FT82-43 Slit Toroid: Construction

The FT82-43 toroid slit easily enough, using the same diamond-wheel Sherline setup as for the smaller toroids:

FT82-43 toroid - slit

FT82-43 toroid – slit

I’m pretty sure that chip at 1 o’clock happened while it was clamped in the vise between two cardboard sheets, but I haven’t a clue as how it got that much force. In any event, that shouldn’t affect the results very much, right up until it snaps in two.

Although the current will come from a (rectified) 120 VAC source, the winding will support only as much voltage as comes from the IR drop and inductive reactance, which shouldn’t be more than a fraction of a volt. Nevertheless, I wound the core with transformer tape:

FT82-43 toroid - wrapped

FT82-43 toroid – wrapped

That’s 3M 4161-11 electrical tape (apparently out of production, but perhaps equivalent to 3M’s Super 10 tape) cut into half-foot lengths, slit to 100 mils, and wrapped ever so gently.

The thickest offering from the Big Box o’ Specialty Wire was 24 AWG, so that’s what I wound on it:

FT82-43 toroid - wound

FT82-43 toroid – wound

That’s 56 turns, which should convert 2.2 A into 1000 G (enough to max out the Hall effect sensor) and is more in keeping with 24 AWG wire’s 3.5 A current rating.

The insulated core requires just under 1 inch/turn, so figure the length at 56 inch. The wire tables show 26.2 Ω/1000 ft, so the DC winding resistance should be 120 mΩ. My desk meter has 0.1 Ω resolution, which is exactly the difference between shorted probes and probes across the coil: close enough.

The inductance is 170 µH, so the inductive reactance at 120 Hz  = 128 mΩ.

Now, for a bit of armor…




  1. #1 by Jason Doege on 2014-08-21 - 08:46

    Bah. An engineer of your calibre should have ginned up a Kelvin impedance measurement rig to get the winding resistance. Slacker! :-)

    • #2 by Ed on 2014-08-21 - 09:05

      I grovel, I abase myself, I kiss your feet, I beg your forgiveness …

      • #3 by Jason Doege on 2014-08-21 - 10:26

        Once upon a time I was using current sensing resistors in high-current situations. I was dismayed to discover that my multimeter couldn’t measure .1ohm resistances and had to do a voltage/current measurement to match resistors. I remember in a later job being highly impressed at having access to bench meters that had 4-terminal probes and could measure down to .001 ohm. That would have saved me a lot of work.

        • #4 by Ed on 2014-08-21 - 10:41

          bench meters that had 4-terminal probes

          Like the HP3455A voltmeter we had in the IBM lab, back in the day, that produced results down to 1 µV. That’s where I learned about guarding & shielding & Kelvin probes & stuff like that…