Wire cross-over arrangement for angular coil assembly

A coil assembly, as for the secondary of a high voltage ignition transformer, includes a coil subdivided into segments in slots defined by radial flanges on a coil bobbin. The bobbin spindle is of a polygonal cross section, preferably having four axially-extending corners along its outer perimeter. Cross-over grooves provided in the bobbin flanges allow the coil wire transitioning from one slot to the next to be relatively isolated from contact with the multiple layers of coil-turns deposited in a slot. The angular geometry of the core spindle and the geometry and positioning of the cross-over grooves in the flanges is such as to afford relatively-easily molded grooves and wide slot widths relative to the thickness of the flanges in which the grooves are formed.

TECHNICAL FIELD 
The invention relates generally to a coil assembly, such as a transformer 
coil assembly, and more particularly to a multiple slot bobbin used in 
such coil assemblies, particularly for high voltage applications. 
BACKGROUND ART 
Electrical coil assemblies exist for numerous applications. In one 
application, that of voltage step-up or step-down, an induced voltage of 
one potential in a primary coil is stepped-up or stepped-down by a 
secondary coil as a function of the turns ratio. High voltage transformers 
of relatively small size exist both in communications equipment, such as 
televisions and the like, and in ignition systems for automobiles and the 
like. In each instance a relatively low potential applied to a primary 
coil is stepped-up to a relatively high voltage, i.e. thousands of volts, 
by a secondary winding. 
In the design of such high-voltage coil assemblies as exist in the 
secondary of an automotive ignition transformer, it is necessary to design 
against electrical shorting or arcing in order to ensure the long life and 
integrity of the coil. Because each coil-turn in the secondary develops a 
potential thereacross and because such high voltage secondaries may 
include a very large number of turns, and thus a large total potential 
across the entire coil, care must be taken to minimize electrical shorting 
between coil-turns. Normally, the wire used in winding a coil will include 
a thin, insulative coating which may be rated for several hundred volts. 
In the winding of such a coil, it is necessary that a coil-turn of a 
relatively low potential not be in contact with a coil-turn of a 
substantially higher potential. To this end, the bobbin structure for the 
secondaries of high voltage transformers have been provided with multiple 
slots which effectively provide numerous small coils of limited axial 
extent. Examples of such coil assemblies are depicted in U.S. Pat. No. 
4,274,136 to Onodera et al; U.S. Pat. No. 4,388,568 to Goseberg et al and 
PCT Application No. DE83-00184 of Worz having International Publication 
No. WO84/02224. 
By employing the winding configuration of the aforementioned patents, and 
assuming for example that each coil-turn develops a potential thereacross 
of five volts and that ten such coil-turns exist in a particular layer in 
each slot on the bobbin, then each adjacent coil-turn in a particular 
layer in a slot will differ by only five volts from that of the preceding 
or following coil-turn and the coil-turns in the layer immediately above 
or below will typically differ in potential by only about 50 volts. 
While the provision of slots in the bobbin does provide a plurality of 
coils of limited axial extent and thus limited difference in the potential 
between successive layers of coil-turns within a slot, there remains the 
possible problem that the coil wire which normally transitions or 
crosses-over from the uppermost layer of coil-turns in one slot to the 
lowermost layer of coil-turns in the next adjacent slot will be placed in 
undesirable proximity or contact with some of the radially uppermost or 
outermost turns in the second slot which have a greatly different 
electrical potential. Moreover, even in situations in which the difference 
in electrical potential is far less, the insulating coating on the coil 
wire which crosses over from one slot to the next may be subjected to 
considerable abrasion by the coil windings as they are subsequently 
formed. The aforementioned U.S. Pat. No. 4,274,136 discloses the use of 
notches or recesses in the flanges which constitute the walls to the 
successive slots. These recesses extend axially the full way through a 
respective flange and radially from the outer surface of the bobbin 
spindle to the radially outer end of the flange. While the provision of 
such recesses does enable the coil wire to transition from one slot to the 
next, it does not appear to provide particularly good separation or 
isolation of the transitioning coil wire from either the outermost 
coil-turns in the slot into which it is transitioning or the inward 
coil-turns in the slot from which it is originating. In this latter 
regard, when the coil-turns are completed in one slot and the transition 
is made from that slot to the bobbin spindle in the next adjacent slot via 
the notched recesses of U.S. Pat. No. 4,274,136, there exists the 
possibility that the winding tension on the wire will cause the 
transitioning wire to be pulled radially downward between the coil and the 
flange of the slot from which it is transitioning. In such instance, it 
will be appreciated that the aforementioned problem is again created, 
however, in this instance in a reversed manner. 
