Toroids with many turns of secondary winding are very useful for AC current sensing and measurement because you can just pass the wire carrying the current to be measured thru them. (We’re not talking about coils with a couple of wimpy turns for RF – though those can be a good source of cores.)
But unless you can find a good, cheap source of well-wound toroids, you’re faced with the really unpleasant task of threading hundreds of turns of fine wire through that core. (Well, unpleasant unless you have one of these cool toroid winding machines. But I don’t.)
After I finally figured out how the machines work, I thought about building one. But there are too many strange and precision parts, especially if you want to wind small cores – which I do. So how do you quickly and simply wind wire on a closed loop toroid?
Well, duh – you cheat. If it’s good enough for Captain Kirk… The idea is to cleanly split the toroid in half, wind it, then put it back together. Yeah, it’s magnetically not quite as good after you glue it back together, but it’s good enough.
Fortunately, a ferrite core is kind of brittle, and if you score it nicely, it will often break fairly cleanly. I’ve tried a couple of different ways to score them, with varying results. The first was to just saw halfway thru before cracking it. (The material cuts very nicely with a hacksaw.) That approach provided a nice break, but lost so much material that it really felt like performance of the core would be compromised. (I never bothered to wind it.)
In the spirit of scoring plastic, I tried a Stanley knife. It felt like it didn’t go deep enough to guide the break as much as I wanted. A triangular file didn’t have any trouble cutting the material, but made a wider score than I wanted. In any event, scoring the outside (and inside!) edges, as well as the flat side might help guide the break better.
The material’s clean breaks make super glue appropriate. By clamping the pieces together tightly, the gap should be small enough to not hurt the magnetic properties too much. When there are chips, it may be easier to glue the chips back carefully as a separate step before you put the two halves back together. If there are chips missing, abort and try a new core. An air gap in the core is exactly what you DON’T want.
While you could wind the secondary by hand, my strong first choice would be to let a drill or something turn the core while I guide the wire. We’ll need to attach some kind of spindle to the core to do that. Most of the winding will be about midway between the two breaks, so that part should be on the centerline of the spindle to minimize wobbling.
You could use a dowel nicely shaped concave to fit up against the core as the spindle, but the bottom would probably (underlap?) interfere with some of the windings. Some copper or aluminum tubing lets you take advantage of the part of the core that won’t get any wire as a glue surface. If you handle it gently while winding, hot melt is a great way to hold the spindle to the core half temporarily.
It’s nice to know about how many turns you’ve put on so you can confidently make an identical one or vary the number of turns as you experiment. Having a helper to count the turns is convenient. Another, probably less accurate approach is using an estimate of the average diameter of one turn of the winding and the desired turn count to compute how much wire it will take. Pre-measure the wire, and when you run out, you’re done. In any event, chuck the spindle in a drill, tie off some wire so you’ll have a nice lead to work with and start winding! Thanks to Ti Leggett at Workshop 88 for the video. It’s also posted on the W88 blog.
Unlike with some fussy RF coils where you have to worry about losses distributed through the bulk of the core, it doesn’t really matter where on the core your secondary windings are. Having them all bunched up together is fine. But if you’d like to spread them out a little more (perhaps to leave more room for the primary wire in a very small core), it seems like if the drill/whatever is turning slowly enough, you should be able to move your wire-guiding hand back and forth in sync with the rotation such that the extended edges of the winding were neatly wrapped square around the core like the ones in the middle. I’ve tried it, but with only very limited success.
Putting it back together
Super glue seems good because the breaks are clean and it introduces such a minimal gap at the joints. Here’s a 1″ O.D. core wound with 200 turns of #30 magnet wire and glued back together. Something like epoxy, that provides filler and some thickness to the glue joint would require that you clamp the core halves together quite tightly to squeeze out as much glue as possible.
If you’d like a little more physical strength and robustness – perhaps for a current transformer you were going to put around the main AC feed wires in your breaker panel – one possible approach would be to include a narrow zip tie on the outside of the core under the winding before you wind it. After you glue the core halves together, bring the ends of the zip tie around the top of the core and pull it tight. If you were quick, it could even provide clamping while the glue dried. (Sorry – no picture. I’ve never tried it.)
