At 08:27 PM 10/09/98 -0500, you wrote:
>>  Aw come on, do a *decent* job!  For phone circuits, use a regulator,
>>e.g. 78L08 (as well as a couple of electrolytics).
>
>A light bulb isn't noisy, is cheap, and will survive line transients. A
>kilometer of wire has some significant inductance.  I've done the constant
>current approach too, but I kept loosing reg chips.
>
>>> Some modems, including ones I've designed, need loop current to bias
>>> the diodes in their line interfaces on, otherwise the distortion is
>>> huge, and they may not work at all.
>>
>>  You've got me there!  The only diodes a modem would normally have are
>>those in the ring detector, and those are DC isolated by a capacitor
>>anyway.  You are talking of line-*powered* (PCMCIA) modems?
>
>
>We did line voltage sensing, so we had a bridge on the input. Some also use
>optoisolated interfaces (no transformer) and they also require loop current
>to run. You gotta light the LEDs with something.
>The telco never gives you a specific voltage, but they do guarantee a
>minimum 20mA, and you have to draw at least 20mA off hook to assure that
>they see you.  It's a current-oriented world.
>
>

BUZZT
On this they do! The line voltage is 48V in the specs (Telstra 1563) and it
has a minimum voltage drop (2V) to the tip and ring lines irrespective of
the exchange battery which can be in the range of 42 to 56V. As for the
current, 20mA is the nominal limit, however exchanges must operate down to
17mA (Austel TS002).
As for the current-orientated world, yes! But does your equipment work down
to 3V (This is what providers would like, so that the exchange lines can be
longer), but on this Austel TS002 says that the voltage across the equipment
should not be less than 8V.

As for the light bulb surviving line transients, well unless it is something
like a 6MW bulb, i don't think it would go past a 20KA pulse test!

As for needing voltage, this is correct, as most modems now don't have a
real transformer, and use opamps directly connected to the line (Via some
effective isolation goodies like caps etc.). Again, this depends on the
country that the device is aimed at, Germany and Australia still have some
of the toughest design rules out there


Dennis