Examples of coil assemblies which do provide grooves in the flanges 
separating adjacent coil segments include U.S. Pat. No. 3,661,342 to Sears 
and U.S. Pat. No. 2,355,477 to Stahl. In U.S. Pat. No. 3,661,342 there is 
disclosed a rectangular core having rectangular flanges and a 
corresponding cross-over groove between successive slots on the bobbin. 
However, the cross-over slot is formed by a complex contouring of the 
flange which defines an "enclosed" cross-over path, such that the coil 
winding is trapped axially, or longitudinally, of the bobbin as it 
transitions from one slot to the next. 
The U.S. Pat. No. 2,355,477 discloses a multi-slot coil form of circular or 
polygonal configuration. The flanges between successive slots on the coil 
form are provided with grooves (or slots 15) in which the windings 
transitioning from one slot to the next may lie. Those grooves (slots 15) 
are directly open in an axial direction. However, the depth of those 
grooves is shallow relative to the greater axial width of the slots. 
As used in the present application, the term "slot" denotes the recess 
extending radially inward of the bobbin's perimeter and into which 
multiple coil-turns are deposited, and the term "groove" denotes the 
generally linear recess formed in a respective flange and which provides a 
cross-over path for a winding between successive slots. 
Recently, U.S. Pat. No. 4,638,282 by Ellison and assigned to United 
Technologies Automotive, Inc., the same inventor and assignee as in the 
present application, disclosed an improved wire cross-over arrangement for 
a coil assembly. In that patent a cross-over groove of relatively simple 
axially-open construction is provided in the wall of successive flanges to 
define a cross-over path for the wire transitioning from one slot to the 
next. More specifically, that patent discloses a specific geometry 
involving both the cross-over grooves and the slots for accommodating a 
"worst-case" winding condition. Specifically, the slots are relatively 
narrow and the axial depth of the cross-over grooves is sufficiently deep 
that even if the lead wire from a supply spool is in contact with the 
interior face of the opposite flange during the initial turn in a slot, 
the wire will remain substantially entirely in the cross-over groove. This 
feature is highly desirable for the purpose of minimizing or eliminating 
contact of the transitioning coil with subsequent coil turns in a slot 
which may be of relatively higher potential. For the generally circular 
bobbin geometry described in that patent, to attain the aforementioned 
characteristic while retaining the full structural integrity of the 
separating flanges it was generally necessary that the axial thickness of 
a flange be substantially as great as the width of a slot or conversely, 
that the width of a slot be relatively narrow. A shortcoming in such 
construction resides in the relatively large amount of material required 
for the flanges and the relatively small remaining space for the slots in 
which the coil turns are deposited. This may increase the overall volume 
and/or cost of the product. 
DISCLOSURE OF INVENTION 
Accordingly it is a principal object of the invention to provide a coil 
assembly, particularly though not exclusively for high voltage 
applications, which minimizes the possibility of electrical shorting or 
breakdown between coil-turns. It is a further object of the invention to 
provide an improved coil assembly of the type employing a multiple-slot 
bobbin. It is a still further object of the invention to provide an 
improved coil assembly which maximizes the slot width available for coil 
turns relative to the material required for the separating flanges, yet 
which also preserves the desirable characteristics of a simple, 
axially-open cross-over groove of a geometry which addresses the 
"worst-case" winding condition. 
