RAMPS 1.4 power input has melted...what is the problem?

I was having issues getting the heated bed to power. Manually or even after i'd run a job Repetier would say "heating bed", and it obviously wasnt heating...no LED light near mosfet, but the extruder and fan works great. My RAMPS setup isnt particularly easy to get to, so after unplugging everything and looking at the board for further inspection i've noticed the main positive power input directly beside D8 has turned black and bubbled up. Dead give away that is the issue, especially after i did a continuity test from where the wire goes in and to the 'brown-halfway-burnt' pin soldered to the backside of the board. No connectivity. I have no issues resoldering a replacement connector on to it. My main question is "Why did this happen in the first place??" Surely it couldnt be my power supply. I'm using a 550 Watt PSU capable of 32A on the 12V rail so it should have been getting enough power...any ideas whatsoever? I'm itching to get back to printing...but the last thing i want to do is wake up on fire. Thanks ahead of time

Edited 2 time(s). Last edit at 01/02/2014 08:18PM by zachisparanoid.

Unfortuantely, this sort of thing is a regular problem, and really a design flaw rather than anything users are doing.

I think the basic problem is that historically RepRap boards don't use connectors that are rated high enough. Of course, higher rating means bigger connectors and more expense, and people rely on anecdotal test like "I ran it for a bit and it didn't get hot". But the point is the rating is for a worst case, if you have a low ambient temperature and free air, then it will be fine. If it's stuck in a box with poor airflow, then eventually it overheats. Also people subsitute cheaper parts without checking the rating.

Additonal problems is that the screws can shake loose, but also the wire creeps in the clamp especially if soldered. This leads to higher contact resistance.

So really you want to make sure you have connectors with a good margin, plus use crimp ferrules and ensure the connectors are not affected by shaking. Improved airflow also helps.

The spec for the Phoenix MSTBA connectors says 12A, so that operates on the limit for a typical heatbed. The TE equivalent rating is 15A, for cheap Chinese sources probably more like 8A. The plug and socket connectors are convenient, but tend to have a lower rating than a straight screw clamp terminal.

If you replace the connector, get a quality part from a reliable source like Digikey etc, with a rating that exceeds the required current.

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Yea, lesson learned!! no more chinese stuff for me. Luckily (for some strange reason) i picked up a pack of Board Terminal Connectors from radioshack the day after i got my RAMPS...honestly not forseeing this problem. Checked the back of the pack, they're rated for 16A MAX. Should be fine, although the pins that would go through the board seem to be smaller. Then again this could be somewhat of an illusion due to the meniscus of solder around the original connector. Anyways, i'll change this out within the hour and post how it all goes. No more printing while i'm asleep! I'll be watching this thing like a hawk from here on out regardless of print time! Thank you so much for the response bobc!

Replaced the board terminals with the Radioshack 16A replacement....works like a charm

Hi Zachisparanoid,

I too got the same issue of melting the connector, but I used a large heatbed(10" x 10") without any relay.

Did you used the same large heatbed? If yes, do you use any relay circuit to draw more current out of PSU?

I tried connecting the #1 on J1 to PSU(ATX 450W) +12v pin on 20 pin connector as shown in image [www.makerfarm.com]

I am using 6 pin relay.

But I was not successful to heat up the bed.

Do you have any configuration/ckt for the relay to be used for large heatbed?

Regards,
Prashant.