Ignition questions & the Nat semi Injector driver

[John S Gwynne]

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| 
|    General: When the field in the primary collapses (the +12V side of the coi
| l) 
| there is a sudden induced voltage, duh... But is it true that the direction o
| f 
| the voltage is the reverse of it's initial condition? 

No. Ideally, voltage equals inductance multiplied by the time
derivative of current; regardless of the initial conditions. Assuming
one side of the coil is attached to +12VDC and a transistor of some
type drives the other side low to "charge" the coil, you will find a
positive induced voltage relative to the 12VDC return. Your diagram
has some sort of sign error.

At idle on an automotive ignition system, I would expect a positive
voltage on the primary of 40-60V during the duration of the spark.
The primary is often clamped at or near 400V to de-energize a coil
with an open secondary. You may fine it helpful to make some 
measurements of an existing electronic ignition system.

                                       John S Gwynne
                                          Gwynne.1 at osu.edu
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