According to the invention, there is provided an improved coil assembly of 
the type which includes a bobbin and a multi-turn winding of coil wire 
disposed on the bobbin. The bobbin includes a central spindle and a 
plurality of axially-spaced annular flanges extending from the spindle to 
define a plurality of respective slots between facing surfaces of adjacent 
pairs of the flanges. The winding is disposed in multiple layers in the 
respective slots on the bobbin and is continuous between successive slots 
in which it is disposed. One flange of each pair of the flanges between 
which the winding is disposed include a cross-over groove formed in the 
facing surface thereof for receiving the coil wire which transitions from 
one slot to another. The diameter of the coil wire is much less than the 
axial width of a slot. The cross-over groove is directly axially open to 
the slot into which the coil wire is transitioning and is of a width and 
depth in the respective flange such as to receive the full diameter of the 
coil wire therewithin throughout the extent of the groove while also 
preserving the separation function of the flange. This minimizes or 
prevents contact of the coil wire transitioning into a slot with the 
radially-outward winding subsequently deposited in that slot and also with 
the radially inward windings in the slot from which it is transitioning. 
The improvement specifically includes the bobbin spindle being of polygonal 
cross section having an outer perimeter which includes at least three 
axially-extending corners. Moreover, the thickness of respective flanges 
in the longitudinal, or axial, direction is less than the width of 
respective slots in that same direction, and a respective cross-over 
groove extends from a position near the radially-outer edge of the 
respective flange in an inward direction substantially tangent to the 
bobbin spindle and to a position of substantial coincidence with a corner 
of the bobbin spindle. Still further, the remaining geometry and 
positioning of the cross-over grooves relative to the geometry of the 
respective flanges is such that the coil wire necessarily lies within the 
respective grooves for substantially the full length of the grooves, even 
under so-called "worst-case" winding conditions. 
The bobbin spindle is preferably of rectangular cross section and the 
contour of the perimeter of the respective flanges conforms substantially 
to the contour of the perimeter of the bobbin spindle. Further, the 
cross-over grooves are preferably positioned to be of minimum possible 
length or extent from the perimeter of the flange to their tangency with 
the bobbin spindle at a respective corner. This is conveniently provided 
in the preferred embodiment by orienting the cross-over groove to extend 
substantially coplanar with the planar surface of the bobbin perimeter 
which extends from the corner with which the groove is coincident to the 
next adjacent corner in the direction in which the coil is wound. By 
providing a pair of such cross-over grooves, a respective one of the 
grooves being at a respective one of a pair of adjacent corners of the 
bobbin, it is possible to accommodate winding of the coil in either 
direction. Further, the transition of the coil wire from one slot into the 
groove of the next slot is guided by at least one, and possibly two, 
radially-outwardly extending ridges at the radially-outward end of a 
respective flange and disposed angularly to the side of the cross-over 
groove. 
In a further embodiment, the cross-over groove is disclosed as being 
inclined to the planar surface of the spindle, though still being 
tangential to the spindle at a corner. A pair of guide ridges are disposed 
to angularly opposite sides of the entry to the cross-over groove.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIGS. 1-4, there is illustrated a coil assembly 16 in 
accordance with a first embodiment of the invention. Coil assembly 16 may 
typically provide the secondary of a high voltage ignition transformer as 
used for automotive ignition. The coil assembly 16 may typically be 
concentrically disposed about a respective primary coil assembly (not 
shown). A laminated core (not shown) may extend through the center of the 
primary and secondary coil assemblies. In a representative embodiment, the 
potential across the terminals of the secondary coil assembly may be in 
the neighborhood of 40,000 volts. For a further understanding of the use 
of coil assembly 16 as the secondary in a high voltage automotive ignition 
transformer, reference may be made to the aforementioned U.S. Pat. No. 
4,638,282. 
