Quoted from
Water Sparkplug - Page 73
The standard "Kettering" type ignition coil supplies
12-14v through the primary winding of the coil while the points are closed, this creates a strong magnetic field in the core of the coil, when the points open and current is abruptly removed from the primary winding,
the field collapses and generates voltages in the region of 250v to 350v. It is the voltage of the collapsing field that is transformed in the secondary winding of the coil up to
100 times.
Quoted from
http://www.jetav8r.com/Vision/Ignition/CDI.html
The coil is fed 12v to the primary winding. This in turn creates a large (enhanced by the iron rod) magnetic field which also surrounds the Secondary windings. The coil is now storing a large magnetic field (a Flux" field). When the +12v to the coil primary winding is turned off the magnetic ("flux") field inside the coil "collapses".
This causes a "Back EMF" (Electro Motive Force) current in the primary wire of about 200-300volts. THIS IS IMPORTANT. Most think the coil converts 12v to 30,000 volts. Not exactly. See,
this back EMF voltage of 300volts is now applied to both windings. When the coil collapses this rapidly changing magnetic field is also transferred to the "Secondary" windings as current (remember the discussion above about magnetism... "a changing magnetic field passing by a coil creates electric current").
The Secondary winding is 100 times longer so produces a voltage about 100 times more than the Primary during collapse. Lets do the math. The Primary ("Low Tension") wire is about 300v during the Back EMF spike. So the Secondary ("High Tension") wire is 100 x 300=30,000 volts. This high voltage is going somewhere, somehow to ground. The faster the power cutoff is in the primary, the faster the collapse, and the faster (more powerful) that spark is. So, when the points open (instantly cutting off power to the coil) 30,000 volts goes to ground from the secondary winding via the spark plug.
...
With the Kettering Induction ignition design, the coils are powered all the time at 12 volts and are commanded to collapse to spark by the ignition module. Here, the ignition module disconnects the primary winding coil ground. The coil secondary winding collapses to spark at about 30,000 volts.
In the CDI design, the coils are not powered. They receive a short high (250 volt) pulse from the ignition module and then amplify that (100:1) to a much larger voltage spike (about 40,000 volts).
Hi Sifu,
Please throw some light on me as I don't understand how "ignition coil" convert voltage. Let's say:-
1. Power input is 12vdc, coil ratio is 100:1 but the ignition coil output becomes about 24kv.
2. CDI output of 350vdc is sent to the input of ignition coil with 100:1 ratio but the ignition coil output becomes about 50kv.
Regards.
