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rolling your own (an adventure in coil winding) part II

Posted 28th October 2011 at 08:13 AM by wintermute

OK so I've been a bit slack getting to the second instalment. I actually finished the last coil about three weeks ago and have made the crossovers.

Tip number 1. Wear Gloves!! After the first coil my fingers were killing me. I'd pulled back the skin on my fingertips so hard that it was coming away from under the fingernails. Ouch!!! The gloves I subsequently wore were just normal rubber dish washing gloves. These worked quite well because they allowed me to put tension on the wire (holding the spool) and I no longer got sore fingers

Tip number 2. If you can make the same value with different size formers, choose the one with the lesser turns per layer. My pancake coils were much easier to make than the taller ones.

I think that the reason for tip 2 is that the more turns per layer, the more chance that small kinks in the winding wire will add up to enough to make it hard to get the last turn on for any particular layer. The coils I made with the 19mm and 21mm formers were much better then the ones with a 27mm former.

Tip number 3. make a test coil first and measure to see how close to your target you are. Make it one of the smaller values. The first coil I wound was supposed to be 200uH but was actually around 180uH.

I ended up adding 10% to the value I needed for each of the coils nominal values, and wound for that. This worked well and all coils were very close to the required value (rather than the 10% over theoretical value).

I used this calculator for working out the coils.

I had 1.6mm wire (pretty much equivalent to 14AWG) I found that I needed to add about 1/2 mm to the size of the former over the recommended size.

The original two 180uH coils that I wound, I ended up redoing because I did not have enough tension on them the first time, and they varied in value each time I measured them.

So below are pics of the resulting coils They aren't perfect but they aren't too bad either if I do say so myself at least the 330uH one

1st pic is the first coil I made. 180uH I ended up redoing those.
2nd pic is the 200uH I fixed up the wonky bit at the top a little.
3rd pic is the 2nd 330uH as you can see the practice on the first five helped
4th pic is a 90uH coil
5th pic is one of the coils soaking in varnish. It was oil based and the coils stank for at least two weeks.
6th pic is the coils in their final resting places.

The last picture shows a sanity check. I measured all of the coils using a Wallin Jig II and speaker-workshop as I didn't have an LCR meter. I started worrying that maybe my 10% out was actually a measurement error!! so I paralleled the 330uH with a 4.7uF cap (which I was confident in the measured value of) and did an impedance measurement. The result matched what I got in a spice sim of the same value components, so I was pretty happy that the values are accurate.

So that's it. I had originally planned to do this (ie wind my own coils) back in 2004 or 2005, it's only taken 7 years
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