The coil assembly 16 includes a bobbin 20 of insulating material on which 
is formed a multi-turn winding of insulated coil wire 22. The number of 
turns may typically be in the range of 10,000-20,000. The bobbin 20 is 
formed of a suitable rigid plastic and includes a tubular spindle portion 
24 of polygonal cross section having an outer perimeter including three or 
more corners 25 extending longitudinally, or axially, thereof. In the 
illustrated embodiment, the bobbin spindle 24 is rectangular and 
specifically, square, and thus contains four corners 25 disposed at 
90.degree. intervals about the centerline or axis of the bobbin 20. 
Because of the relatively high electrical potential associated with the 
coil assembly 16, the bobbin 20 is axially segmented by the provision of a 
series of annular, axially-spaced flanges 26 which are integral with and 
extend transversely, or radially, outward from the bobbin spindle 24 to 
form respective slots 28 therebetween. The contour of the perimeter of the 
flanges 26 corresponds with that of the perimeter of spindle 24. In the 
illustrated embodiment, the bobbin 20 may be provided with ten 
equally-sized slots 28 defined by eleven flanges 26. Approximately 1100 
turns of coil wire 22 are disposed in each of the slots 28, with a 
somewhat lesser number of turns being provided near the end and a somewhat 
greater number of turns inwardly thereof. The coil wire 22 including its 
insulating coating, may typically have a diameter of about 0.002 inch and 
the width W.sub.S of a respective slot 28 may typically be about 0.060 
inch such that approximately 30 coil-turns may be wound in a single layer 
of common radius within a particular slot. Thus, there may be 30 or more 
layers of such windings within a respective slot 28. The difference in 
electrical potentials between radially inner and outer layers of windings 
of wire 22 in a particular slot 28 may be several thousand volts. 
Each bobbin 20 is provided with a pair of tie-off terminals 30 and 32 
disposed at opposite ends of the coil assembly 16. In accordance with the 
present embodiment of the invention, the coil wire 22 may be wound on the 
bobbin 20 in either direction such that either tie-off terminal 30 or 32 
may represent the beginning of the coil winding and the other would then 
represent the end. 
In accordance with the invention, there is provided at least one cross-over 
groove 35 formed in the interior wall of one flange 26 of each pair which 
define a respective slot 28. In the embodiment depicted in FIGS. 1-4, the 
bobbin 20 is provided with two such cross-over grooves 35 and 35' 
associated with each slot 28 to facilitate winding the coil assembly 16 in 
either direction as will be hereinafter described. The cross-over grooves 
35, 35' are formed in the interior wall of each flange 26 that is followed 
by a respective slot 28. The cross-over grooves 35 are utilized during the 
winding of coil wire 22 in a clockwise direction about the bobbin 20 as 
represented in FIG. 1. Conversely, the cross-over grooves 35' are utilized 
when the coil wire 22 is wound on bobbin 20 in a counterclockwise 
direction as viewed in FIG. 1. Obviously, the coil wire may be deposited 
on the bobbin 20 by rotating the bobbin relative to the coil wire 22, in 
which case the direction of rotation of the bobbin 20 is reversed. Each 
cross-over groove 35 or 35' facilitates the transition of the coil wire 
from the top of a completed winding in a preceding slot 28 into the next 
adjacent slot 28 and more specifically, to the spindle 24 of bobbin 20 to 
begin the winding process in that slot. 
Importantly, a cross-over groove 35 or 35' is structured so as to 
facilitate the avoidance of contact of the transitioning coil wire 22 
entering a slot 28 with the later-completed coil-turns of the winding in 
that slot. This minimizes or eliminates abrasion of the insulating coating 
on the transitioning coil wire 22 and thus is desirable even for coil 
assemblies of relatively lower voltage ranges. Moreover, for a high 
voltage coil assembly it minimizes the possibility of breakdown or arcing 
in the region of the coil-turns which are positioned relatively toward the 
radially-outward end of the slot. The grooves 35, 35' have a depth D.sub.G 
(seen in FIGS. 3 and 4) in the axial direction into a flange 26 and a 
corresponding width W.sub.G (seen in FIG. 4) transverse thereto. Both the 
depth D.sub.G and the width W.sub.G of the grooves 35, 35' are sufficient 
to receive the full diameter of the coil wire 22 (0.002 inch) therewithin 
over its length. 
