Using "Free" telephone power
Greg Bell
This is in response to the person who was talking about lighting an LED from
the 50 VDC phone line voltage as an indicator for hook status:
That's the easy project! But, how 'bout making a circuit that will light an
LED when the line voltage DROPS (ie. the phone goes off hook). I tried it
once, and got it to work moderately well. I used a combination of voltage
dividers to keep a transistor off if the voltage is 50 or so. Worked alright
until the phone rang... the circuit answered the phone! Got a little
annoying.
I'm still amazed at how phones run entirely off the phone line power. They
have lit dials, and audio amplifiers... and I had trouble lighting an LED!
(trouble means the phone company equipment gets loaded down enough that it
shuts off your line for a while!).
Can anybody offer insite on the secrets of running audio amps and chips off
the meagre phone line power? How 'bout the ammount of current the phone line
can source?
Greg Bell_________________________________________________________
Hardware hacker |
Electronics hobbyist | UUCP: uunet!serene!pnet12!gbell
EE major at UC San Diego |
Larry Lippman
> This is in response to the person who was talking about lighting an LED from
> the 50 VDC phone line voltage as an indicator for hook status:
>
> That's the easy project! But, how 'bout making a circuit that will light an
> LED when the line voltage DROPS (ie. the phone goes off hook). I tried it
> once, and got it to work moderately well. I used a combination of voltage
> dividers to keep a transistor off if the voltage is 50 or so. Worked alright
> until the phone rang... the circuit answered the phone! Got a little
> annoying.
No kidding! :-) That's what I'm been trying to tell sci.electronics
readers for quite some time.
> I'm still amazed at how phones run entirely off the phone line power. They
> have lit dials, and audio amplifiers... and I had trouble lighting an LED!
> (trouble means the phone company equipment gets loaded down enough that it
> shuts off your line for a while!).
>
> Can anybody offer insite on the secrets of running audio amps and chips off
> the meagre phone line power? How 'bout the ammount of current the phone line
> can source?
Again, here are some assorted comments about telephone loop voltages
and currents:
1. The on-hook (i.e., idle) voltage of the vast majority of central
office telephone loops ranges between 48 to 52 volts, with 50 volts
being a pretty typical value. However, this is NOT ALWAYS the
case, since on-hook voltages of lines supplied from various
subscriber line concentrators, subscriber line carrier apparatus
and subscriber lines on long loops with loop extenders can vary
from a low of 6 volts (that's right, 6 volts using Continental
FDM AML subscriber line carrier) to a high of 96 volts (using
Cook Electric or equivalent loop extenders). A few electronic
PABX's may have 24 volts instead of 48.
There are two morals to the above:
(a) Only a 500-type telephone set with passive network will work
on all telephone loops at all times; anything else will fail
under some circumstances.
(b) If in doubt, measure the on-hook voltage of the telephone loop.
2. In general, good telephone circuit design practice dictates that
no less than 100,000 ohms should be bridged across a telephone
line in an on-hook state. That means you can draw a maximum of
0.5 milliampere of current with _assurance_ of not causing trouble
with your telephone and not causing telephone company automatic
line insulation test (ALIT) apparatus to print false trouble
reports.
0.5 mA is, by the way, sufficient current to keep a well-designed
reperatory dialer memory or clock circuit alive.
3. In general, good telephone circuit design practice dictates that
no current path to ground should ever exist between tip and/or ring
of the telephone loop and ground. From a practical standpoint,
a value of at least 100,000 ohms leakage resistance can be used.
The only exception to the above is equipment for use on ground-start
PBX loops and for test purposes.
An additional exception to the above involves party line circuits,
but most telephone company tariffs prohibit the use of ANY apparatus
on a party line circuit that is not directly obtained from the
operating telephone company.
4. From a practical standpoint, bridging less than 20,000 ohms across
a telephone line (i.e., a current flow of more than 2.5 mA) in an
on-hook state is an immediate invitation to trouble, especially with
respect to false ring-tripping as described in the referenced
article.
