[Fwd: Re: [GreenKeys] To snub or not to snub?]

[Peter Gottlieb]

The MJE transistor is not all that fragile actually.  What is an "EMP"?

The circuit I am using is a simple current source using the MJE13005 as 
the active element.  I have an emitter resistor which has a voltage drop 
across it which reaches the level of a zener from base to ground.  Any 
more current and the transistor starts backing off from full 
conduction.  Classic textbook circuit.  To switch it all I did was 
install a 2N2222 from base to ground, when it saturates at a couple 
hundred millivolts, shorting out the zener, the MJE can never see enough 
base voltage to turn on.  I hooked this to a signal gen and it works 
nice and sharp up to a kHz, didn't try higher.  The whole circuit, 
including power resistors (surplus chassis mounted Dale aluminum ones) 
is very small.  If I had a smaller isolation transformer I could easily 
mount this entire thing into the base of the 2B but since the 
transformer is overkill I think I will make at least two RS232 to 60ma 
converters and put the thing in a separate box so I can run the 28s that 
I am getting shortly.  I have a power resistor pot for the emitter 
resistor so I can set the current to 20, or 40, or 60 or whatever in 
that range as needed.

So I'm thinking, if I don't avalanche the MJE it will run forever, but I 
don't want to slow the coil too much, so maybe a diode in series with a 
snub resistor, or might experiment with the cap instead and see if the Q 
of that RLC is high enough to cause ringing.  Shouldn't ring with the 
diode.  In any case I could allow the spike to get to say 500 volts, 
I'll have to see how bad the QRM is. 

I already wired everything with shielded cable, maybe the QRM won't be 
an issue.

Peter