Wire cross-over arrangement for coil assembly

A coil assembly, as for the secondary of a high voltage ignition transformer, includes a coil subdivided into segments by radial flanges on a coil bobbin. Cross-over grooves provided in the bobbin flanges allow the coil wire transitioning from one coil segment or slot into the next to be relatively isolated from contact with coil-turns of greatly different electrical potential. The formation of the cross-over groove in a respective bobbin flange is such that the cross-over of the coil wire from one segment to the next is forced to occur and remain at a radially-outward position relative to the formed coils. Guide ridges facilitate the cross-over of the coil wire at the entry to a respective cross-over groove.

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. 
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. 
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 radially outward 
from the spindle to define a plurality of respective slots between the 
axially-inner surfaces of adjacent pairs of the flanges. The coil winding 
is disposed in multiple layers in the respective slots on the bobbin and 
is continuous between the successive slots in which it is disposed. 
The improvement specifically includes providing a cross-over groove formed 
in the axially-inner surface of one flange of each pair of flanges between 
which a winding is disposed for receiving the coil wire which transitions 
from one slot to an other. The 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. The width and depth 
of the groove in the respective flange is such that, at least toward the 
radially-outer end of the flange, the full diameter of the coil wire is 
received therein while it also preserves the separating function of the 
flange. By allowing the diameter of the wire to be fully received within 
the groove, its contact with the various layers of the coil winding in the 
slot into which it is transitioning is minimized or eliminated. Moreover, 
by preserving the separating function of the flange, as by preventing the 
groove from extending axially all the way through the flange, the wire is 
also prevented from being forced radially inward in the slot from which it 
is transitioning. 
The cross-over groove extends to a position of substantial tangential 
coincidence with the bobbin spindle and its axial depth in the wall of the 
flange gradually decreases in a direction toward the bobbin spindle; 
however, the groove depth is preferably always greater than the diameter 
of the coil wire. To ensure that the coil wire remains in the groove 
during a "worst-case" winding operation, the depth of the groove at the 
radially outer end of the flange is substantially as great as the axial 
width of the respective slot. The transition of the coil wire from one 
slot into the groove of the next slot is guided by a pair of 
radially-outwardly extending ridges at the radially-outward end of a 
respective flange and disposed on angularly opposite sides of the 
cross-over groove.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to FIG. 1, there is illustrated an automotive ignition unit 10 
comprised of twin, high voltage ignition transformers 12 and 112 contained 
within a molded plastic housing 11. Each transformer 12, 112 includes a 
respective primary assembly 14, 114, a respective secondary assemblies 16, 
116 and a respective lamination assembly 18, 118. The secondary assemblies 
16, 116 are concentrically disposed about the respective primary 
assemblies 14, 114. The lamination assemblies 18, 118 aid in magnetically 
interconnecting the respective primary and secondary assemblies 14, 16 and 
114, 116 in a known manner. In the illustrated embodiment, the primary and 
secondary windings may have a ratio of about 1:100 such that an induced 
voltage of about 400 volts in a primary coil will result in a 40,000 volt 
potential across the terminals of the respective secondary coil assembly. 
For the purposes of describing the present invention, only the secondary 
coil assembly 16 will be described in detail. To obtain the 40,000 volt 
output from the secondary coil assembly 16, the secondary 16 will be 
required to have approximately 10,000 coil-turns. Accordingly, referring 
additionally to FIGS. 2 and 3, the coil assembly 16 includes a bobbin 20 
of insulating material on which is formed a 10,000-turn winding of 
insulated coil wire 22. The bobbin 20 is formed of a suitable rigid 
plastic and includes an annular spindle portion 24 having a radially-outer 
surface on which the coil wire 22 is wound. 
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-shaped flanges 26 which are integral with and 
extend radially outward from the bobbin spindle 24 to form respective 
slots 28 therebetween. In the illustrated embodiment, the bobbin 20 is 
provided with ten equally-sized slots 28 defined by eleven flanges 26. 