5. In the off-hook state power to operate amplifiers and other circuit
elements may be obtained from the voltage drop across a series
resistor (i.e., in series with the talk circuit). From a practical
standpoint, any such series resistance must be limited such that
no less than 30 mA of loop current flows in an off-hook state.
Taking an example value of a 250 ohm resistor on a 30 mA loop, we
have 7.5 volts available with a DC power of 225 mW. 225 mW is
actually a decent amount of energy to play with (in this day and
age of CMOS circuits) - but you will ONLY have it available when
the telephone is off-hook.
If you attempt to obtain a larger voltage drop through a higher
series resistor, such that the off-hook loop current drops below
30 mA you run the risk of: (a) having the central office apparatus
fail to detect an off-hook condition; (b) being unable to trip
ringing; (c) being unable to send rotary dial pulsing signals to
the central office; and (d) having insufficent current available
to operate telephone set DTMF dialing and talk circuits.
6. The "series resistor" described in (5) above may be effectively
replaced with solid-state circuit elements, such as a constant
current regulator set for a minimum of 30 mA.
7. Any series element for the purpose of obtaining DC power as described
above should be bridged with a non-polarized capacitor to minimize
the voice-frequency insertion loss of such element.
It should be obvious from the above guidelines that any BRIDGED
connection of an LED will draw excessive on-hook current, but that a series
connection (i.e., "in use" indicator) is okay.
<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/
<> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"
Mark Zenier
> have lit dials, and audio amplifiers... and I had trouble lighting an LED!
They don't , If you dig around in your installation, you'll probably find
a plug in the wall transformer to run the incandescent dial lights.
Mark Zenier uunet!nwnexus!pilchuck!ssc!markz ma...@ssc.uucp
uunet!amc!
uw-beaver!tikal!
David Robins
>
>I'm still amazed at how phones run entirely off the phone line power. They
>have lit dials, and audio amplifiers... and I had trouble lighting an LED!
>(trouble means the phone company equipment gets loaded down enough that it
>shuts off your line for a while!).
I don't know how it is now, but the old Princess and other Bell System
phones had dials with incandescent bulbs, whcih were powered off AC by
a small wall transformer. The power was carried throughout the house
phone wiring on the yellow/black pair (red/green reserved for signal).
However, my phone dialer IS powered by the phone line, but no light.
--
David Robins, M.D. (ophthalmologist / electronics engineer)
The Smith-Kettlewell Institute of Visual Science, *** net: uunet!skivs!dr
2232 Webster St, San Francisco CA 94115 *** 415/561-1705 (voice)
The opinions expressed herein do not reflect the opinion of the Institute!
Greg Wageman
>have lit dials, and audio amplifiers... and I had trouble lighting an LED!
>(trouble means the phone company equipment gets loaded down enough that it
>shuts off your line for a while!).
An off-hook telephone is *required* to draw a minimum amount of
current. I believe that it is permitted to draw about 20 milliamps
from the line in an *off-hook* condition. The Central Office's
switching equipment takes current draw above a certain limit as an
indication of an off-hook condition. If you were trying to light that
LED with the phone on-hook, you were actually taking the line
off-hook. After 15 seconds or so, the dial tone generator would time
out and you would have an open line.
Please note that the (Western Electric) telephones (e. g. "Princess")
with lighted dials are supplied 6 VAC by a small line transformer
which is typically connected to the black and yellow "unused" wires in
the phone jack. Multi-line phones are a whole 'nother ballgame, with
a closet full of electronics and power supplies to support them.
>Can anybody offer insite on the secrets of running audio amps and chips off
>the meagre phone line power? How 'bout the ammount of current the phone line
>can source?
I've used line-powered modems. They use all CMOS parts to keep
current consumption down, and as I said, since an off-hook device is
required to draw a certain amount of current, why not use it to power
the modem? Don't expect to be able to use phone line current and keep
the circuit on-hook, however. Even if you could, you would have to
cope with the possibility of an incoming ring signal, which is
typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house
current.)
