Darren Gibbs wrote:

>So the answer appears to be the diode.  When it's in parallel with  
>the transistor as the book shows, it causes the latching behavior.   
>When I put the diode in parallel with the solenoid,  no more latching.
>
>So now I know what's wrong, but not why it does what it does.  Can  
>anyone explain why the transistor would stay on because of a path for  
>current to get from E back to C?
>
First things first,  You HAVE to provide a  inductive dump path for the 
energy stored in the coil!
Think of it as a current flywheel, you get the current flowing in the 
coil to cause the solenoid action.
When you turn off the current source, the stored magnetic field MUST 
collapse and the coil becomes
a generator.  (With potentially a VERY high voltage compliance!)
The generated current is equal to the applied current , and it WILL find 
a place to go!
In your case, It found your Collector Base junction, as the voltage VERY 
rapidly climbed as the transistor turned
off, causing an avalanche.  So then it finds the Base Emitter junction 
turning the transistor ON, vola! latchup.

Put the diode across the LOAD (like you old me!) and it will supply a 
path for the load dump current.

>
>mystery half solved,
>
>darren
>
>  
>

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