Coil Application Notes
BY: T. H.
Eyerman and L. Schmitz
5-12-2003, Updated: 7-01-2012
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:
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.
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 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.
Coils intended for tuning applications require some spacing between turns to
allow for the movement needed. These so called "tuning" applications occur
when the circuit requirements call for inductance tolerances smaller that
those achieved in normal circuit manufacturing yields. Usually the tuning is
accomplished in circuit by moving the individual turns actively, while
observing the frequency response in an analyzer. Once they are "tuned" for
the desired response they are usually secured using doping compound that
does not affect inductance or Q variations.
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.
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.
Tinning leads immediately adjacent to the windings of a coil many times
results in "fusing" the first winding or two of the coil 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).
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
Most applications are wound clockwise but not always. It is important to
specify winding direction (clockwise or counter-clockwise) and not assume that
it is always clockwise.