The ignition coil should convert a primary voltage of 12 V with several Amperes of current, into a high tension of 5000 - 30.000 V with an accordingly, very much lower current. For the following explanation, change over to the picture - 'Older type'.
The plus pole on the battery is connected to the igintion switch through terminal 15 (on the left). When closing the power switch, the terminal 1 (on the right) is additionally connected to the earth, the primary circuit is now closed. In the following closing period the magnetic field is built up. It breaks down when the power switch is opened. The high tension flows through the cable 4 (center on top) and the high tension distribution to the spark plug.
In an ignition coil there is no visible movement. Here, there are two, very different windings coiled around the same iron core. The primary coil, supplied by the battery, causes changes in the iron core. It consists mainly of finned iron plates which cause less swirling currents. The plates are made up of easily changeable mini-magnets, called dipoles. These are abruptly aligned by the magnetic field. The ability to change the dipoles quickly is a special charcteristic of this type of soft iron.
The primary winding in the Motronic-ignition coil has a resistance of about 0,7 Ohms in contrast to the 8 kOhms of the secondary winding, which means a factor of more than 10.000. In this second winding, an induction tension is generated. If the magnetic field breaks down through switching off the primary current, this higher current can, e.g., in the spark plug, be compensated for by at least this factor.
The amount by which the current is increased, depends on the ratio between the primary- and the secondary windings. Of course, the expected tension increase is also amplified by the time available for charging through the primary coil and with the type or the time for the change in the magnetic flow. Thus, the generated secondary tension, e.g., becomes distinctly higher if it is connected with a spark plug in a condensed medium. Even a strong drop in the compression can be seen in the current amplitude.
The primary windings can be recognised by the distinctly thicker wire and fewer coils, the secondary windings by its thin wire and a great many coils. An ignition coil can, particularly through the primary windings, become quite hot, which is why it is more often installed around the secondary windings. Older ignition coils even have a fuse against overheating, this however, is only a protection against fire and cannot be reset. From the safety point of view, one should touch neither the primary- nor the secondary circuit of an ignition coil during operation, the danger could be fatal … 09/12
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