I'm fairly new at this and self-taught, so assume I'm ignorant and my apologies for any incorrect terminology.

I'm repairing my 3D printer for the third time, and the previous 2 breakdowns and repairs, including my terrible soldering, have left the negative terminal for the power in really bad shape. (More details on that below, in case it's relevant) I'm wondering if I can just attach the negative wire for power to one of the other negative terminals on the board. With my very limited experience, it makes sense to me that all of the negative terminals would connect to each other, and it wouldn't really matter which I connected to. Is that true? This is a cheaply made and cheaply assembled board, so I can't guarantee that it complies with best practices, in case that matters.

More details on the situation: There was originally a power connector with screws to attach the wires, but the first breakdown (due to poor design) caused it to melt. I added a MOSFET board to fix the bad design (a common upgrade for cheap 3D printers) and soldered the power wires directly to the pins that were inside the melted connector. I did a bad job, and after a couple of years, one of the joints broke. I discovered that the pin had become quite lose, and I decided to try to re-solder it more securely. The pad seems to be totally gone, though, and the best I could manage still left it wobbly. I believe a combination of the breakdowns and my poor soldering has damaged the board a bit around the pin. I don't think I have the skills to repair it, and if attaching to another negative terminal will work, I would prefer to do that. If not, I'll attempt to repair the board, but that's probably beyond me.

Here's a guide to the MOSFET upgrade I did with some decent pictures of the board in question, in case that's helpful: https://thatnerdchannel.blogspot.com/2017/06/mosfet-upgrade-wanhao-duplicator-i3-v21.html

The reprap page on the board: https://reprap.org/wiki/Melzi

  • Includes schematic and a decent picture of the full board
  • My specific board appears to be the "eBay hybrid"

Side question: If I scratch off the outer layer of the PCB near the burnt negative terminal, am I right to think that I will hit the layer of copper the terminal connects to, and that I could attach to that? (I believe this is basically how repairing the board would work.)

  • 2
    \$\begingroup\$ Do you have a link to the schematic of the board? With it, this question can be answered pretty solidly, as either yes, yes-with-caveats, or no. Otherwise, all we can say is that it's common for them to be the same, but we can't guarantee it. \$\endgroup\$
    – nanofarad
    Aug 17, 2021 at 3:05
  • 2
    \$\begingroup\$ No need to wonder. Just resistance test with a multimeter to find out. If it's truly connected it will read under 2 Ohms. \$\endgroup\$
    – DKNguyen
    Aug 17, 2021 at 4:17
  • \$\begingroup\$ @nanofarad I found that link. (in the "edit" section above) Is that sufficient for a definitive answer? \$\endgroup\$
    – Josh
    Aug 17, 2021 at 12:51
  • \$\begingroup\$ @DKNguyen I'll have to order a new multimeter, but that's on my to-do list, so I may take that route - it'll just take time. \$\endgroup\$
    – Josh
    Aug 17, 2021 at 12:52
  • \$\begingroup\$ @Josh It should be, but I don't have the software to view the KiCAD schematic on my work computer and won't have my personal machine with KiCAD until I'm finished moving. This image suggests, but does not prove that there's a large ground pour that could likely bond the negative terminals all together, which is what you want. \$\endgroup\$
    – nanofarad
    Aug 17, 2021 at 14:05

2 Answers 2


I don't think I have the skills to repair it, and if attaching to another negative terminal will work, I would prefer to do that. If not, I'll attempt to repair the board, but that's probably beyond me.

Step one is to check if those "negative terminals" are electrically connected with each other. Using a multimeter, you should measure 0 ohm between them. That's the bare minimum.

But more relevant here, the module you linked with a fat heat sink on it is designed to work with a lot of current, the product brief says 15A. This means that the connector and the PCB traces where you connect it must also be designed to handle that kind of current. Otherwise they'll burn up and you get a potential fire hazard.

In order to tell if the connector can handle that current, you need to check the datasheet for the connector.

In order to tell if the PCB traces can handle that current, we would need a picture of them or the PCB layout. Generally it's quite easy to see, they will be much thicker than normal PCB traces, some ~5mm wide at least. Though for a multi-layer PCB, a ground pin via will typically be connected directly to the ground plane - if so, one will see "thermal relief" traces around that pin.

  • 1
    \$\begingroup\$ It's definitely a potential fire hazard. I learned that soon after buying the printer. The MOSFET upgrade supposedly helps, but I still never run it unattended. On my board, the hot bed and hot end connectors are identical to the power connector, so at the very least, they can't be any worse than it was. \$\endgroup\$
    – Josh
    Aug 17, 2021 at 12:57
  • \$\begingroup\$ The multimeter I ordered came in today. I tried it, and I'm a little confused on a few points. You mentioned 0 ohm is the bare minimum. Can you have negative ohms? My second point of confusion is that between the negative for power and the negative for the hot bed, it measured 0 in one direction, but if I swapped the leads, it measured about 0.5. I wouldn't have thought that was possible. Every other pair of negative terminals measures the same when I swap the leads. \$\endgroup\$
    – Josh
    Aug 21, 2021 at 22:47
  • \$\begingroup\$ I did some research. I think you're "0 ohm" measurement was likely a typo, as everything I've seen points to a value around 0.2. I still don't know what's going on with the one pair of terminals measuring differently depending on which lead touches which terminal - as far as I can tell that shouldn't happen, but I did prove to my satisfaction that the other terminals on the board are NOT connected and can't be used. There is one on the MOSFET board I added in the upgrade that looks like it will work, though. \$\endgroup\$
    – Josh
    Aug 21, 2021 at 23:14
  • \$\begingroup\$ My previous comment is incorrect - there is not a terminal on the MOSFET board that can be used. I think I'm out of luck unless I can manage to repair the terminal or find a large ground area on the board to tap into. \$\endgroup\$
    – Josh
    Aug 21, 2021 at 23:40
  • \$\begingroup\$ @Josh Generally, you can't have negative resistance. If you are using the instrument correctly, you should have somewhere between 0 to a few ohm and that's enough to be considered a connection. (Use the beep feature, usually found at the same location as diode measurement.) Start with connecting the probes together. If it shows significant resistance then (>10ohms), then you either got a crap multimeter, crap probes or it ain't calibrated correctly. \$\endgroup\$
    – Lundin
    Aug 23, 2021 at 6:31

It depends on what negative terminal you are talking about. But I'll give a general idea here.

We generally call the negative terminals as Grounds. Ground or the negative terminal is generally the return path for the current coming from the corresponding positive terminal.

For example, let's say you have a 5V D.C system. Now when you connect an small Fan between the 5V positive and negative lines, the fan starts to work. The current travels from the positive terminal of the 5V system, through the fan and all the way back to the source via the negative terminal.

Now when you hook up a voltmeter between the positive terminal and the negative terminal, a potential of 5V is shown on the voltmeter. Now this 5V is w.r.t is it's corresponding negative terminal which is ideally at 0V.

Now let's say your system has both 5V D.C and 240V A.C on the same board. Now you cannot connect the negative of 5V D.C system to the negative of the 240V A.C system. It is like connecting your fan's positive wire in the above example to the battery's positive terminal and the fan's negative wire to the A.C wall outlet in your home. It doesn't form a closed circuit and the fan doesn't work.

I hope you got it.

  • \$\begingroup\$ There's only one AC power source involved, (the power cord to the wall outlet) but there may be multiple DC voltages supplied to the board - I really don't know. Is this what the 0ohm multimeter measurement that nanofarad and Lundin mentioned would be confirming, or is this a separate concern? \$\endgroup\$
    – Josh
    Aug 17, 2021 at 13:10

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