Longish .signature follows. Skip now, or don't complain.
Greg Wageman DOMAIN: gr...@sj.ate.slb.com
Schlumberger Technologies UUCP: ...!uunet!sjsca4!greg
1601 Technology Drive BIX: gwage
San Jose, CA 95110-1397 CIS: 74016,352
(408) 437-5198 GEnie: G.WAGEMAN
------------------
"Live Free; Die Anyway."
------------------
Opinions expressed herein are solely the responsibility of the author.
Gerry Wheeler
" the meagre phone line power? How 'bout the ammount of current the phone line
" can source?
You'd probably get lots of info on stuff like that by posting to
comp.dcom.telecom if you can. They discuss all kinds of telephone things
there.
--
Gerry Wheeler Phone: (519)884-2251
Mortice Kern Systems Inc. UUCP: uunet!watmath!mks!wheels
35 King St. North BIX: join mks
Waterloo, Ontario N2J 2W9 CompuServe: 73260,1043
Al Zantow
>the 50 VDC phone line voltage as an indicator for hook status:
>
>
>have lit dials, and audio amplifiers... and I had trouble lighting an LED!
>(trouble means the phone company equipment gets loaded down enough that it
>shuts off your line for a while!).
>
>Can anybody offer insite on the secrets of running audio amps and chips off
>the meagre phone line power? How 'bout the ammount of current the phone line
>can source?
>
There are some problems with trying to use phone power to moniti or
the phone line. The phone company equipment "watches" the current in you
phone line, and if it goes above a few mA's it assumes you are off hook and
want dial tone. When you realy are off hook the current in the line is limited
to about 50 mA's. This is partly for your protection so you don't cook
if you get across the phone line, but mostly since bipolar transistors are
current operated devices. A "regulated" current makes them happier than a
regulated voltage source, which allowed the earlier touch tone phones to be
more stable.
The dial lights and such are probably powered by the yellow and black
wires that have about 12 VAC on then from a transformer wired into your
house (or aprtment) AC system. This is so the phone company does not need
to provide all the power for the "goodies", just for the actual phone circuits.
The red and green wires are the ones that have the connection to the phone
company, and normaly are not dependent on comercial AC power to work. This
is so that you can use the phone after power is knocked out to yell at the
electric company for being such clods and letting the lights go out ;-)
Not all houses have the "extra" AC power circuit wired up, you can
always check this with a meter though. The power availabe from the
seconday AC is abount a few hundred mA's, so you can power just about
anything from it.
"tell me, tell me about Subscriber Trunk Dialing !!!" from Time Bandits
(the Devil was asking one of his minions for really EVIL technology like
fast breeder reactors, comercial television, ect)
Larry Lippman
> >Can anybody offer insite on the secrets of running audio amps and chips off
> >the meagre phone line power? How 'bout the ammount of current the phone line
> >can source?
>
> I've used line-powered modems. They use all CMOS parts to keep
> current consumption down, and as I said, since an off-hook device is
> required to draw a certain amount of current, why not use it to power
> the modem?
Interestingly enough, the Western Electric 113-type data set in
its "plain vanilla" form was totally telephone-line powered. This is
a 300 baud originate-only modem which was developed around 1970. It
used NO IC's whatsoever, and employed discrete germanium and silicon
transistors. I always thought it was a pretty clever design, but also
sort of silly to go to all that trouble for a box twice the size of
a 500-type telephone set that was ALWAYS used where AC line power was
available. Musta been someone's pet project at Bell Labs... :-)
> Even if you could, you would have to
> cope with the possibility of an incoming ring signal, which is
> typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house
> current.)
Sorry, but you canNOT ring a phone with "house current". Virtually
all conventional telephone ringers are frequency selective, and operate only
between 16 and 32 Hz; 60 Hz won't cut it. Some ringers used on older
party line systems (like the "harmonic", "decimonic" and "synchronic") were
_very_ freqeuncy selective, with a "bandpass" of 5 Hz or less.
