LED spec for phone line?

[Arfa Daily]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
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"spamtrap1888" <spamtrap1888@gmail.com> wrote in message
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On Jun 21, 7:12 pm, Robert Macy <robert.a.m...@gmail.com> wrote:
On Jun 21, 6:05 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:









vjp2...@at.BioStrategist.dot.dot.com> wrote in message

news:jrt8d5$hmp$1@reader1.panix.com...

Ok, I set up a 2p2t switch that turns off my in-home lines and
instead
goes to an
RJ11 jack. The telco says I should test this to see if the problem
is mine
or
theirs. SO far this is SOP.

So then me thinks, why should I have to scramble for a tel set to
make a
test? Why not just put an LED there? So I need a 2700 ohn resistor
in
series with the LED. I can still plug in the tel set, but the LED
will
tell
me immediately what is going on as soon as I swich from whole-house
to
test-circuit.

- = -

You would still need to watch out for the AC ringtone, as this will
try to
stick a huge reverse voltage across your LED on every half cycle. Your
resistor can be much higher than 2.7k with modern high efficiency
LEDs. I
think I would go for around 10k as the series resistor, and also put a
1N4007 diode in inverse parallel with the LED. Of course, this only
all
works at all, if your telephone operator doesn't use line polarity
reversal
for line status signalling. It's been a lot of years since I learnt
all this
stuff at college, but I seem to recall that this used to be the case -
at
least here in the UK anyway - and was the reason that there was a
bridge
rectifier in the telephone, so that the polarity inside remained the
same,
irrespective of what was going on polarity-wise on the line. Perhaps
Robert
would be more current on that.


Here in the US the telco sometimes would also do line reversal AFTER
you acquired the connection! Translates to Touch Tone Dialer didn't
work anymore! I haven't heard of it happening lately, but moot point,
every phone I've seen simply bridge rectifies and is done with it.

My 1970s WECo phone will not generate dialing tones if tip and ring
are reversed -- no rectifier there.

When I worked for the telephone circuit company, two-party lines had
tuned ringers: 20 Hz for one party and 30Hz for the other. Is that
what line reversal was used for in the UK? to distinguish between
parties? Here you had to go to single party service if you wanted
touch tone dialing.


I honestly can't remember why it was done. I never worked directly in
phone service provision, but back 40 years ago, college courses were 'well
rounded' affairs, and as well as the radio TV and electronics course that
was my primary module, we also had classes in math and telecom principles.
As an apprentice, I did a full 12 hour day at college, one day per week,
for five years ...

However, as Robert says, I seem to recall that the line reversal took
place after connection acquisition, and was for some kind of signalling
purpose. Maybe, it was used by other equipment in the chain to determine
that the line had been looped ?

I don't recall ever being taught anything about how party line routing
worked here, but it does seem as though that might be a valid way of using
line reversal signalling. There is an ex British Telecom engineer lives a
couple of doors up from me. He was an exchange engineer as far as I
recall, so I guess he should know the system backwards. Next time I see
him out in his garden, I'll ask.

Arfa

I just had a look on t'interweb and it says that in the UK, as well as in
other countries, a line polarity reversal takes place to inform caller ID
equipment that the CID data burst is the next thing that's going to be
placed on the line. Following the data burst, the AC ring voltage is placed
on the line, and everything then proceeds as expected for an incoming call.
It further said that a line polarity reversal can also be used to signal a
remote party call termination, and can be used by some PBX equipment to
initiate a cleardown.

So I would expect that there is a distinct possibility that similar
signalling is used by at least some carriers in the U.S. , which means that
the OP should definitely place his LED across a bridge, and feed the bridge
with a suitable value resistor to provide current limiting for the LED, as
well as a degree of isolation from the phone network. If he uses a high
efficiency LED and a resistor of at least 10k, all the line conditions of
idle, looped and ringing will be catered for, and as an added bonus, the
voltage level can probably be estimated by eye, as a function of how bright
the LED is.

Arfa

 

[David Lesher]

klem kedidelhopper <captainvideo462009@gmail.com> writes:

And as soon as you tell them you did that they will have justification
to blame YOU for the whole damn thing, or whatever it is that you're
complaining about. You might even find a charge for a service call on
your bill as well.

Quite True.

If you understand anything about telephone equipment, your line sits
at 48VDC when the phone in on hook, (hung up). When it rings, an AC
voltage, usually at a frequency of 20HZ is superimposed on the line.

Well, ~130VAC atop the -48V.

When you, your fax machine, or modem etc. "answers" the call your line
goes "off hook".

Detected by the line relay looking at the -48.

At this point your line is sitting at about 6VDC until you hang
up, whereupon the line returns to 48VDC.

The exact off-hook loop current is a function of the
length/gauge of the outside plant. It may be ~5-30 ma; seldom
more. Some badly designed phones/systems get upset if the loop
is too short/current high. Distorted audio is one issue.

Another is the busy-detector in some phones/Zon credit card
machines/faxes; it looks at line voltage. If they see <~35V;
they think it's busy. With short outside plant, the off-hook
voltage may only drop to say 40V. I've put 330 ohms in series
with each side as required.

An LED, (along with the appropriate series resistor) will only
draw a few milliamps from a 48V line so I don't think that it
would trigger an off hook detection, however I really don't
understand what it is you are actually trying to accomplish.

ISTM anything more than ~0.5ma may trip, or maintain, the line relay.

Another old wives tale was you should not short the pair to force a busy;
it will start a fire. Piffle. Work out the math. 0 ohms * 30 ma = 0 watts.


--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

 

[spamtrap1888]

On Aug 7, 11:08 am, David Lesher <wb8...@panix.com> wrote:

If you understand anything about telephone equipment, your line sits
at 48VDC when the phone in on hook, (hung up). When it rings, an AC
voltage, usually at a frequency of 20HZ is superimposed on the line.

Well, ~130VAC atop the -48V.

As I recall from days gone by, the ringing current waveform was 130 Vp-
p. It was also far from a perfect sine wave, so what the rms voltage
was was anybody's guess.

 

[David Lesher]

spamtrap1888 <spamtrap1888@gmail.com> writes:

On Aug 7, 11:08=A0am, David Lesher <wb8...@panix.com> wrote:

If you understand anything about telephone equipment, your line sits
at 48VDC when the phone in on hook, (hung up). When it rings, an AC
voltage, usually at a frequency of 20HZ is superimposed on the line.

Well, ~130VAC atop the -48V.


As I recall from days gone by, the ringing current waveform was 130 Vp-
p. It was also far from a perfect sine wave, so what the rms voltage
was was anybody's guess.

Depends on the source. Large CO's had motor-driven 20Hz generators.
C.P Stocker invented the SubCycle, a no-moving-parts 20 Hz generator
that replaced all but the largest m-g sets.

In any case, the ringer was not picky about waveform.




--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433