Cleaning up the power connection

I'm not sure if my power supply uses thicker wires but it feels really sketchy trying to fit 12 ground wires into the single Melzi terminal. On my first build I connected each of the power supply bundles to two of the same gauge wire and put that into the Melzi, so 6 +12V wires -> 2 wires -> Melzi, and 12 COM wires -> 2 wires -> Melzi. I checked the wires and the terminal frequently for heat and so far have never had a problem. But I don't think it's an ideal setup, especially after reading this conversation, so I would like to improve things and make sure all my wiring is rated properly. If I need to accommodate 16A, then, according to the AWG chart on wikipedia, I should use 14 gauge copper stranded, although other AWG charts seem to say as low as 10 gauge. Someone at my hackspace suggested 2 x 14 gauge for each bundle to provide a safe margin and minimize loss. I would appreciate another opinion on the matter. FYI, the PSU has 18 gauge stranded aluminium.

On a related note, I am curious what terminals are being used on the Melzi supplied with the kit and what there rating is. The Melzi BOM here shows terminals that are rated to 8A which is only half of the required 16A. Plus they are blue whereas mine are green so I'm thinking they are not the same.

Edited 1 time(s). Last edit at 09/24/2014 02:03PM by ashleywebster.

The important thing is having a good connection to the terminal, as a loose or partial connection may cause arcing or at least increase the resistance of the connection.

When bundling together my main power wires I worked out the maximum diameter or wire that would fit into the terminal and left this amount extending from the main bundle - I think this worked out to be about 90% of the wires coming out of the PSU. I then soldered the main bundle together, being careful to leave the section that went into the terminal unsoldered (there's a separate thread about why we shouldn't tin the wires that go into terminals) - I then put heatshrink over the soldered section to prevent accidental shorting. This has worked fine for me for the two years or so that I've had this printer.

Arguably if I'm only using 90% of my wires then there's no point in having the remaining 10% at all, so I could just as easily have cut them short and insulated the spare ends.

Speaking of the importance of a solid terminal connection, I learned that first hand working on the printer at my hack space last night. It's a "homemade" Prusa with wiring everywhere. One of the power connections had come lose in the terminal and fried the Arduino and all the drivers. So yes, good point.

At any rate, I'm likely over thinking this whole thing. Based on what you said, there is really no point in using 12 wires if I just solder them down to 2 wires. So I bundled three +12V and three ground, because that's as thick as I can fit in the terminal, and capped all the rest. I hooked it up this evening and it measures a little over 12V across the power terminal and the resistance across the terminal is also lower than on my first printer where I only used two wires. So far so good.

And I answered my own question about which power terminals are being used. I looked again and found the model number, it was just hard to see in the dim light of late last night. They are rated for 16A as expected.

Thank you for your help.

>there is really no point in using 12 wires if I just solder them down to 2 wires.

There is because the two wire section can be very short, so its resistance is negligible compared to the long wires.

The ACE 500BR PSU we supply has 14/0.15 strand wires, which is why I can get all 12 in. Obviously six is enough to carry the current because the six +12V wires don't get warm. Using more gives a lower resistance which helps with the max bed temperature and in the case of the ground reduces the noise on the USB cable.

If you have filled the terminals then you must have the same amount of copper in three wires, so it should behave just as well. If you have three spare then you could solder the bed positive wire to those, splitting the load and avoiding it going through the terminals and PCB. However you would still have the full current through the ground terminal.

>PSU has 18 gauge stranded aluminium

Really? Aluminium wires needs special handling. I have heard of it in buildings but never seen it used in equipment.

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[www.hydraraptor.blogspot.com]

Thanks Nophead. Yes, not aluminium, my mistake. Just tinned copper.

About the spares (I've got way more than 3 spares), having it joined directly to the +12V coming out of PSU and not going into the Melzi, wouldn't that have the bed always on? Or no because the voltage is controlled by the ground side which is still going through the terminal. What is the benefit to doing that?

Edited 2 time(s). Last edit at 09/25/2014 12:32PM by ashleywebster.

Only the grounds are switched, so the +12V goes in and straight out again to the bed, heater and fan terminals. By wiring it to the spare wires it will get slightly more voltage because those wires will have less current flowing through them and if you have more than three spare then less resistance as well.

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[www.hydraraptor.blogspot.com]