Larry Lippman
> There are some problems with trying to use phone power to monitor
> the phone line. The phone company equipment "watches" the current in you
> phone line, and if it goes above a few mA's it assumes you are off hook and
> want dial tone.
The minimum off-hook threshhold current ranges between 10 and 20
milliamperes, depending upon the type of central office apparatus.
> When you realy are off hook the current in the line is limited
> to about 50 mA's.
I don't know about AT&T #5ESS, Northern Telcom DMS-series or
other current production electronic central office apparatus, but older
ESS and electromechanical central office apparatus had no active current
limiting whatsoever.
Current limiting was achieved by a combination of a 200 ohm
resistance on the ring lead to central office battery, and a 200 ohm
resistance on the tip lead to central office ground. Assuming a 50 volt
office battery, the maximum short circuit current in the central office
at the distributing frame is 125 mA. Loop resistance of outside cable
plant further reduced this 125 mA to lower levels; however, if you
were close to the CO, you could easily get 100 mA of short-circuit
current across the telephone loop.
> This is partly for your protection so you don't cook
> if you get across the phone line, but mostly since bipolar transistors are
> current operated devices. A "regulated" current makes them happier than a
> regulated voltage source, which allowed the earlier touch tone phones to be
> more stable.
This is simply not true; the central office apparatus regulated
absolutely nothing. Earlier touch-tone telephone sets used a _single_
germanium transistor to generate the DTMF tones. This tone generator was
an _extremely_ clever design which would oscillate under virtually ANY
condition (provided the tip/ring polarity was correct; later telephone sets
were equipped with bridge rectifiers called "polarity guards" by WECO to
solve this problem).
> The dial lights and such are probably powered by the yellow and black
> wires that have about 12 VAC on then from a transformer wired into your
> house (or aprtment) AC system.
The voltage is actually 6 to 8 volts AC for WECO sets using a
2012A transformer.
Interestingly enough, in the early 1970's GTE/Automatic Electric
manufactured a "Trimline" lookalike telephone called "Styaline" which had a
dial illuminated by an electroluminescent (EL) panel. The EL panel was
powered by a "plug" adapter which plugged into the AC power line and
contained two 1 megohm resistors in series with each leg. NO TRANSFORMER -
just two series resistors. Hard to believe, but true.
Duke McMullan n5gax
> Sorry, but you canNOT ring a phone with "house current". Virtually
>all conventional telephone ringers are frequency selective, and operate only
>between 16 and 32 Hz; 60 Hz won't cut it. Some ringers used on older
>party line systems (like the "harmonic", "decimonic" and "synchronic") were
>_very_ freqeuncy selective, with a "bandpass" of 5 Hz or less.
Wellll -- I do it on occasion. I generally pull the voltage down to about
90 VAC with a variable autotransformer for safety, but the 'phone rings loudly
and continuously.
If you close the hookswitch, you get a loud buzz. ;^)
I've never tried it on one of the new solid state (easily damaged, cheap,
poorly made, etc., etc.) 'phones so widely available, but ALL of the old-style
Western Electric sets I've tested (just under a dozen, I'd imagine) ring
just fine at 60 Hz. The ring does seem a little "buzzy" or rapid, as you'd
expect.
I don't speak for every 'phone, but you can ring _some_ of 'em with 120VAC.
Call me a 'phoney,
d
"To be or not to be -- that is the square root of 4 B^2." -- Anon.
Duke McMullan n5gax nss13429r phon505-255-4642 ee53...@hydra.unm.edu
joseph.g.klinger
Subject: Re: Using "Free" telephone power
Summary:
Expires:
References: <6...@serene.UUCP> <9...@snjsn1.SJ.ATE.SLB.COM>
Sender:
Reply-To: j...@cbnewsc.ATT.COM (joseph.g.klinger)
Followup-To:
Distribution:
Organization: AT&T Bell Laboratories
Keywords:
In article <9...@snjsn1.SJ.ATE.SLB.COM> gr...@sj.ate.slb.com (Greg Wageman) writes:
>In article <6...@serene.UUCP> gb...@pnet12.cts.com (Greg Bell) writes:
>>
>>I'm still amazed at how phones run entirely off the phone line power. They
>>have lit dials, and audio amplifiers... and I had trouble lighting an LED!
