My Arc Reactor (Winding a Toroidal Inductor)

Now it’s time to use that toroidal core to construct the main inductor that will be used in the power transfer controller. I needed an inductor with 12 turns of heavy wire on it, tapped at every turn. That meant 13 evenly-spaced divisions on the toroid.

paper guide
After measuring the outer circumference of the toroid with a strip of paper, LibreOffice Draw was used to print out a precision strip with 13 divisions on it.
marking toroid
The strip was taped around the toroid, and used to mark off the 13 divisions where the turns of wire would lie.
winding completed
The winding of #12 wire is completed. It wasn’t easy, doing this by hand.
adding legs
The outside edge of each turn of wire is sanded and tinned. A “3rd Hand” jig is extremely handy for holding the legs in the right position for soldering.
The finished inductor
The finished inductor.
land pattern check
Again, LibreOffice Draw was used to draw a precision circle of 70 mm diameter. The inductor leads were pushed through this printout to verify fit. Everything seemed in order, so I could then proceed to construct a custom land pattern in DesignSpark (PCB design software).

After completing the inductor, I measured the inductance of each turn. These are raw numbers, before accounting for jig inductance:

Between pins Measured Inductance
1 – 2 4.0 μH
2 – 3 3.9 μH
3 – 4 3.9 μH
4 – 5 3.9 μH
5 – 6 4.0 μH
6 – 7 4.0 μH
7 – 8 4.0 μH
8 – 9 4.0 μH
9 – 10 4.0 μH
10 – 11 4.0 μH
11 – 12 4.0 μH
12 – 13 4.0 μH
1 – 3 8.4 μH
1 – 4 15.6 μH
1 – 5 25.8 μH
1 – 6 38.9 μH
1 – 7 55.0 μH
1 – 8 74.3 μH
1 – 9 96.5 μH
1 – 10 122.5 μH
1 – 11 152.7 μH
1 – 12 188.0 μH
1 – 13 0.234 mH

I’m pleased that the inductance between turns is fairly consistent. That will translate into substantially equal voltages across the windings, which is exactly what is needed for equalizing the voltages across supercapacitors. I feel like Tony Stark creating the first miniature arc reactor in a cave!

The 234 μH across the entire inductor may be too much for my application. That would force the supply frequency way down, and necessitate using more input capacitance, making everything bigger. I might have to drive the inductor at the 8- or 9-turn tap, instead of the full 12 turns.

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