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

[Jeffrey Angus]

John wrote:
>      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.

Those are usually available in a variety of surplus stores at a
reasonable price. 

However, I was using a whole bunch of them to control a LOT
of heaters on a synthetic petroleum project in Bakersfield. And
we STILL had to add snubbers across the outputs to keep 
from killing them switching resistive heater loads at 240 and 
440 volts at 15-30 amps depending on which heaters we were
controlling.

I'm thinking the best solution for switching a loop supply is a
high speed relay. With some light snubbing across the contacts.

My memory is a bit fuzzy now, it's been 35 years, but I remember
building a loop keyer using an SCR and relying on the inductive
kick back to turn the SCR back off. 

I believe I copied the design out of the GE SCR manual at the 
time.

Jeff-1.0
wa6fwi