[GreenKeys] Loop Supply

[Sam Hallas]

UEOGUY wrote:
>  Wow!  That was the logic behind the "black box", which allowed
>  conversation to a called number, between the ring cycles, by making
>  the connection, without tripping the ring!   The good old days when
>  they had Phone Police!  And every phone savvy person would disconnect
>  the ringers in phone so they could have extra extensions!  I always
>  wondered how they current limited a single phone line.  I've seen the
>  Submarine cell batteries used, the solid copper buss bars,  the
>  rotary DC generators, and solid state rectifiers used in the old No.5
>  XBAR central offices to charge the batteries, I guess it was those
>  ballast tubes, that limited the current in the line relays.  Wonder
>  what they use now? Bob
>

The usual circuit arrangement for line feeding was two 250 ohm relay 
coils, one in each leg fed from -48v and ground. On the shortest of 
lines this would limit the current to 100 mA, but a typical carbon 
microphone circuit would only draw 20-40 mA. Loop resistance might be up 
to 1000 ohms too, so even less current available. These days it's all 
electronic with limited current feeds.

Some of the early push-button phones contained a NiCd battery which was 
charged from the line, but the current had to be pretty low to avoid 
operating the line relay, say less than 10 mA.
Cheers,
Sam