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

[Bob Camp]

Hi

If you snub a loop supply you will slow things down. That's not a  
good thing since you are introducing distortion into the loop signal.  
You need to have something that will protect the switching  
transistor. You don't want to over do it.

One very simple way to do it is with a string of NE-2 bulbs. Mount  
them directly on the magnets and they make a nice light show.

Bob

On Apr 10, 2007, at 12:45 AM, Peter Gottlieb wrote:

> 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
>
>
>
> _______________________________________________
> GreenKeys mailing list
> GreenKeys at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/greenkeys
>