steell wrote:
I'm familiar with autotransformers, but never seen one with a single tap, maybe for special uses, don't know.
Never heard of one being used by the manufacturer in any production vehicle as an ignition coil, and I've been working on this stuff for a "long" time, electronics has just been a hobby.
The autotransformer configuration was always the standard for conventional auto ignition coils, and still is to my knowledge.
news.carjunky.com/how_stuff_works/how_ig...l_works_ic2315.shtml
An ignition coil, which is also known as a spark coil is an induction coil in an automobile’s ignition system. This particular form of the autotransformer (an electrical transformer with only one winding and it has at three electrical connection points called taps)
This design was used to keep the voltage difference between the two windings lower and reduce arcing between them:
www.burnyourbonus.info/sci.electronics.design/thread1427.html
auto ignition coil: why autotransformer?
I want to use an old automotive ignition coil to generate a spark. I'm
puzzled that the (HV) secondary is connected to the primary.
ANSWER: Yes. It minimizes the chance that the two windings will have a high potential difference. This was much more important back when insulation wasn't so great, but it works so why change it?
Here is one reference where a guy is using a standard ignition coil for a project (diagram shown, coil is clearly an autotransformer):
www.sentex.net/~mwandel/cannon/sparky.html
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.
Here's another reference:
This place shows the Ohm test for measuring the resistance of the secondary winding on a standard auto coil:
autorepair.about.com/od/troubleshooting/ss/coil-test-ohm_3.htm
It shows the meter connected from the center (HV tap) across to the +12V terminal to measure the secondary resistance. That is an autotransformer wired coil, since the LV end of the secondary winding and the end of the primary winding are tied together.
When you think about it, a standard automotive coil has only three wired terminals: +12V, LV (points), HV (spark). If the secondary was isolated from the primary, it would have to have one end of the secondary winding connected to ground somehow. That would make the coil harder to manufacture since it couldn't be wound on the center core and just slid into the case. There would have to be a wire connected to the case somehow.
So, using the autotransformer design has two advantages:
1) Less voltage difference between the primary layer winding and the secondary winding which is wound going outward from the common tap (+12V). The portions of the winding laying on top of each other are nearly the same voltage.
2) It does not require a ground connection for return current to the windings. No internal connection to the case needed (cheaper to make) and it can't fail if the ground to the case is dirty.
That's why it has always been the standard for automotive spark.
The "floating secondary winding" topology with a spark plug on each end will work in cases where two plugs are being fired at the same time. So, it can be used in some motorcycles but would not be advantagous in automotive applications.