[GreenKeys] low Voltage Loop Supply

[Gerry Block]

... to exemplify in the days when I had machines operating my loop supplies were 140v hp lab supplies with the current limiters turned down to 60.  The supply would sit at 140v (the “compliance voltage”) and during Mark the current would instantly rise to 60ma (the supply now in cc mode) and the output voltage would go to something like 12v with one selector magnet.  Today   this can all be done with a single ic and no power resistor would be required.  It would be mighty small. 
Gerry

Sent from my iPhone

> On Mar 24, 2022, at 3:53 PM, Gerry Block <gblock at sbcglobal.net> wrote:
> 
> 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."
>>   "No it ain't! No it ain't!  But ya gotta know the territory."
>>       Meredith Willson, The Music Man
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