[GreenKeys] (Fwd) Re: RS-232 to 60mA current loop conversion f

[Howard Weeks]

I have a lot of equipment around here with MJE 340 keyer transistors. 
Mine have worked well with or without the snubber circuits and have 
for many years.  I use a diode bridge as the interface to get away 
from worrying about polarity.  I always use an opto isolator to keep 
the loop voltage off the low level circuitry.  

The MJE 340 is inexpensive and there are other more modern 
equivalents - maybe the TIP 48.  

Howard
------- Forwarded message follows -------
Date sent:      	Mon, 26 Jan 2009 11:31:03 -0500
To:             	greenkeys at mailman.qth.net
From:           	Jon Schlegel <ews265 at rochester.rr.com>
Subject:        	Re: [GreenKeys] RS-232 to 60mA current loop 
conversion for Model 15
	TTY?

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John,

That's a great Idea.  As you point out it is a bit overkill.  A 
probable red herring is that the spec seems to require a snubber 
diode to protect its output stage from loads that are inductive.  The 

diode is likely to allow magnet current to continue to flow for too 
long a time after transition to "space" (current off) and stretch the 

preceding marking pulse for too long of a time.

I have an RS-232 keying circuit I built ages ago but it's buried in 
the back of a 28RO cabinet and I can't get to it to see what I 
did.  No doubt I scribbled its design on a sheet of paper and it's 
anyone's guess where that is by now.  Actually I'm working on a new 
keyer design right now.   If I'm happy with the results, I'll pass it 

along but don't know when that might be yet.

The keying circuit itself can usually take the form of a few 
transistors along with a small handful of "pieces" parts for 
biasing/bypassing. Other than that, conventional wisdom requires the 
use of an opto-isolator to keep the loop supply out of the RS-232 
equipmemt (OUCH!).  The catch is that the output device has to 
tolerate the high voltage inductive kick when the current goes 
off.  Joe, WL7AML recently suggested using the Horizontal Output 
transistor from a TV set to handle the voltage.  Got any old TV sets?

Sorry I can't offer anything more concrete right now.

Regards,
Jon Schlegel, WA3MVM



Howard Weeks
Harlem, GA 
K5JCP