>>(trouble means the phone company equipment gets loaded down enough that it
>>shuts off your line for a while!).
>
>Please note that the (Western Electric) telephones (e. g. "Princess")
>with lighted dials are supplied 6 VAC by a small line transformer
>which is typically connected to the black and yellow "unused" wires in
>the phone jack.
I have an AT&T 1300 feature telephone which only has connections to
tip and ring (I took it apart), no transformer, no yellow/black pair.
It also has two high intensity (and obviously high efficiency) LEDs which
light the translucent pushbuttons when the phone is off hook,
drawing current from the telco line. It's a current design, 1986.
Joe Klinger
att!iexist!jgk
John Opalko, N7KBT
>
> I don't know how it is now, but the old Princess and other Bell System
> phones had dials with incandescent bulbs, whcih were powered off AC by
> a small wall transformer.
The newer phones use an electroluminescent panel that is, indeed, powered
by the CO battery.
What I'm trying to figure out is how you folks who hang voltage dividers
and LEDs across your phone pairs keep from frying things when the ~100 VAC
ringing current arrives. I imagine you can get a big, honking capacitor
that has a low reactance at 20 Hz and shunt it around the LED, but... :-)
John Opalko
uunet!nwnexus!thebes!mcgp1!jgo
PS:
How much do 8,000 mfd non-polarized caps cost, anyway? :-)
Greg Wageman
>
>> cope with the possibility of an incoming ring signal, which is
>> typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house
>> current.)
>
> Sorry, but you canNOT ring a phone with "house current". Virtually
>all conventional telephone ringers are frequency selective, and operate only
>between 16 and 32 Hz; 60 Hz won't cut it. Some ringers used on older
>party line systems (like the "harmonic", "decimonic" and "synchronic") were
>_very_ freqeuncy selective, with a "bandpass" of 5 Hz or less.
You are correct that the ring signal's frequency is nominally 20 Hz,
but you can ring a telephone with house current. I have done it with
several different (Bell) telephones of varying vintage. It isn't very
efficient, since the coils are tuned for 20 Hz, but it works. (Of
course the set must be on-hook for this to work.)
Larry Lippman
> > Sorry, but you canNOT ring a phone with "house current". Virtually
> >all conventional telephone ringers are frequency selective, and operate only
> >between 16 and 32 Hz; 60 Hz won't cut it. Some ringers used on older
> >party line systems (like the "harmonic", "decimonic" and "synchronic") were
> >_very_ freqeuncy selective, with a "bandpass" of 5 Hz or less.
>
> Wellll -- I do it on occasion. I generally pull the voltage down to about
> 90 VAC with a variable autotransformer for safety, but the 'phone rings loudly
> and continuously.
Well, I suppose anything is possible if you drive the ringer hard
enough. Ringing voltage as provided from central office apparatus is well
limited as to current. For many years the method of current limiting was
a resistance lamp; this method was also used in earlier ESS systems. Newer
apparatus uses solid-state current limiting circuits, although resistance
lamps are still used in some apparatus because they are so _simple_ and
reliable.
A WECO 13G resistance lamp was typically used for current limiting
on ringing circuits and has a current limit of around 400 mA. At a typical
20 Hz ringing voltage of 100 VAC, a 500 ohm subscriber loop would of itself
limit ringing current to 200 mA. In addition, the central apparatus trunk
circuit introduces further resistance from the ring-trip relay or series
resistor used for electronic current sensing.
Ringing current is further limited by a series capacitor in the
telephone set itself to 50 mA or less.
A typical mechanical ringer is well intentionally designed for
frequency selectivity though the use of a careful core design, a permanent
magnet for magnetic bias, and an adjustable spring for mechanical bias.
