Secondary winding bobbin for an ignition coil for an internal combustion engine
FIELD OF THE INVENTION 
     The present invention relates to a secondary winding bobbin for an ignition coil for an internal combustion engine, and more particularly to such a bobbin of the type that includes a tubular winding core and a plurality of fins substantially perpendicular to the axis of the core, the fins defining between them annular winding compartments of which the core forms the bottom, and each including a passage to allow the secondary winding wire to pass through in the winding direction, from the compartment located upstream of this fin to the compartment located downstream. 
     BACKGROUND OF THE INVENTION 
     Secondary bobbins of this kind are known, for example from European Patent Application EP-A 0 375 502. 
     In internal combustion engines with controlled ignition, the combustion of the gaseous mixture in the cylinder is induced by the spark that is produced between the electrodes of a spark plug. 
     In order to produce this spark, the terminals of the spark plug are connected to the ends of the secondary (high-voltage) winding of a transformer, such as an ignition coil, whose primary winding is connected to a voltage source by way of a switch, such as a transistor. 
     With this switch closed, an electric current circulates in the primary winding. If at a given moment the switch is then opened, an abrupt overvoltage occurs in the primary winding, which by induction generates a voltage spike within the secondary winding. Once this voltage reaches a sufficient value, the spark is produced, causing the combustible mixture to ignite. 
     The voltage at the terminals of the secondary winding may be as high as several tens of thousands of volts. The aforementioned fins, by defining separate winding compartments, make it possible to limit the risk of a spark developing between two turns of this winding. In fact, the voltage between the starting turn and the ending turn of the same compartment is limited to several thousand volts, in the typical case where the number of compartments is on the order of about 10. 
     Nevertheless, the beginning and end of a compartment is formed by a slot in which each of the fins define this compartment, and so the voltage between two turns located facing one another on either side of this slot in two adjacent compartments still remains on the order of several thousand volts, so that the risks that a spark may develop are nevertheless not completely averted. 
     The aforementioned document proposes to overcome this disadvantage by providing insulating compartments between the winding compartments in such a way as to increase the distance between the turns of two successive winding compartments. 
     Such an arrangement nevertheless has the disadvantage of increasing the axial size of the secondary bobbin, and consequently of the ignition coil that includes it. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome this disadvantage. 
     To that end, the subject of the invention is a secondary winding bobbin of an ignition coil for an internal combustion engine, of the type including a tubular winding core and a plurality of fins substantially perpendicular to the axis of the core, the fins defining between them annular winding compartments of which the core forms the bottom, and each including a passage to allow the secondary winding wire to pass, in the winding direction, from the compartment located upstream of this fin to the compartment located downstream, characterized in that said core has a shape such that said passage opens into the upstream compartment at a certain distance from said core and into the downstream compartment at the level of said core. 
     Hence at the level of the passage, the windings of two adjacent compartments are staggered, in such a way that it is possible to limit the voltage between two turns located facing one another. 
     In particular, the passage may be constituted by the bottom of a slot extending from the edge of the fin. 
     More particularly, the bottoms of two adjacent compartments may be radially offset from the level of said passage by a distance equal to the thickness of one winding. 
     In other words, the secondary winding wire, at the time of the passage from the upstream compartment to the downstream compartment, is routed from the upper level of the winding to a lower level. 
     In that case, only the last winding turns of the upstream compartment are located face to face with the first turns of the downstream compartment, such that there is virtually no potential difference whatever at the level of the passage slot. 
     In a particular embodiment of the invention, the core has a generally rectangular cross section, said passage being formed in an angle of the core, and the corresponding angle of the upstream compartment being cut. 
     Other geometries are possible, the essential point being that the bottoms of two adjacent compartments are not superimposed completely in projection on one another, so as to allow the passage of the wire from a certain level of the upstream compartment to the bottom of the downstream compartment. The bottoms of the compartments may in particular be identical and in projection, between two adjacent compartments, may be staggered in their plane or angularly about their axis. 
     Hence the bottoms of two adjacent compartments may be eccentrically circular or elliptical with angularly staggered long axes, or rectangular and staggered in their plane parallel to one of their sides or to their diagonal, or rectangular with their diagonals angularly staggered. 
     It may also be provided that the starting passage in a compartment be staggered angularly from its ending passage. 
     The starting and ending passages may in particular be diametrically opposed. 
    
    
     A particular embodiment of the invention will now be described by way of non-limiting example, in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a secondary bobbin according to the invention; 
     FIG. 2 is a superposition, intended to provide better comprehension of the invention, of sectional views taken along the lines IIa--IIa and IIb--IIb of FIGS. 1 and 3; 
     FIG. 3 is an axial sectional view taken along the line III--III of FIG. 2; 
     FIG. 4 is a diagram in axial view of the winding in a winding compartment; 
     FIGS. 5a-5d illustrate the passage of the wire in the successive winding compartments; and 
     FIG. 6-10 are figures similar to FIG. 2, illustrating variant embodiments. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIGS. 1-3, the secondary bobbin 1 of an ignition coil of an internal combustion is seen, formed of a tubular core 2 with an axis 3 and made in a known manner of plastic insulating material. The mounting and connection devices 4 of the bobbin are made in one piece with the core 2. 