You must have had some reason to wind that coil in the first place. If it’s for a current transformer to measure AC current, you’ll need to terminate the secondary in a very low resistance, and figure some way to measure the secondary current. You’ll probably also need to do some calculations (or at least testing) based on the material and dimensions of the core to see if it will stay linear (not saturate the core) over the current range of interest.
If you just need to sense AC current – for some kind of logging or perhaps automatically turning one device on when another is on, you can probably get away with most any small core. Here’s a note on how I built an interface to an Arduino.
And if you wound wire on both halves of the core, you could make a more traditional transformer. I’ve never tried that.
Good luck with your new toroid!
Update 2/14/13: The W88 blog post made it to Hackaday! The commentors there always keep people honest, and this was no exception. One poster said he had broken cores several times, and there was no way to get the core back together tight enough. He indicated his cores lost an order of magnitude in inductance! I had no quantitative handle how much the gluing cost, but I didn’t think it would be that much. Time to try to measure the reduction I’m seeing.
I hand wound exactly 30 turns on one of the same cores (without breaking it first). I tried a couple of indirect ways to measure the inductance, since I don’t have an inductance meter. (I do have a capacitance meter, and tried using that both by switching the test leads and even by rotating the meter 180 degrees, but neither one worked.)
Extrapolating from the 30- to the 200-turn coil
The first thought was to measure the inductance of the intact 30 turn test toroid and compare it to the 200 turn glued-together toroid. Since inductance is proportional to the square of the number of turns, I would expect the inductance of the big coil to be about (200/30)²=44.4 time that of the small coil.
I tried making a tank with a 47 nF cap in parallel with the coil, driving it with my dear old (old being an operative term) Southwest Technical Products function generator and looking at it with a scope. (The 22 Ω across the generator output was to lower the output impedance from I think 75 Ω.) I was going to use the known cap and the resonant frequencies to compute and compare the inductances of the large and small coils. I aborted this after I realized this added a new variable of frequency, and I had no idea how the toroid material behavior changed with frequency.
To eliminate the frequency variable, I found resonance with the large coil and 1 μF at just about 2 KHz. I changed to the small coil and tried different cap values until I got resonance at 2 KHz again. That was adding a 10 μF electrolytic across the 1 μF film cap for around 11 μF. Plugging those values into a nice LC resonance calculator gives inductances of 570 μH for the small coil and 6300 for the large one. That’s hardly the expected factor of 44, and using the test of a factor of √10, is in fact (barely) an order of magnitude off. But the methodology was poor: Twisting a knob and looking for a peak magnitude on a scope for a low-Q circuit is hardly precise. And electrolytic caps have notoriously inaccurate values at best, and using them at zero volts is completely out of spec. Next approach?
Apples and apples
The best approach would be to get something with the 30 turn coil I could measure with a decent meter, break and glue it, and measure again. I put a nice carbon resistor in series with the coil, cranked the signal generator to 1 KHz, and measured the voltage across the resistor and the coil with a multimeter whose AC ranges were known good for that frequency.
I scored the coil with a jeweler’s file and broke it. The break was one of the cleanest I’ve had (fortunately). I used some old (read: thicker than expected) no name cyanoacrylate, wiggled the pieces to be sure they seated as well as possible, squeezed hard and put a spring clamp on it. Then I put it back in the test circuit.
The break/glue happened over maybe 5 minutes, and the test setup was untouched, including the signal generator. I measured the voltages across the resistor and coil again. (Twice for each, as I had done the first time.) This is about as clean a comparison as I can make. So at 1 KHz with a 22 Ω resistor here are the results:
Vres Vcoil Zcoil L Before 0.367 0.097 5.81Ω 925μH After 0.372 0.068 4.02Ω 640μH
The inductance is reduced by the gluing by about 30%. Quite noticeable, but quite acceptable for a hack.
I also did measurements of the tank circuit resonant frequency with the parallel combo of 10 μF electrolytic + 1 μF film caps before and after the break. While there is a frequency difference, I’d guess it doesn’t contribute a large error.
freq L Before 2.0 KHz 576μH After 2.7 KHz 316μH
That test indicates a 45% decrease in inductance. Not identical, but in the same ball park. Now we have a rough quantitative handle on the impact of break/glue.