The tale of two saturations...

[Frank_Marrone]

On Mar 29,  3:01pm, Peter Wales wrote:
> Subject: Re: The tale of two saturations...

>
> 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?

> Peter

I'm not sure where the diagram came from but let me offer a few hypothesis...

The transistor is in saturation during charging (B to C) and the voltage is in
the .25 to .5 volt range which would be consistant for a non-darlington coil
driver.

There are two (at least) possibilities for the period from C to D...

1.  The coil has saturated and the current has risen abruptly and is limited by
a ballast resistor or the impedence of the wiring and the coil windings.  The
base drive is not large enough to keep the transistor in saturation at this
limiting current and the Vce voltage has risen accordingly.

2.  There is solid state limiting, the coil has saturated and the voltage noted
from B to C is the new Vce required to keep the current in the primary at the
specified limit value.  The Vce voltage need only rise to a value where the
remainder of the supply voltage can not induce a current in the rest of the
primary circuit above the specified limit value.


I'd like to clarify that in a typical automotive coil, not only will the coil
be as saturated as it will ever get after a 1 second application of battery
voltage but it is as saturated as it will ever get period regardless of applied
voltage.
Another way to think of inductor saturation is that it happens at a specified
volt second product (V*dt) and you can't determine if an inductor will saturate
or not without knowledge of both quantities. Purists will note that even after
the iron core has saturated there is additional energy stored and flux increase
in the air core (assuming the current is free to rise unchecked) but this is
negligeable with respect to the energy in the iron.

Correction:  There was an error in my original post  E = 1/2 * L * I^2

not E = L * I^2

sorry.



-- 
Frank Marrone at marrone at optilink.dsccc.com
1965 Sunbeam Tiger B9471116
1960 Sumbean Pintopine Series I B9009330
1966 Ford LTD 4-door family barge.