     In addition, fins 5 perpendicular to the axis 3 define between them winding compartments 6. Each compartment 6 accordingly forms a throat in which the lateral walls 7 are formed by the two fins that define this compartment, and whose bottom 8 is formed by the core 2. 
     The winding is effected from one compartment to the next, from right to left in FIG. 1. Consequently, the fin 5b of FIG. 1 separates its upstream compartment 6a, located to its right, from its downstream compartment 6b, located to its left. 
     As can be seen in FIG. 2, the bobbin 1 and in particular its core 2 have a substantially rectangular shape. 
     This figure also shows how the passage of the winding wire from one compartment to another is done via slots 9a and 9b that extend from the edge of the fins and are substantially tangent to the core. More particularly, this figure shows the slot 9a formed in the fin 5a, to allow the entry into the compartment 6a, and the slot 9b formed in the fin 5b, to allow the passage of the wire from compartment 6a to compartment 6b. The slot 9a is tangent to the bottom 8a of compartment 6a and arrives at the level of this bottom, and similarly the slot 9b is tangent to the bottom 8b of the compartment 6b and also arrives level with this bottom. 
     It is also seen in FIG. 2 that at the end of the slot 9b for passage from compartment 6a to compartment 6b, the core bottom 8a of compartment 6a has its angle 10 cut, in such a way that at this angle 10, the end of the slot 9b extends a certain distance e from the bottom 8a of the compartment 6a. 
     FIG. 4 shows only the bottoms 8a and 8b of compartments 6a and 6b, the slots 9a and 9b, the winding 11a in compartment 6a, and the first winding turn 12 in compartment 6b. It can also be seen from this figure that the distance e equals the thickness of the winding 11a. Consequently, the first winding turn 12 in compartment 6b is face-to-face, at the level of the slot 9b, with the last winding layer in compartment 6a, such that there is no difference whatever in potential between the windings in the two compartments in the level of this slot. 
     FIGS. 5a-5d, which in simplified fashion represent sections taken along a line such as V--V of FIG. 2, illustrate the winding of a wire 20 whose diameter will be equal to the thickness of a compartment 6 and which accordingly actually schematically represents several winding layers. 
     FIG. 5a shows the wire 20 filling a compartment 6 n  and penetrating the compartment 6 n+1 , crossing the slot 9 n  formed in the fin 5 n . 
     Filling of the compartment 6 n+1  by the wire 20 is done as shown in FIG. 5b. This figure shows that no potential difference whatever exists at the level of the slot 9 n  between the winding in the compartment 6 n  and the winding in the compartment 6 n+1 . 
     When the winding in this compartment 6 n+1 , in the angular zone of the slot 9 n+1  of the fin 5 n+1 , arrives at the level of the bottom 8 n+2  of the compartment 6 n+2 , the wire 20 crosses this slot, and the winding process in the compartment 6 n+2  has begun, as shown in FIG. 5c. 
     The winding thus continues one compartment at a time, with FIG. 5d showing the winding in the next compartment 6 n+3 . 
     FIGS. 6-10 show other embodiments and are similar to FIG. 2, except that only the bottoms of the compartments have been shown, the passages from one compartment to another being illustrated by arrows. 
     In FIG. 6, the bottoms of the compartments are circular, and the bottoms 61 and 62 of upstream and downstream compartments, respectively, are staggered in their plane. The passage from the upstream compartment to the downstream compartment takes place at the level of the arrow 63, and the passage to the next following compartment is diametrically opposed. 
     In FIG. 7, the bottoms 71 and 72 of the upstream and downstream compartments form ellipses whose long axes are perpendicular, and the passage from the upstream compartment to the downstream compartment takes place at the level of one of the arrows 73 or 73&#39;. The passage to the next following compartment is offset here by 90° from the one preceding it. 
     In the case of FIG. 8, the bottoms 81 and 82 of the upstream and downstream compartments are square and are staggered in their plane parallel to one of their sides. The passage from the upstream compartment to the downstream compartment takes place at the level of the arrow 83, in the zone of the middle of the staggered sides 84. The passage to the next following compartment takes place in the zone of the side 85 opposite the side 84. 
     FIG. 9 also shows the embodiment where the bottoms 91 and 92 of the upstream and downstream compartments are square, but here, they are angularly staggered by rotation in their plane about their axis. The passage from the upstream to the downstream compartment takes place at the level of one of the arrows 93, 93&#39;, 93&#34;, 93&#34;&#39;, and the passage to the next following compartment takes place at the level of one of the arrows 94, 94&#39;, 94&#34;, 94&#34;&#39;. 
     Finally, in the embodiment of FIG. 10, the bottoms 101 and 102 of the upstream and downstream compartments, respectively, are still square but are staggered in their plane parallel to one of their diagonals. The passage from the upstream compartment to the downstream compartment takes place at the level of the arrow 103, and the passage to the next following compartment is offset by 180°.