About CEMF/BEMF spikes
#12 ·
Hi,
I found a good schematic of the auto ignitioncoil.
As you can see, it is actually 1 coil.
An ignition coil is essentially an autotransformer with a high ratio of secondary to primary windings. By "Autotransformer", I mean that the primary and secondary windings are not actually separated - they share a few of the windings.
The ratio of secondary to primary turns in an ignition coil is somewhere around 100:1. The ignition coil is operated directly off a 12 volt source. However, the ignition coil does not work like an ordinary transformer. An ordinary transformer will produce output current at the same time that input current is applied. An ignition coil actually does most of its work acting as an inductor. When the ignition coil is connected to the battery, the inductor is 'charged' with current. It takes a few milliseconds for the current to build up the magnetic field - this on account of reverse voltage caused by the increase in magnetic field. During this short charging period, maybe a thousand volts are produced at the high voltage terminal, not enough to actually cause a spark.
The actual spark is generated when the breaker contacts open.
For an ideal inductor, the current and voltage relate by:
V = L dI
Where V is voltage, L is inductance (in henrys) and dI is the rate of change of the current.
Thus, seeing that L is constant for the inductor, the abrupt change in current will cause a very large voltage to be produced. This produces a very short, very high voltage spike. Of course, the change in current is on the primary side, but because the primary and secondary coils have a large mutual inductance (this is where the transformer part comes in), you get a spike on the order of 100 or more volts on the primary, and 10000 volts on the secondary. Even the primary side of the coil can give you a bit of a jolt if you hold the wires wile disconnecting power. Also note that any contacts you will use will get a lot of sparks, also on account of this.
A less theoretical way of looking at it is that disconnecting the current source will cause the magnetic field in the coil to collapse (because nothing is forcing it magnetic anymore), and the rapid change of magnetic field in turn induces a large voltage in the windings.
In an engine with four or more cylinders, the high voltage terminal of the coil is connected to the distributor, which is just a fancy high voltage rotating switch, for selecting which of the spark plugs needs to be fired. This is much cheaper than having one ignition coil for each cylinder.
Modern cars of course all use electronic ignitions instead of the unreliable mechanical breaker points points for interrupting the current to the ignition coil. However, the ignition coil still works the same way, so you should be able to use an ignition coil even from a modern car. The main thing that has changed is that the 'points' have been replaced with fancy computer gadgetry and power electronics.
One thing that cars usually have that I didn't include in the circuit is a small capacitor across the breaker points, often called the "ignitor". This capacitor in cars before electronic ignition had a tendency to fail over time, so you may have heard people refer to it. The capacitor helps to protect the points by liming voltage across the contacts, so the contacts themselves won't spark. It also forms a bit of a resonant circuit with the coil, and that way enhances the spark, although it probably cuts down on the output voltage a little bit.
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Steve
Wow, you was looking into the auto ignition atleast a week before i stumbled upon it. Thats strange isn't it.
When you said the back emf from the coil may not contain a high enough voltage to cause an arc, well it does! I'm glad to see that you stumbled upon this so much sooner than i.
Must be something in the air?