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The 125kHz RFID scanner / reader device I have, has a coil like this:

enter image description here

NB> Don't have a good camera handy at the moment, so had to share a cropped image from Alibaba, but the pic is very close to the one I have, mine being slightly thicker.

Now the problem is that this RFID reader has seen some abuse, and on opening it up, I figured that the 2 coil ends that were soldered to the PCB have snapped. While soldering them back, I have unfortunately broken the thin copper wires bit more, such that, I have very little free length remaining to solder them to the board.

Question now is -

  1. If I undo one round of the coil (which BTW, seems pretty difficult, as the thin wires are stuck together pretty hard), and solder the leads back on PCB, would the reduced coil turn impact functioning of the RFID reader ?

  2. Alternatively, if I solder 2 offboard wires (26AWG single strand copper) to the remaining tiny lengths of leads, and then solder those offboard wires back to PCBA, can it impact the functioning of the RFID reader ?

  3. Are these thin copper wires insulated from each other by something like transparent lacquer / clear varnish ? If so, while soldering, I believe I might burn-off some of it, so can I use standard clear-varnish available at hardware stores (for woodworking), to restore the insulation layer ?

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    \$\begingroup\$ If you pull out some wire to get more length, run the new wire along the coil to where it used to leave from. The thin wires are insulated. As long as your new wire is insulated it will be ok in contact with the coil. Being thicker will not make much difference. \$\endgroup\$ – Russell McMahon Feb 4 '13 at 7:18
  • \$\begingroup\$ Thanks @RussellMcMahon, that's a very useful suggestion, and I might try it. BTW, does the shape of the coil have anything to do with "tuning". \$\endgroup\$ – icarus74 Feb 4 '13 at 13:48
  • \$\begingroup\$ Found this excellent answer explaining the shape of RFID antennae. \$\endgroup\$ – icarus74 Feb 4 '13 at 13:50
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  1. If you unwind one turn of the coil, you will detune the antenna. At 125 kHz, the coil should have many turns and maybe this won't throw it off so far that it doesn't work. But if you have two broken ends, you would be tempted to remove two turns. That would be even worse. You might be able to add some capacitance to bring it back in tune.

  2. Reattaching the wires by, in effect, splicing in some more wire, will work, especially at this low frequency. There are some mechanical issues with those #26 wires; they will seem huge and will want to pull out the tiny wires, but electrically, it's sound. It's the approach I would take, especially if I really just wanted to see if that really was the problem in the first place.

  3. Some types of insulation will burn off so you can solder, but many times it won't, and then you have to scrape the wire. That's a challenge in itself. I would not bother trying to apply more insulation unless I didn't think I could prevent the wires from touching.

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  • \$\begingroup\$ Thanks for the very pertinent answers. I'll probably take approach #2 as it clearly sounds safer. I will try to find some thinner wire, but AWG26 is handy so I mentioned that. As for the insulation thingy, the only thing that is handy (and comes to my mind) is standard electrical insulation tape, or a piece of soft rubber tape. My worry about insulation was however about the possibility of the the thin copper wire from the coil, which've broken quite close together, and soldered quite close together as well. \$\endgroup\$ – icarus74 Feb 4 '13 at 5:08
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    \$\begingroup\$ It's OK to pull some wire free of the coil, as long as you don't remove a turn. The wire you add in will take the place of the length that broke off. \$\endgroup\$ – gbarry Feb 4 '13 at 5:17

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