[sci.electronics] Drawing DC phone current

raoul@eplunix.UUCP (Nico Garcia) (05/28/91)

In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes:
> Draw less than 4mA from the loop and you will not have any 
> problems.

Only works in the off-hook position, Tom. A standard phone is about 600
ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA:
the standards are deliberately extremely broad. It's not clear at all to me
that 4 mA will not maintain the "off-hook" status in the phone company's
circuits. However, the phone company does *not* want DC paths available for
the on-hook state. 

In fact, let me go try this (putting a 2700 ohm resistor across my office
phone line). Hmmm: goes off-hook as the voltage drops to 18 volts. Maybe my
line is picky, but forcing it off-hook is not what *I* want from an
in-use detector.

-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu

stevem@specialix.co.uk (Steven Murray) (05/29/91)

raoul@eplunix.UUCP (Nico Garcia) writes:

>In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes:
>> Draw less than 4mA from the loop and you will not have any 
>> problems.

>Only works in the off-hook position, Tom. A standard phone is about 600
>ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA:
>the standards are deliberately extremely broad. It's not clear at all to me
>that 4 mA will not maintain the "off-hook" status in the phone company's
>circuits. However, the phone company does *not* want DC paths available for
>the on-hook state. 

I'm sure it is true the phone company does not want the DC path - nevertheless
they do allow for some leakage across the circuit.  The original poster was
about right I think.

>In fact, let me go try this (putting a 2700 ohm resistor across my office
>phone line). Hmmm: goes off-hook as the voltage drops to 18 volts. Maybe my
>line is picky, but forcing it off-hook is not what *I* want from an
>in-use detector.

You used 2700 ohms - but the off-hook voltage was something like 18 volts -
thats not 4 milliamps.   Just leave the 4 milliamps bit for a moment, try
a resistor of value 22k.  Its still not 'proper' but I think you will find
it works.  I'm not suggesting you do this on anything but your own private
telephone exchange :-)
 

>-- 
>			Nico Garcia
>			Designs by Geniuses for use by Idiots
>			eplunix!cirl!raoul@eddie.mit.edu

-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
I am speaking, but  | If these are your opinions, then we are in agreement!!
not for my employer.| Flames, spelling errors, complaints > /dev/null

raoul@eplunix.UUCP (Nico Garcia) (05/30/91)

In article <1991May29.084859.5324@specialix.co.uk>, stevem@specialix.co.uk (Steven Murray) writes:
> You used 2700 ohms - but the off-hook voltage was something like 18 volts -
> thats not 4 milliamps.   Just leave the 4 milliamps bit for a moment, try
> a resistor of value 22k.  Its still not 'proper' but I think you will find
> it works.  I'm not suggesting you do this on anything but your own private
> telephone exchange :-)

Yeah, OK. I used 2700 ohms to mimic 10 Volts (what folks refer to as
"off-hook" voltage, although this is not correct) delivering only 4 mA, and
to see if it would force an off-hook condition. It did so. Voltage in the
off-hook state depends on voltage drop from your telco, and their line
voltage. 22 kohm might work at one home (2 mA at -48 Volts worst case in the
States' phone system), but not in another, due to a lower impedance from the
telco permitting more current. And what if you put one on every phone, and
have ~5 phones? All of a sudden you`re drawing 10 mA. Enough for off-hook
detection, I'm certain.

Typical operating current, according to my copy of "Understanding Telephone
Electronics", is 20 to 120 mA. That doesn not mean that 4 mA or less won't
trigger off-hook or maintain it. Doesn't mean it will: I believe the original
poster that it didn't happen for 4 mA, on *his* line. Also, the telco has to
provide that current. I can only assume they won't notice half a dozen users
drawing it.  Widespread use, however, and I'll *bet* they notice such a
constant drain of a few percent of the active load. And they could start
pulling phone service because of it, until it stops.

It's kind of like operating things beyond spec: I don't like relying on such
behavior, or designing circuits that do so. It feels like really bad design
to me. This is a common example of every new gadgetmeister solving the
problem, and most of them doing it *badly* because of subtleties they don't
think about.
-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu

grayt@Software.Mitel.COM (Tom Gray) (05/30/91)

In article <1991May29.084859.5324@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes:
>raoul@eplunix.UUCP (Nico Garcia) writes:
>
>>In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes:
>>> Draw less than 4mA from the loop and you will not have any 
>>> problems.
>
>>Only works in the off-hook position, Tom. A standard phone is about 600
>>ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA:
>>the standards are deliberately extremely broad. It's not clear at all to me
>>that 4 mA will not maintain the "off-hook" status in the phone company's
>>circuits. However, the phone company does *not* want DC paths available for
>>the on-hook state. 
>

 Telephone line circuit switch hook detectors are designed to operate
at currents ABOVE 12mA. They are also designed NOT TO operate at
currents LESS than 7mA. 

