Air Wound Coil Application Notes

By: T. H. Eyerman and L. Schmitz

Date: 5-12-2003, Revised 6-13-2008

PREFACE: Over a period of several decades of talking to many users and designers of wound coils we have come to the conclusion that there are a number of aspects about these simple devices that are understood differently from person to person. We are therefore presenting the following to clarify these points:

1. NUMBER OF TURNS:  Counting turns, as simple as it seems, differs from person to person. One definition is that the number of times that the wire crosses the form or mandrel that it is being wound on is the number of turns. The other method is to count the number of turns "showing" on the top of the finished coil. Please note that the number of turns showing on the bottom of the finished coil is not always equal to the number of turns showing on the top. The counting method must be specified.  
2. INSIDE DIAMETER: This applies primarily for air wound coils. The question here relates to the form or mandrel diameter the coil is being wound on v/s the finished coil inside diameter. When winding heavier wire gages on larger mandrels or forms many times a phenomenon many call "spring out" occurs. This is also influenced by larger numbers of turns coupled with heavier wire gauges and larger diameters. More turns under these conditions result in more "spring out". This results in a larger inside diameter than the mandrel or form that the coil was wound on. Of course, this changes the coil inductance as well. If the coil design assumes no, so-called, "spring out" then compensations must be made to allow for this phenomenon.  The finished dimension needs to be specified.
3. SPACING: The spacing between turns is also influenced by the so-called "spring out" phenomenon described above. When winding heavier gauge wires on larger diameter forms or mandrels the "spring out" also manifests itself in larger spacing between turns. If the coil design assumes no such "spring out" compensations for the resulting increased spacing must be made.
4. TUNING: Coils intended for tuning applications require some spacing between turns to allow for the movement needed. When the turns are "touching" they are assumed to be "close wound" and turning is either very difficult or impossible. This spacing may be small to allow for some tuning. Of course the wider the spacing the greater the tuning range. The coil design should allow for any significant spacing between turns in order to maintain the desired nominal inductance.
5. LEADS: The leads on wound coils are part of the device. They contribute to the overall inductance of the part. Consideration must be made for the lead contribution when designing the coil.
6. TINNING: Tinning leads immediately adjacent to the windings of a coil many times results in "fusing" the first winding or two of the coil together. Essentially the coating on the wire is what is being fused together. The leads are a very good thermal conductor. Of course, once these turns are "fused" together they can no longer be tuned. Cleaning parts that are "fused" together is not a solution. The more mass the greater the problem (bigger wire gauge).
7. SOLDER TEMPERATURE: Many applications call for high temperature solder for lead tinning because the resulting device will be subsequently wave soldered and reflow of the lead could be a problem. Using high temperature solder on small gauge coils presents another problem in that leads could be compromised (at the least will be difficult to solder at these temperatures).
8. WINDING DIRECTION: Most applications are wound clockwise but not always. It is important to specify winding direction (clockwise or counter-clockwise or anti-clockwise) and not assume that it is always clockwise.