[GreenKeys] RS-232 to 60mA current loop conversion for Model 15 TTY?

[Jon Schlegel]

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



At 11:24 AM 1/25/2009 -0800, John Nagle wrote:
>      What's considered good present technology for converting
>RS-232 levels to 60mA current loop for a Model 15?  Most
>of the current loop converters available are for 20mA loops,
>and even the ones that are rated for 60mA aren't intended
>to handle a 120VDC loop or the inductive kickback from a
>Model 15 selector electromagnet.
>
>      What about DC-DC solid state relays?  The HDD-6V15
>("http://www.power-io.com/products/hdd.htm") ought to
>work, since it can handle 600VDC at 15A with a switching
>time of 25 us, but it's overkill at 15A and overpriced at $93.
>
>                                 John Nagle
>
>
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