Approximately 1,000 turns of coil wire 22 are disposed in each of the ten 
slots 28 to provide a total of 10,000 turns. 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 0.050 
inch such that approximately 25 coil-turns may be wound in a single layer 
of common radius within a particular slot. Thus, there may be 
approximately 40 layers of such windings within a respective slot 28. Each 
coil-turn may represent an associated difference in electrical potential 
of approximately four volts, with a particular layer within a slot 28 
representing a difference in electrical potential of approximately 100 
volts. Obviously then, the difference in electrical potentials between 
radially inner and outer layers of windings of wire 22 in a particular 
slot 28 may be approximately 4,000 volts. 
Each bobbin 20 is provided with a pair of tie-off tees 30 and 32, disposed 
at opposite ends of the coil assembly 16. With reference to the sequence 
with which the coil wire 22 is wound on the bobbin 20, the tie-off tee 30 
seen uppermost in FIG. 2 represents the beginning or start of the winding 
of wire 22 and the tie-off tee 32 represents the end. Thus, the end 22a of 
coil wire 22 is tied to tie-off tee 30 to begin the coil winding process 
and the end 22b is finally tied to the tie-off tee 32 to complete that 
process. 
In accordance with the invention, a cross-over groove 35 is formed in the 
interior wall of one of each pair of flanges 26 which define a respective 
slot 28. More specifically, a cross-over groove 35 is formed in the 
interior wall of each flange 26 that is followed by a respective slot 28, 
starting with the end of bobbin 20 at which the first coil segment is 
wound, as represented by the tie-off tee 30. Each cross-over groove 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 is structured so as to facilitate the 
avoidance of contact of the transitioning coil wire 22 entering a slot 28 
to begin a winding on spindle 24 with the later-completed coil-turns of 
the winding. 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 groove 35 has a depth D.sub.G (seen 
in FIG. 4) in the axial direction into a flange 26 and a corresponding 
width W.sub.G transverse thereto. Both the depth D.sub.G and the width 
W.sub.G of the groove 35 are sufficient to receive the full diameter of 
the coil wire 22 (0.002 inch) therewithin over its length. The cross-over 
groove 35 extends from a position at the radially outer edge of a flange 
26 in a direction which is substantially tangential to the outer surface 
of the bobbin spindle 24. The groove 35 extends at least to a point of 
tangency with the bobbin spindle 24 and typically, a small distance 
beyond. 
As a further feature of the invention, the radially-outermost or "entering" 
end of cross-over groove 35 has a depth D.sub.G which has approximately a 
1:1 correlation with the width W.sub.S of a corresponding slot 28, such 
that the coil wire 22 lies within the groove 35 over its full length even 
if the lead of the wire from a supply spool is in contact with the 
interior face of the opposite flange 26 as depicted in broken line in FIG. 
4. Such situation might be characterized as a "worst case" winding 
condition and the objects of the invention are met by providing such 
adequate depth to the "entering" end of the cross-over groove 35. 
Although the cross-over groove 35 might have a uniform depth D.sub.G 
throughout its 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. Such arrangement allows continued support of 
the coil wire 22 by the base of the cross-over groove 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 is sufficiently continuous along 
its length that the coil wire 22 transitioning from a preceding slot 28 to 
a following slot 28 may not be drawn radially inward along the side of the 
winding in the preceding coil. 
As represented in FIG. 3, a supply spool 50 of coil wire 22 is represented 
in broken line for supplying coil wire for the winding of the coil 
assembly 16. The bobbin 20 is rotated in the direction indicated by the 
arrow 52 depicted thereon and begins at the far end represented by tie-off 
tee 30 and proceeds toward the near end represented by tie-off tee 32. 
Viewing FIGS. 4 and 5, the winding of bobbin 20 proceeds from left to 
right such that as shown therein, when a winding is completed in one slot 
28 the coil wire 22 is led, via a cross-over groove 35 to the next 
adjacent slot therebelow. To further facilitate the transitioning of coil 
wire 22 from one slot to the next during the winding operation, pairs of 
lobes or guide ridges 60 and 62 are positioned adjacent the entry to 
cross-over groove 35 on angularly-opposite sides thereof. The guide ridges 
60, 62 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. 
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.