In accordance with the invention, the cross-over grooves 35, 35' extend 
from respective positions at the radially-outer edge of a respective 
flange 26 in a direction which is substantially tangential to the outer 
perimeter of the bobbin spindle 24. Since the outer perimeter of the 
bobbin spindle 24 includes four corners, each interconnected by a 
respective flat, or planar, surface, the cross-over grooves 35 and 35' are 
also inherently tangential with a respective corner of the spindle 24 and 
their respective innermost ends are substantially coincident with those 
same corners. Since it is desirable to avoid sharp edges at the corners 25 
of the spindle 24 to minimize stressing of the coil winding 22 and promote 
appropriate seating thereof, the corners 25 are provided with a small 
radius to facilitate the 90.degree. transition from one plane to the next. 
Thus, each of the corners 25 of the spindle 24 is represented by an 
axially-extending arcuate surface, and the cross-over grooves 35, 35' are 
tangent to a respective pair of those corners. The two cross-over grooves 
35 and 35' associated with a respective slot 28 are preferably not 
associated with the same corner, but may conveniently be associated with a 
pair of adjacent corners 25 on the spindle 24. Such arrangement of grooves 
35, 35' facilitates the molding operation, places them relatively close to 
the appropriate tie-off terminal 30 or 32 also positioned intermediate 
that particular pair of adjacent corners, and thus groups to one side of 
coil assembly 16 all parts extending outward of flange 26, thereby 
facilitating design of a housing. 
Although the cross-over grooves 35, 35' might have a uniform depth D.sub.G 
throughout their length, it will be advantageous to gradually decrease 
that depth substantially to zero in the direction toward the point of 
tangency with the bobbin spindle 24 at a respective corner 25. Such 
arrangement allows continued support of the coil wire 22 by the base of 
the cross-over groove 35 or 35' throughout its length and further ensures 
the structural integrity of the respective flange 26 in which it is 
formed. It will be noted that although the groove 35 has a substantial 
depth D.sub.G at its "entering" end, it nonetheless preserves the 
integrity of the respective flange 26. Importantly also, the base or root 
of each cross-over groove 35, 35' is sufficiently continuous along its 
length that the coil wire 22 transitioning from a preceding slot 28 to a 
following 28 may not be drawn radially-inward along the side of the 
windings in the preceding coil. 
In accordance with the invention, it is intended that the width W.sub.S of 
each slot 28 be greater than the width of axial thickness T.sub.F of the 
bounding flanges 26 in order to optimize the space available for 
depositing coil-turns and minimizing the requirement of plastic material 
for flanges 26. However, it is also important that the geometry of the 
bobbin 20 and the cross-over grooves 35, 35' therein be such as to ensure 
that the transitioning coil wire 22 resides substantially entirely within 
a cross-over groove 35 or 35' throughout its length for the reasons stated 
earlier. This characteristic must also be ensured during the so-called 
"worst-case" winding condition, depicted in FIG. 3, in which the supply of 
wire 22 to the bobbin 20 during the winding operation is longitudinally or 
axially offset relative to the bobbin to such extent that the wire is 
forced into engagement with the axially-interior wall of the opposite or 
facing flange 26 which defines that slot 28. 
