This will be an odd one to describe, but from a high power APC Symmetra UPS, these inductors are used in the AC output filter (HF sine wave). There are 3 of these per AC leg, in series, with no capacitors.

They are wound really odd, and I want to know why! (pictures below)

If you take a length of wire (about 10 feet?), feed 1/2 of the length through the core, and start winding BOTH ends away from the initial wire insertion point... BUT BOTH ENDS GO IN THE SAME DIRECTION around the toroid core... just wound in opposition (one wire leg goes CW, the other CCW).

When you finish winding (same number of turns on each wire leg), you end up with both ends of the wire arriving at the same point... and these are the terminations.

It somewhat reminds me of the Tesla Bifilar Flat coil... but on a toroid, and only one wire.

(If you're up for one more oddity, explain that 4-path small copper trace on the bottom side of the filter board - last image below. There's no connections to them)

enter image description here enter image description here enter image description here Filter board Filter board-bottom side


1 Answer 1


All the turns go thru the core in the same direction, so electrically it is just an inductor, with slightly more interwinding capacitance then an RF guy would prefer (This is a low frequency application so the extra C probably does not much matter).

It is probably done to accommodate sufficient turns while leaving a gap so the core can be epoxied to that mounting plate.

The thin traces are probably provided so that at PCB manufacture an electrical test will catch some more etching problems (These will be more sensitive to over cooking it then the big copper areas), probably something added after they had a problem with a batch.....

Regards, Dan.

  • \$\begingroup\$ Thanks, Dan... I think I let that mounting "gap" get in the way of clear thinking! The toroid winder started at one end like normal, would on "x" turns, then the core rotation stopped and reversed direction, while still winding the wire as normal. I clearly overthunk it!! \$\endgroup\$
    – ronbot
    Jun 25, 2016 at 17:08

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