[GreenKeys] low Voltage Loop Supply

[Jim Haynes]

I've seen some published circuits for a selector magnet driver that
takes advantage of the constant-current characteristics of a transistor
to make the loop current rise faster than it would with a simple
resistor.  What I've been wishing for is someone who knows how to
run one of the circuit simulation programs to see just how fast it
is, compared to the resistor.  Seems like it was said to work 
satisfactorily with a 45 volt supply.  See for example "The Modern
Teleprinter Local Loop" by Frank Merritt, QST January 1972 page 40
and feedback in QST March 1972 p. 57.

Also I was told by Walt Zenner, the late V.P. for R&D at Teletype, that
the original purpose of the holding magnet selector (he inventd it)
was to allow TWX switchboards to operate with only 48V on the cord
circuits.  There was concern about switchboard operators accidentally
touching the bare plug tips (which is hard to avoid in a manual 
switchboard) and being exposed to 120V which could be deadly.  So if
there is an experimenter among us perhaps you can take rangefinder
readings on both types of selectors when operating at reduced loop
voltages.  Of course in Walt's day they didn't have transistors, so
couldn't use the constant-current properties.

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 	"Ya can argue all ya wanna, but it's dif'rent than it was."
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 		Meredith Willson, The Music Man