more sat

[FMarrone at aol.com]

>I suggested that a coil would be come saturated when the 
>voltage on the collector emitter junction showed a rise to the 
>Vce sat level. Thinking about it a little more, I may be wrong, 
>or I may not be.

I think you are confused a bit (or I don't understand your 
statement).  Try and think of transistor saturation and inductor 
saturation as two separate phenomenon.  Transistor saturation 
voltage is a function of collector current and the collector current will 
rise when the inductor saturates but this is a second order thing 
and for all practical (DYI EFI) purposes the transistor 
saturation voltage is constant.

>If there is no current drawn through the collector then will 
>here be a Vce sat voltage?

If there is NO current then the voltage is indeed zero but there 
will always be some small leakage current.  If you try and 
measure the voltage the meter will supply the current.  There is 
also Icbo leakage current.

>My original contention was that the transistor goes into 
>saturation immediately (sic) on application of the drive 
>current, but the significant Vce sat voltage will not occur until 
>the coil is fully saturated and there is no change in current 
>flow. This, because the current and voltage are out
>of phase until the coil is saturated.
>
>What do you think?

Vce sat voltage is, by definition, present as soon as the 
transistor enters saturation.  Vce sat IS a function of collector 
current and will increase as the collector current increases  
BUT with adequate overdrive this increase is small.  Since 
current gain is also a function of collector current you must be 
sure to provide enough base drive for adequate overdrive at all 
currents of interest.  You ARE concerned with the Vce sat 
reliance on collector current in that you choose your device 
and heatsink for the worst case conditions expected.

Frank Marrone at fmarrone at aol.com