[GreenKeys] low Voltage Loop Supply

[Gerry Block]

The absolute best way to do this without kluges is to built a step-up converter with a current limiter set to 60ma.  But I’ve not seen anyone do it. With modern parts it’s a piece of cake.  
Gerry Ad6mc

Sent from my iPhone

> On Mar 24, 2022, at 3:32 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
> 
> 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.
> 
>    ---
> 
>    "Ya can argue all ya wanna, but it's dif'rent than it was."
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>        Meredith Willson, The Music Man
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