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Melzi power cabling -- confused

Posted by dawa 
Melzi power cabling -- confused
November 17, 2013 07:04PM
I have almost finished my new mendel90 kit.
I really liked the manual.
The black power cables to the Melzi, how can they fit the hole in the green little plastic connector?
I feel the connector can easily break if I tighten it too hard.
How do I know if it is tight enough?
How can this little tiny board deliver 10 Ampere, for instance for the heater bed?
And all in all, how much current is supposed to flow through the twelve black parrallel power cables?
I am also surprised that there are no heatsinks on the Melzi. I can see no power transistors. Well, there are a few really small ones. No TO-3 cases. Strange.

I don't dare to turn on the power.

Actually, I messed up the soldering on these black cables, so I soldered four times three and connected each group to thicker wires, that I put into the Melzi connector without solder.
Would that work?
Otherwise there is still room to cut the 12 black wires and try again. My soldering iron is a bit weak too, so I am hesitant to that.

Best regards,

Edited 1 time(s). Last edit at 11/17/2013 07:05PM by dawa.
Re: Melzi power cabling -- confused
November 18, 2013 04:27AM
The Manual shows how the wires fit into the connector with photos. Sometimes I have to shave the solder with a knife to make a nice square end.

The power connection needs to deliver 16A so it has to be a very good connection or it will get hot. If you can't get them all in then use a few less. The wires easily carry the current as only 6 are use for the 12V feed. The issue is the lower the resistance the ground connection is the less noise you get on the USB but I suspect a few less won't make a noticeable difference. It is better they fit well. Four thicker wires should be OK, and no it isn't necessary to solder them as long as you can get all the strands to go in.

Hold the connector to stop it twisting and you should be able to get the screws tight without breaking it.

The board can easily deliver 10A because the MOSFETs used have a very low on resistance compared to other RepRap boards and plenty of copper heatsinking on the PCB. The motors drivers can deliver 1.2A without heatsinks because the PCB acts a heatsink, again better than most other RepRap boards which use the tiny Pololu drivers.

Re: Melzi power cabling -- confused
November 18, 2013 06:20AM
I really think the kits should ship with a single large block junction to put the black PSU wires in, and a short, but thick, piece of cable to connect that to the melzi.

I had a friend who worked as a electronic engineer help me with my build and he was very sceptical about that part of the build.
I also took about 1/2 hour of messing around to get the thing to fit. A single block junction would be a lot better.

Edited 1 time(s). Last edit at 11/18/2013 06:20AM by Traveltrousers.
Re: Melzi power cabling -- confused
November 18, 2013 07:13AM
I can't see the point of an extra screwed junction as it is just another possible point of failure. Why not just solder to a short thick section if you want one?

Another way of doing it is to strip six of the 12 wires shorter so they meet and are all soldered together but only six continue into the connector.

Re: Melzi power cabling -- confused
November 18, 2013 04:50PM
Thanks for the advice, information, and good arguments, nophead. It all makes sense.
I soldered the cables two by two, and then put in olny six of the ends in the connector hole, twisted and soldered together. The heatshrink sleeving fitted well that way too. It worked fine and looked much better!
I think you should mention something like this in the manual.


(But I havent powered up yet... more connections remain, maybe tomorrow :-)

Edited 2 time(s). Last edit at 11/18/2013 04:52PM by dawa.
Anonymous User
Re: Melzi power cabling -- confused
November 20, 2013 04:08PM
I took a single strand thick copper wire and soldered all the black wires around that then put shrink wrap over it to insert into the green connector. For soldering I use an 80 watt iron with a temperature control.
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