The tale of two saturations...

[Peter Wales]

Love the title Frank!

I'm getting lots of theory behind ignition drivers, but not the answer to
the question. I obviously phrased the question badly so let me try again.

Firstly, I think we all agree that when a transistor is driven into
saturation it has a Vce sat voltage, size doesn't matter (so they tell me)

Secondly, when a coil has 12v across it for a significant time (say 1 sec)
it's as saturated as its going to get with that voltage across it. This is
defined by the faxt that the current is not changing and the current is
proportional to the flux.

Now, the diagram
               |
---|           |
   |           |
   |           |
   |_____|-----|
A  B     C     D

A-B is steady state, no current through the coil. B-C has the coil
"charging" and then C-D, the current is steady and no further increase in
flux will occur.

OK, the question is, what is happening at C where the voltage on the
collector WRT 0v rises by 1-2v typically a Vce sat voltage.

My original hypothesis stated that it was the Vce sat voltage, it just
didn't show until the coil saturated. Is this right, and if so, could it be
used?

Back to you guys

Peter