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

[gil smith]

Hi Peter:

If you have the switching transistor in the same chassis as the HV supply, 
just use a diode clamp -- anode to the MJE's collector and cathode to HV.  See:
www.baudot.net/tty-connect/tc-schematic-2of4.pdf

I tried the collector-to-base RC snubber idea and found that it distorted 
the waveform enough to lower range.  But the diode clamp works great.

gil


At 09:45 PM 4/9/2007, 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
>
>
>
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