No problem will result if you draw less than the 7mA from the loop.







=================================================================
Tom Gray



=====================================================================

stevem@specialix.co.uk (Steven Murray) (05/31/91)

rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:

>In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:
>+---------------
>|  Telephone line circuit switch hook detectors are designed to operate
>| at currents ABOVE 12mA. They are also designed NOT TO operate at
>| currents LESS than 7mA. 
>| 
>| No problem will result if you draw less than the 7mA from the loop.
>+---------------

>No *immediate* operational problem... However, in the nightly automatic test
>scans [forgot the Telco acronym], your line will be flagged as "leaky", and
>may eventually spit out a trouble ticket. When they figure out the "leak" is
>inside your house, they'll send you the "repair bill" for a "false service
>call due to customer premises equipment".

Hey! Thats neat! Do you know what current the test flags as a leak?

-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
I am speaking, but  | If these are your opinions, then we are in agreement!!
not for my employer.| Flames, spelling errors, complaints > /dev/null

gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR) (06/01/91)

>>In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:
>>| No problem will result if you draw less than the 7mA from the loop.
>
>>No *immediate* operational problem... However, in the nightly automatic test
>>scans [forgot the Telco acronym], your line will be flagged as "leaky", and
>>may eventually spit out a trouble ticket. When they figure out the "leak" is
>>inside your house, they'll send you the "repair bill" for a "false service
>>call due to customer premises equipment".

Everyone interested in this thread should look at (Usenet) Comp.dcom.telecom,
which (among other things) discusses the nuts and bolts of telephony.

(foggy memory alert...) I think the leakage resistance of a (legal) on-hook
unit with a REN (ringer equivalence) of 1.0 is 22 Megohms.  That means a leakage
current of only a couple of >micro< amperes.  There's an FCC spec & everything.
This requirement makes a line-powered off-hook light much more interesting.

I have seen a phone alleged to have been FCC registered which has a line
powered off-hook light; it seems that they cheat a little.  When another
instrument goes off-hook, what looks like a CMOS comparator turns on the
light.  At this point the lighted instrument is off-hook.  The tricky part
is shutdown when all the other instruments go on-hook.

	geoff steckel (gwes@wjh12.harvard.EDU)
			(...!husc6!wjh12!omnivore!gws)
Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line.
This posting is entirely the author's responsibility.

jon_sree@world.std.com (Jon Sreekanth) (06/01/91)

In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:


    Telephone line circuit switch hook detectors are designed to operate
   at currents ABOVE 12mA. They are also designed NOT TO operate at
   currents LESS than 7mA. 

   No problem will result if you draw less than the 7mA from the loop.

Even if the hook detector is not confused by < 7 mA, won't that 
trigger some automated test equipment that's supposedly looking for
leaky (hence faulty) lines ? 

/ Jon Sreekanth

Assabet Valley Microsystems			Fax and PC products
346 Lincoln St #722, Marlboro, MA 01752		508-562-0722
jon_sree@world.std.com

gbell@sdcc13.ucsd.edu (Greg Bell) (06/02/91)

In article <1991May29.084859.5324@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes:
>
>You used 2700 ohms - but the off-hook voltage was something like 18 volts -
>thats not 4 milliamps.   Just leave the 4 milliamps bit for a moment, try
>a resistor of value 22k.  Its still not 'proper' but I think you will find
>it works.  I'm not suggesting you do this on anything but your own private
>telephone exchange :-)
> 
>

Never mind the fact that if you design something that places a
resistive load across the phone line, and later your phone rings,
the current you're drawing jumps because the phone line ring voltage
is higher than the on-hook voltage.  So, your resistor answers the
phone on the first sign of a ring, then drops it...



-- 
-----------------------------------------------------------------------------
  Who:  Greg Bell                            Address:  gbell@ucsd.edu
 What:  EE hobbyist and major                  Where:  UC San Diego
-----------------------------------------------------------------------------

wolff@duteca (Roger Wolff) (06/02/91)

jon_sree@world.std.com (Jon Sreekanth) writes:

>Even if the hook detector is not confused by < 7 mA, won't that 
>trigger some automated test equipment that's supposedly looking for
>leaky (hence faulty) lines ? 


I don't think so. We had a full "a" to "ground" short that they
didn't come and fix before we called and told them about it. Guess
what? They didn't beleive that it was their cable! Anyway, I'm glad
that it finally was their cable, because we didn't have to pay a thing!
they have been digging a few days for us!

					Roger.

