Melzi Power Terminal Failure

This post is to document a problem that I had with my RepRapPro Mendel with Melzi 2.0 electronics board.

Problem description:
* Computer could connect to printer using pronterface
* Power supply was supplying 12 V
* Fan operational
* Stepper motors would not respond
* Thermal sensors would not respond
* Melzi would not respond to gcode commands

Problem identification:
* Discoloration was noticed on the plastic casing of the "power" terminal block


* Voltmeter readings indicated that the Melzi board was no longer grounded

Problem solution:
* The terminal block on the Melzi board had a manufacturer defect, and did not close completely when the screw was tightened. When assembling the wiring, I assumed that the ground was attached well since the srew seemed tight. I was able to print correctly for a while, but after moving my machine around, it did not work. The incomplete connection caused power to be deoposited into the body of the plastic, which casued the melting.
* The terminal block was de-soldered from the board, and the power supply was temporarily wired up to the Melzi board using some extra wire, or "pig tails". This fixed the problem, and saved me from having to buy another Melzi board. I have ordered a replacement terminal, and plan on installing it on the board, for a cleaner looking solution.


In summary, try diagnosing your Melzi board with a voltmeter, if you think this might be your problem.

The connector works by pulling up on a plate, trapping the wire at the top of the terminal. It could be the plate was already in the up position when you put the wire in, so the wire was below the plate and just hanging in the terminal. Doing up the screw may not have made any difference. Could you take a picture of the front of the terminal, showing the area where you put the wires in?

You can see discoloration of the metal plate from the back:


The plates can be seen from the front:


After removing the wires from the terminal, the ground side plate would raise partially up, then get stuck. It was physically harder to turn the screw on that side (after the failure), than the + side.

When originally connecting the power wires to board (before the failure), the terminal wasn't large enough to fit my soldered wires and fan wires in, so I had to trim down the ends of the mains wiring to make it fit. I ordered a slightly larger (taller) terminal that I will be placing on the Melzi board, instead of the original size.

Thanks for the feedback. It is a bit of a squeeze getting the wires in, especially if you tin the wires with solder. I usually twist the ends as tight as possible before tinning, so it fits easier, and lightly solder the wire fan on, otherwise it is difficult to put in, and often falls straight back out!

Apologies for resurrecting this thread but I just had a similar failure on my RepRapPro mendel hotbed connector.

I've been printing fine for a while and then last night the bed stopped heating. Checked the connector and it looked like the +ve wire had worked loose. I took a great deal of care trimming and tinning the hotbed cable and ensuring both cores were secure but I guess these things loosen up with the motion of the cable and vibration.

This caused a melt and a shorting of the socket. Fingers crossed the board is OK , I just need to replace the terminal and then print a strain relief clip so that there's zero movement of the wire at the terminal end.

And I'll be checking my connections on a regular basis!

Rob

ihad/have same problem with bed wires. but after i soldered wires directly to board the main power connector did the same thing 2 months later. so i soldererd them as well. after extensive almost 24 hour printing for 2 weeks i realized i needed a heatsink and fan directly on wires. which i did. today the hot end fan comes on but the red led does not. the imput wires do not read 12v or anything for that matter. but the terminal pin farthest from connector side reads 5v and the pin closest to terminal side reads .02 or 0..... troubleshooting reprap says processor is screwed but possibly voltage regulator??? i dont know where that is can anyone help? or should i just humbly order another crap board?

I'd say the answer is that you are running more current than the board will handle. You need an external relay or something similar to handle part of your load.

The board can handle the current and so can the connectors but the wire needs to be thicker to do so and the connection has to be well made.

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[www.hydraraptor.blogspot.com]

We occasionally do hear of Melzi's that melt the connector, usually because of poor contact in the screw terminal - either the wire is loose in the terminal, or has been stripped too short. It does carry a lot of current, so the connection does have to be good. Once they have melted, you are best off replacing them; it's not too difficult a soldering job, just be careful not to lift the tracks on the board when you desolder the old connector. Use a 'solder sucker' and/or solder braid to remove as much solder as possible from the pins, before removing the terminal block.

Our supplier has used a couple of different terminal blocks, these are the most common:
[uk.farnell.com] - this is the most common one
[uk.farnell.com] - this one is a bit taller, and easier to get the wires into.
Unfortunately, we don't stock these.

When we build printers, we tend to twist the stripped (8mm minimum) power wire ends as tight as possible, then 'tin' the end with solder, and finally flick the end of the wire while the solder is still hot so there are no lumps of solder protruding. This usually makes a neat end that fits straight into the screw terminal, but it can be a tight fit.

Ian
RepRapPro tech support

Turns out one of my tamiya wire connectors fizzled into a black charcoal mess so I did as suggested and doubled up the wires from board to bed and it all worked like a charm! Don't need the heatsink on wires but still have fan on board. Thanks guys !

I burned out the terminal block for my heatbed power connector and my heatbed connector on my RAMPS 1.4 board as well and it uses the same kind of terminal block. If you're not careful and move the machine around those connectors will come loose and get hot/melt once you try to pass sufficient current through them. I just replaced the terminal blocks and it's been working great ever since. I really don't like those connectors for this application though and I feel that a more solid connector should be used. Those screw-in connectors shouldn't be used for high current applications, despite what the datasheet says.

If you use them as specified with solid wire and tighten them to the specified (high) torque. They work quite well. With stranded wire and who knows what torque - not so much. For about 4X the cost and 4X the real-estate on the board you can go with the next better connector. Of course you then would not have room for half the connectors or the FET's ....