I don't doubt that it will ring to 60 Hz if you drive it hard enough to
overcome the magnetic and mechanical bias elements.
Under the circumstances, I bet you were either: (1) driving the
ringer at current levels >> greater than 50 mA, and/or (2) driving the
ringer without the usual series capacitor.
> If you close the hookswitch, you get a loud buzz. ;^)
See any smoke? :-)
Isaac S Wingfield
Ni-Cads in your telephone, you could charge them all the
time the 'phone was on hook, and have plenty of "free energy"
during off-hook use.
Isaac i...@cup.portal.com
Isaac S Wingfield
of the things we never could "realistically" record and play back
was the sound of a ringing phone on stage. We regularly rang
standard issue phones from the power line. (We always wired the
ring voltage through the hook switch, and instructed the performers
to pick up "during a ring"; it was more realistic that way).
Of course these were POTS phones, probably 500 series.
Isaac i...@cup.portal.com
Larry Lippman
> I'll bet that if you used a lead-acid gel cell or some
> Ni-Cads in your telephone, you could charge them all the
> time the 'phone was on hook, and have plenty of "free energy"
> during off-hook use.
The maximum amount of current which one can draw from a telephone
line while on-hook without running the risk of telephone line trouble is
about 1 milliampere (this actually exceeds usual loop leakage specifications
by a factor of two, but I'll be generous).
(.001 ampere) X (50 volts) X (8760 hours) = 438 watt-hours per year
At typical electric utility rates of 7 cents/KWH, this power is
worth about 3 cents.
Assuming that you can save 3 cents per year, let's do the ROI
calculations for a 4 ampere-hour 48-volt battery string; this will give
you the capability of saving up all your energy for a 2-hour consumption
spree once per year.
Using 4 Globe JC1240-1 12-volt 4-AH gel cells @ $ 34.00 will cost
you $ 136.00. Neglecting the cost of money, changes in future energy costs
and useful life of the batteries, the batteries should pay for themselves
by the year 6522.
:-)
Jon R. Vermilye
>> > Sorry, but you canNOT ring a phone with "house current". Virtually
>> >all conventional telephone ringers are frequency selective, and operate only
>> Wellll -- I do it on occasion. I generally pull the voltage down to about
>> 90 VAC with a variable autotransformer for safety, but the 'phone rings loudly
>> and continuously.
>
> Well, I suppose anything is possible if you drive the ringer hard
>enough.
> Under the circumstances, I bet you were either: (1) driving the
>ringer at current levels >> greater than 50 mA, and/or (2) driving the
>ringer without the usual series capacitor.
>
>> If you close the hookswitch, you get a loud buzz. ;^)
>
> See any smoke? :-)
I have to add my 2 cents. We have been ringing phones with 60
Hz AC for years. I just finished measuring the ringing current on a
standard C/D 500 (old black non-electronic) phone - 40ma. Of course
the ring is not normal, but it does ring, and the current is reasonable.
The best solution for ringing phones for the theatre is to get
a ringing generator. We use a Tellabs 8101, which produces 90 - 120 v
AC at 30Hz. The phone will sound normal and the ferro-resonant saturable
core reactor limits the output to about 5 watts. It cost about $60.00
in 1980, and since it is is a mechanical, ie transformer, device, it puts
up with the abuse that theatrical technicians can provide. By the way,
if you plan to answer, ie pick up the reciever of the phone, be sure to
remove the earpiece. You don't have the CO to shut off the ringing
voltage, and 30 or 60 hz at ringing current will annoy the actors ;^).
I also suggest that you modify the phone so that lifting the
handset will open the ringing circuit. You can use some of the
normally closed contacts on the hanger switch. This will prevent the
situation where the phone is ringing while the actor holds it in
his hand. I suspect that many of these modifications swould be more
difficult on an electronic phone, however is should still ring with
the ringing generator.
Jon R. Vermilye 315 341 2138
Department of Theatre verm...@oswego.Oswego.EDU
SUNY Oswego rutgers!sunybcs!oswego!vermilye
Oswego, NY 13126