-- 
               EMail:  wolff@duteca.et.tudelft.nl
*  There used to be text here but because people complained that my  *
*  signature file was too long I deleted it. Now only this short     *
*  message, explaing what happened to my signature remains.          *

larry@kitty.UUCP (Larry Lippman) (06/03/91)

In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:
> Telephone line circuit switch hook detectors are designed to operate
>at currents ABOVE 12mA. They are also designed NOT TO operate at
>currents LESS than 7mA. 
>
>No problem will result if you draw less than the 7mA from the loop.

	There could well be a problem if rotary dial pulsing is used.
Such current leakage of 7 mA or less can well result in dial pulse
distortion, especially if pulsing an "A-relay" on a longer loop in an
electromechanical central office.  Loop leakage effects are much more
evident due to residual magnetism in a relay, as opposed to rotary dial
pulsing with electronic loop current detection.

	This is not, of course, a problem when DTMF signaling is used.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231       {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635   [note: ub=acsu.buffalo.edu] uunet!/      \aerion!larry

larry@kitty.UUCP (Larry Lippman) (06/03/91)

In article <107170@sgi.sgi.com> rpw3@sgi.com (Rob Warnock) writes:
>|  Telephone line circuit switch hook detectors are designed to operate
>| at currents ABOVE 12mA. They are also designed NOT TO operate at
>| currents LESS than 7mA. 
>| 
>| No problem will result if you draw less than the 7mA from the loop.
>
>No *immediate* operational problem... However, in the nightly automatic test
>scans [forgot the Telco acronym], your line will be flagged as "leaky", and
>may eventually spit out a trouble ticket. When they figure out the "leak" is
>inside your house, they'll send you the "repair bill" for a "false service
>call due to customer premises equipment".

	In general, automatic line insulation test apparatus (ALIT) is
concerned with leaks to ground or presence of foreign EMF from another pair.  
Such conditions are indicative of cable faults.  The primary purpose of
ALIT is as an early warning of outside plant cable trouble - and not as
an indicator of subscriber instrument trouble.

	While it is possible to configure more intelligent ALIT apparatus
to perform a tip-to-ring resistance test, this is generally not done due
to the possibilty of false alarms.  Such false alarms are most commonly
caused by old, leaky series ringer capacitors.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231       {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635   [note: ub=acsu.buffalo.edu] uunet!/      \aerion!larry

larry@kitty.UUCP (Larry Lippman) (06/03/91)

In article <1991May31.135017.13702@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes:
>>No *immediate* operational problem... However, in the nightly automatic test
>>scans [forgot the Telco acronym], your line will be flagged as "leaky", and
>>may eventually spit out a trouble ticket.
>
>Hey! Thats neat! Do you know what current the test flags as a leak?

	Convention in the telephone industry is to measure leakage to
ground in terms of resistance in ohms rather than current.

	The alarm threshhold for automatic line insulation test apparatus
is generally set between 50,000 ohms and 150,000 ohms - depending upon the
type of central office and the existing condition of its outside plant.
This corresponds to a current leakage of 0.3 to 1.0 mA.

Larry Lippman @ Recognition Research Corp.  "Have you hugged your cat today?"
VOICE: 716/688-1231       {boulder, rutgers, watmath}!ub!kitty!larry
FAX:   716/741-9635   [note: ub=acsu.buffalo.edu] uunet!/      \aerion!larry

grayt@Software.Mitel.COM (Tom Gray) (06/03/91)

In article <1991May31.135017.13702@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes:
:rpw3@rigden.wpd.sgi.com (Rob Warnock) writes:
:
:>In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:
:>+---------------
:>|  Telephone line circuit switch hook detectors are designed to operate
:>| at currents ABOVE 12mA. They are also designed NOT TO operate at
:>| currents LESS than 7mA. 
:>| 
:>| No problem will result if you draw less than the 7mA from the loop.
:>+---------------
:
:>No *immediate* operational problem... However, in the nightly automatic test
:>scans [forgot the Telco acronym], your line will be flagged as "leaky", and
:>may eventually spit out a trouble ticket. When they figure out the "leak" is
:>inside your house, they'll send you the "repair bill" for a "false service
:>call due to customer premises equipment".
:

The periodic tests on the subscriber loop will not normally look for faults 
between tip and ring.

Also to render a little bit of reality back into this discussion, there 
is automatic equipment designed to operate at the end of telephone loops.
In the days before microprocessors and integrated op amps (when dinosaurs
ruled the world and I travelled daily past the brontosaurus store), these
detectors consisted of high resistance relays. If your tip to ring
resistance is greater than 10koms nobody is going to get excited
(ie 4mA). If your tip to ring resistance is greater than 30kohms,
nobody will ever know.