It will now be noted that the length, or extent, of a cross-over groove 35 
or 35', represented by the dimension B in FIG. 3, is relatively short when 
compared with the remaining distance A, measured from the point of 
coincidence of that groove with the spindle 24 to the point of contact of 
the wire 22 with the opposite flange 26 as it is fed from an offset supply 
reel. This being the case, the depth D.sub.G of cross-over groove 35 or 
35' at its outermost end need not be particularly great in order to ensure 
that the transitioning wire 22 lies entirely within the groove to its 
point of coincidence with the spindle 24, even for a significant width 
W.sub.S to the corresponding slot 28. Because the depth D.sub.G of slot 35 
or 35' needn't be particularly great, the thickness T.sub.F of the 
corresponding flange 26 in which that groove 35 is formed may also be 
relatively small. 
As depicted in FIG. 3, it is only necessary that the fraction, or 
relationship, 
##EQU1## 
be greater than the relationship, or fraction, 
##EQU2## 
Such relationship in effect ensures that a transitioning wire 22 is forced 
to lie entirely within a groove 35, 35' throughout its length, even under 
a "worst-case" winding condition. 
To facilitate the transitioning of coil wire 22 from one slot to the next 
during the winding operation, lobes or guide ridges 60 and 60' are 
positioned adjacent the entry to cross-over grooves 35 and 35', 
respectively. The guide ridges 60 and 60' are positioned angularly, or 
circumferentially, of the flange 26 to that side of the respective groove 
35 or 35' from which a transitioning coil wire 22 is entering. The guide 
ridges 60, 60' are smoothly-contoured lobes which extend radially outward 
from a respective flange 26 and aid in guiding the coil wire 22 into a 
respective cross-over groove 35, 35'. 
Referring to FIGS. 5 and 6, the invention is depicted in the context of a 
second embodiment in which the bobbin 120 is similar to the bobbin 20 of 
FIGS. 1-4 in that it includes a spindle 124 of rectangular or square cross 
section having four corners 125 connected by respective planar surfaces as 
the perimeter. Moreover, bobbin 120 includes rectilinear flanges 126 
extending transversely, or radially, from the spindle 125 and axially 
spaced from one another to define slots 128 therebetween. Bobbin 120 does 
reveal, however, that the cross-over groove 135 formed in the axially 
inner surface of successive flanges 126 may be oriented somewhat 
differently than the cross-over groove 35 in bobbin 20. Specifically, 
while cross-over groove 135 remains oriented such that it extends tangent 
to the perimeter of the spindle 124 and is coincident therewith 
substantially at a corner 125, the groove need not extend in a direction 
which parallels one of the planar surfaces of the spindle 124. Such change 
in orientation may be desirable for the purpose of facilitating the 
transitioning entry of a coil wire 22 from one slot into the next. For 
instance, the orientation of cross-over groove 135 in FIG. 5 subjects a 
transitioning coil wire 22 to a shallower angle at the region of 
cross-over than does the earlier embodiment, thereby possibly lessening 
stress in that region. It will also be understood by reference to FIG. 5 
that the increased length of cross-over groove 135, as represented by the 
dimension B, concomitantly indicates the need for a relatively narrowed 
width to slots 128 and/or an increased thickness to flanges 126 to 
accommodate a corresponding increase in the depth of the grooves 135. 
Because of complexities introduced in the molding operation by the 
inclining of a cross-over groove 135 relative to the four normal planes of 
the rectangular spindle 124, the bobbin 120 has not been provided with a 
further cross-over groove analogous to groove 35' in FIG. 1 which would 
permit winding in the opposite direction. Further, because the entry to 
the cross-over groove 135 is relatively to the left of center as depicted 
in FIG. 5, a pair of lobes or guide ridges 160 and 162 are positioned 
adjacent the entry to the cross-over groove on angularly-opposite sides 
thereof. While guide ridge 160 is analogous to guide ridge 60 in the FIG. 
1 embodiment, the secondary guide ridge 162 is added for the purpose of 
ensuring that the transitioning coil wire 122 is caused to enter the 
cross-over groove 135 rather than being pulled relatively rightward along 
the outer periphery of a flange 126 without entering the groove 135. A 
tie-off terminal 130 is mounted on each of the end flanges 126 near the 
entry to the cross-over groove 135. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and scope of the claimed invention.