. Is it still safe for me to use the heatbed again if I put a new insulation on the melted part?

Re: Melted Terminal Block for Heater Bed

I concluded that soldering directly to the board would severely limit my ability to reassemble the board and connect all the wires in the future. doing so would complicate matters. I would prefer to find an alternate solution or perhaps an new controller board. Today I attempted to flash the updated drivers but downloading the .zip revealed nothing but source code - no .exe file as explained on the directions. Although I am familiar with make files and building software I think this is not something for users to do.

Re: Melted Terminal Block for Heater Bed

Hello, after a few weeks of happily printing on the machine (great fun, btw!), I noticed my heatbed was starting to take longer and longer to heat. I smelled a bit of burnt plastic near the Rambo, and after taking it apart found a crispy heatbed connector (see image below). As far as I can tell, screws and plugs were both on tight. I've cleaned out the PCB terminals and am going to go try another connector to see if that fixes the issue, but this is just an FYI that one more person is having this issue. If the next Molex burns out then I'm going to have to brave soldering it - do you have any good instructions or guidelines for doing so? (I'm pretty sure I don't want to do this wrong, with the heat and electricity and all.)

Re: Melted Terminal Block for Heater Bed

Thanks for adding this. This was my experience as well.

Re: Melted Terminal Block for Heater Bed

It seems to me that there is a decision to make when designing the RAMBo. Either include screw terminals soldered directly onto the board or to include a socketed connection.

Both have pro's and con's . Obviously, the best option would be to solder the heater cables directly onto the board, but I am not sure if the wires are copper (solderable) or steel (or similar - not solderable).

I also have a Chinese clone, which uses screw connectors on the board. I had a big problem with this as the connectors moved a lot when tightening the screws and I ended up with one solder joint underneath the board breaking which required re-soldering.

Personally, I think the connector/socket arrangement is a good compromise, but even a small piece of dust can prevent a good connection, cause a high-resistance joint and therefore heat up.

Also cable movement during printing can also cause a similar high-resistance joint, hence the importance af routing the cables correctly and especially the cable tie on the RAMBo housing to prevent cable movement inside.

Peter

Re: Melted Terminal Block for Heater Bed

I am not expert but heat is caused by excessive current. Since this issue seems to get worse over time could it be that the heat-bed's resistance gets lower somehow? Maybe a scratch or something? Just thinking out loud here.

Re: Melted Terminal Block for Heater Bed

Hi Tim

It's actually quite simple (especially so when working with DC). The Resistance of an object determines how much Current will pass through that object at any given Voltage.

The Resistance of the hot bed is approximately 1 Ohm, therefore at 12 Volts, it will allow 12 Amps of Current to pass. The heat generated by this is determined by Volts x Amps, so approximately 144 Watts of heating power. The 15 Amp fuse on the RAMBo allows for manufacturing tolerance of the heated bed.

That's all great, but it there is a high resistance joint at the connector, that may also measure at 1 Ohm, thus the total resistance of the heater and connector will be 2 Ohms allowing only 6 Amps to pass and therefore generate only 72 Watts of power BUT only half of that power will be dissipated as heat by the hot bed - 36Watts. The remaining 36 Watts will be dissipated across the connector.

Now, 36Watts is not a lot of heat; it would cause the hot bed to warm up very slowly and possibly not reach the full temperatures. But that 36Watts being generated at the connector is a huge amount concentrated in a very small space. It would definitely cause the plastic to melt and char.

Peter

Re: Melted Terminal Block for Heater Bed

So are you saying that soldering the wire **would** solve the problem?

Re: Melted Terminal Block for Heater Bed

Tim

Assuming the wire is a solderable wire, then soldering it would remove 2 potential causes of high resistance joints from the system as long as the joint was soldered correctly.

Peter

Re: Melted Terminal Block for Heater Bed

OK got a new connector in and it's wired up and the bed seems to be coming to temp nicely. Hopefully this time it won't char on me.

I would like to point a fan on the Rambo to reduce the possibility of future crispies - does anyone know if there are any spare pins on it to add one?

Re: Melted Terminal Block for Heater Bed

Marco

There is a spare 12Volt outlet in the PSU. I find that better than tinkering with a working RAMBo.

There is also a housing re-design here: http://shop.prusa3d.com/forum/improvements-f14/housing-for-rambo-t253.html

That seems to have decent cooling.

Peter

Re: Melted Terminal Block for Heater Bed

We were trying to replicate this thing multiple times here checking with our Flir-E8 to not much success.

What we know is that, once this happen, only long term stable solution is to solder the cables on the back. The pins on the board side oxidise when the problem occurs.
Once you do that, you are still able to disassemble the printer as the heatbed is not screwed in to the printer 💡

We are working on next revision of the RAMBo mini with soldered terminals on the board (similar are used on the PSU but covered).

What is the ambient temp and humidity in your print room?

Re: Melted Terminal Block for Heater Bed

Thank you both! I have not observed the issue again (though it's only been 24 hours) but if it does recur I will be soldering as you suggest. As to your question - the room is a garage that is currently on the cold side (15-20C) so the heated bed may be working a bit harder than it should to maintain temp.

FWIW, I am having a great time on this very well-built product - thank you!

Re: Melted Terminal Block for Heater Bed

Marco

It may be worthwhile making a box out of plywood and plexiglass to keep the printer warm and away from any drafts.

Peter

Re: Melted Terminal Block for Heater Bed

Just finished up with a long print, when i noticed my heatbed was no longer hot (thank goodness it was PLA)

Pulled the Molex plug for the heatbed and found exactly whats described in this thread. Wires were screwed very tightly into the molex connector, As other folks have described i noticed it taking longer to heat up and it finally stoped heating completely.

Re: Melted Terminal Block for Heater Bed

I have a issue with my heated bed as well, but mine never worked from the start, not melted, very tight connection. the printer trips and resets when its plugged in and I try to heat the bed. with the bed unplugged everything worked flawlessly, I am 15 hours into my first print with no issues. what can I do about this, the printer was just shipped to me brand new kit 2 days ago.

Re: Melted Terminal Block for Heater Bed

Hi Bryan

I see that you have checked the washers and PSU voltage settings with no luck.

Can you please try swapping over the cables for the bed heater on the RAMBo connector. Also check out the resistance of the heater - unplug the connector and put probes from a multimeter set for resistance across the screw connectors. You should get a reading of 1 Ohm.

Also check the 15 Amp fuse for continuity.

Other than the above, I don't have any more ideas, so if that doesn't work, you will have to contact support to see if they can help.

Peter

Re: Melted Terminal Block for Heater Bed

@erron.w what version of Rambo are you using? 1.0 or 1.3?

Re: Melted Terminal Block for Heater Bed

Erron

That looks very much like a cable management issue - did you cable tie at the entry to the RAMBo housing to prevent cable movement?

Peter