[GreenKeys] Two very newbie questions

[Charles]

One additional point... there is another reason for using a high voltage and 
a series resistor. You're likely familiar with the RC time constant 
dictating charging voltage across a capacitor.
There is an analogous L/R time constant for current through an inductor, and 
the L of the selector magnets is large.
So in order to have a fast rise of current, R must be large too - which 
requires a large voltage :)

(This isn't an issue on more recent TTY's like the 33. Well, it is, you 
can't change the laws of physics, but the selector magnet is driven by a 
transistor amplifier, about 500 ma at 120 volts IIRC, and accepts the loop 
standard 20/60 ma at its input).

Charles
WB3JOK/0

-----Original Message----- 
From: Harold Hallikainen
Sent: Tuesday, June 18, 2019 8:03 AM
To: Joe Herdler
Cc: GreenKeys Mailing List
Subject: Re: [GreenKeys] Two very newbie questions

An answer to one of the questions:

It's a current loop. Either 60 ma for series pulling magnets or parallel
holding magnets, or 20 mA for series holding magnets.

We make a current source by using a high voltage and a high series
resistance. As long as the "load" resistance is small compared to the
fixed series resistance, the current is approximately constant (until you
open it with a keyboard).

Mark and Space date back to the Morse telegraph, which was originally a
recording telegraph ('til they found people could read the messages by
sound, which made the receive side a lot simpler). When there was current
through the loop, the tape would be marked. When there was no current, the
tape would not be marked (hence, a space).

So loop current = mark, no loop current = space.

Good luck!

Harold
Tokyo Japan

-- 
FCC Rules Updated Daily at http://www.hallikainen.com
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