How long can a telephone extension-cord be?

^^^^^^^^^^^^^^^^^^^^

Yup. That's what I was referring to as "Loop current is supplied through a resistance". I didn't make it clear enough that it is indeed part of the line card in the switch.

Originally that was the resistance of the relay (two balanced windings, one for tip and one for ring, each with 200 Ohms resistance) that pulsed with the loop current to detect dialed digits.

Today that is usually replaced with a current regulator, and the effective resistance is whatever is required to limit the current to whatever that particular equipment uses. Common values are everything from 25mA to 45mA.

--
Floyd L. Davidson            
Ukpeagvik (Barrow, Alaska)                         floyd@apaflo.com

And in addition to the above there is resistance in the Central Office equipment that was about 400 Ohms. Don't know what it is in modern CO equipment.

Bill K7NOM

your

volts

=

Ohms

Not only that, but that's 21 miles round trip, or 10.5 miles loop length since it's a pair. And the average phone works with much less than

12VDC. Usually they work with 6V, but a true Bell 2500 set should work down to even less. The current is typically around 30mA. and for those of you who have AoE, there is a good chart of this on page 936.

your

volts

=

Umm, that's what he just said, above (I quote):