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I am rewinding a transformer that got fried. It is a single phase, reducing transformer.

On the primary we have mains, and the secondary is tapped at different points. See figure for details.

Unfortunately the windings had been destroyed at several points, and I could not figure out what the GND pin was connected to.

The ground pin was physically located on the primary side of the transformer. That doesn't mean that it is electrically connected to the primary, though.

When unwinding the coils, first there was the secondary. Next came the GND wire, but it had only a short length of wire, which had been severed from wherever it connected to. Finally there was the primary, with a bit missing at the beginning.

Any idea of where to connect this GND pin? TransformerDiagram Picture

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  • \$\begingroup\$ In your picture, the wire already is connected on the inside of the bobbin, and that would be an electrostatic shield between primary and secondary. The wire is already connected on that side, so I’m not sure what you’re asking about. The question mark is on the inside of the transformer, but the only free end of the wire is the ring terminal outside, and you presumably took notes to know where it was connected before you pulled the transformer out of the device? Please clarify by editing the question to make it clear which end of the wire you’re unsure about. \$\endgroup\$ Commented Mar 27 at 13:35
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    \$\begingroup\$ That ground wire most certainly does NOT connect to the primary. \$\endgroup\$
    – MOSFET
    Commented Mar 27 at 13:37
  • \$\begingroup\$ Thank you for your comments/questions. I have added a bit more information. I didn't find any shielding as such, if with that you mean anything metallic. \$\endgroup\$
    – DrD
    Commented Mar 27 at 14:12

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That's quite possibly connected to the shielding winding which is usually an unshorted copper foil or a few turns of magnet wire located between the primary and the secondy. The wire connection then goes to the mains GND / PE (protective earth).

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  • \$\begingroup\$ This makes sense. It explains what I found. It carries no current. And it offers protection in case of primary meltdown. Thank you! \$\endgroup\$
    – DrD
    Commented Mar 27 at 16:42
  • \$\begingroup\$ @DrD it does carry current as its main purpose is to help filter out the noise, specifically common-mode noise. Quite common in medical applications. \$\endgroup\$ Commented Mar 27 at 16:46
  • \$\begingroup\$ But if it is not connected to anything other than ground, can a current flow? (we're talking 50Hz here) \$\endgroup\$
    – DrD
    Commented Mar 27 at 18:02
  • \$\begingroup\$ @DrD no, if it's floating no current will flow. It normally carries a noise current which appears on both live (L) and neutral (N) w.r.t. GND (common-mode noise such as ESD or lightning or some other external source). It may also carry a fault current in case of a short between the primary and the shield windings due to an internal failure such as overheating which may lead to melting the isolating coating of the wires and finally an inevitable short. \$\endgroup\$ Commented Mar 27 at 18:44
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The ground needs to connect to the transformer's iron core. This is why it has a ring lug connector at the end.

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    \$\begingroup\$ The question is more about what this wire connects to inside the winding stack I believe. \$\endgroup\$ Commented Mar 27 at 13:30
  • \$\begingroup\$ Thanks for your comment. The ring you see goes to the metal chassis, and its corresponding connector (by the GND label) goes to the mains ground cable. My question is about where the internal wire connects to. \$\endgroup\$
    – DrD
    Commented Mar 27 at 14:15

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