POTS Phone Line Power Capability

I know phones are capable of operating off hook without separate power sour ces, but I was wondering if it might be possible these days to power a phon e/answering machine entirely off the phone line. So many require AC power and fail to operate, might it be possible to vampire enough power from the phone line so a cordless phone/answering machine might work from the phone line alone?

I had a little trouble finding any reference specifically indicating what p ower level draw is allowed on a phone line. I did find a Remote Subscriber Terminal POTS equipment spec from Tellabs that specified it would detect o n hook at 9 kohms. It also specifies a 25 mA loop current at 600 ohms whic h I take to mean in combination with the internal current limiting resistan ce in line with the 48 volt PSU. Doing the math I figure the in line resis tance is 1,320 ohms which gives a 10 kohm load up to 4 mA at around 42 volt s and 180 mW.

That sure seems like a generous budget to keep a phone charged even if not in real time operation.

Is there a regulation for phones that precludes this?

--
  Rick C. 

  - Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricketty C
Loading thread data ...

formatting link

Reply to
Lasse Langwadt Christensen

On Saturday, June 6, 2020 at 9:20:56 PM UTC-4, Lasse Langwadt Christensen w rote:

/

You seem to have some serious google-fu. But I don't understand the requir ement of 5 Mohms. The ringger circuits I've seen certainly present less th an 5 Mohms to the line. I guess I'm forgetting about some DC blocking cap or something and the allowed ring impedance is lower.

Thanks,

--
  Rick C. 

  + Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricketty C

Rick, you won't need any power when the answering machine is idle (on hook) and when it rings, you will have a high-voltlage AC signal to let you know when it needs to start. So that ring current/voltage can start things off.

I was going to say that there isn't nearly enough power available from the line when off hook but I'm sure these days a solid state machine could be made to work.

The only reason I think this is possible is because when I was a kid (i.e. 50+ years ago), I ran a transistor radio off the POTS line.

So, I am guessing that this should work. If there is any loose ends to tie up when the call ends, I suppose extra energy could be drawn from a capacitor charged from that ringing voltage at the beginning of the call.

I think the most power needed might have to be used for the amplifier that is actually presenting the outgoing message but that might be able to come from the short term energy captured from a couple of ring cycles.... Maybe,.. This would all be dependant on how much the POTS system has changed over the decads and years since I used to play with phones.

Reply to
boB

-it/

uirement of 5 Mohms. The ringger circuits I've seen certainly present less than 5 Mohms to the line. I guess I'm forgetting about some DC blocking c ap or something and the allowed ring impedance is lower.

If you only want to record the messages and not play them back I guess you don't need any on hook power. Or the machine could take the line off hook to play messages. But the point is to store energy when on hook within the requirements.

The article Lasse found pretty much indicates what might be required. It i s interesting that it prevents over charging the cap by draining current of f rather than stopping current from flowing in.

The circuit might be optimized by allowing a lot more current to flow while the phone is off hook when the spec allows and, in fact, requires 10's of mA to be drawn. Charging at 10 mA for even a few minutes is worth hours of charging at 10 uA. So every incoming call powers the device during the ca ll and charges the supercap for playback.

The calcs I did show it is unlikely to have an effect on the line until the load resistance is below about 10 kohms, so I suppose on low volume or wit h headphones a radio might work fine. Draw more than around 5 mA and the l ine can go off hook!

There is a "lot" of power (relatively speaking) available once the line is off hook. It is required to draw enough to bring the voltage down to aroun d 12 V, so some 10's of mA. While the electronics for implementing the POT S line has changed, the spec on the POTS line has not.

--
  Rick C. 

  -- Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricketty C

They'll detect "water" on your line and send a high voltage to clear it, so be sure to include a fuse that's good for a few hundered volts.

--
  Jasen.
Reply to
Jasen Betts

48V at 5 Meg-Ohms will get you about 34 milliwatt-hours of energy storage, IF the leakage of the storage capacitor doesn't drop it down. That's 9.6 micro-amps. At 100% efficiency conversion, at 12V you get around 50 micro-amps.

It used to be that you had to pull in a relay contact at least to go off hook.

I think I would just harness the energy of the ring voltage for a short burst

Reply to
boB

sources, but I was wondering if it might be possible these days to power a phone/answering machine entirely off the phone line. So many require AC po wer and fail to operate, might it be possible to vampire enough power from the phone line so a cordless phone/answering machine might work from the ph one line alone?

at power level draw is allowed on a phone line. I did find a Remote Subscr iber Terminal POTS equipment spec from Tellabs that specified it would dete ct on hook at 9 kohms. It also specifies a 25 mA loop current at 600 ohms which I take to mean in combination with the internal current limiting resi stance in line with the 48 volt PSU. Doing the math I figure the in line r esistance is 1,320 ohms which gives a 10 kohm load up to 4 mA at around 42 volts and 180 mW.

not in real time operation.

already has to handle that when the phone is ringing with ~90V AC added to the -48V DC

Reply to
Lasse Langwadt Christensen

OR

Build or buy a small UPS (battery backup) plug it in and forget it.

Sorry, to much extra thinking in the thread!! :-)

Reply to
ABLE1

Ricky is always looking for ways to steal.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

is off hook. It is required to draw enough to bring the voltage down to ar ound 12 V, so some 10's of mA. While the electronics for implementing the POTS line has changed, the spec on the POTS line has not.

I don't think using the ring voltage is viable because you don't know when that might happen. A device can always go off hook itself to get some char ge as needed to prevent going inoperative although I don't think that is ne eded. The ratio of power available when off hook to on hook is huge. But even so, I don't think it would be difficult to maintain a device charge st ate even while on hook. 10 uA at 48 volts is more than enough to keep a pr ocessor running at a low level. These days this is no trivial amount of po wer.

But there is no reason to not harness the ring energy. The circuit would h ave to be protected against that energy and a diode bridge is needed to all ow either polarity to work. That pretty much provides energy from the ring for free.

--
  Rick C. 

  -+ Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricketty C

Hmmmm- wonder if I could build a little box with a Big Red Button, to _FRY_ the spoofing equipment of the #*&%! nuisance callers!

Reply to
Wond

It will only work if their equipment is full of water.

--
  Rick C. 

  +- Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricketty C

Here in the UK it is illegal to connect anything to the POTS line which has not been tested and approved by the telecom authorities.

However, "power over POTS" may be short-lived here even if possible, as there is quite a bit of discussion about switching off POTS, and using VOIP via broadband (eventually using fibre, so no copper at all).

--

Jeff
Reply to
Jeff Layman

sources, but I was wondering if it might be possible these days to power a phone/answering machine entirely off the phone line. So many require AC po wer and fail to operate, might it be possible to vampire enough power from the phone line so a cordless phone/answering machine might work from the ph one line alone?

at power level draw is allowed on a phone line. I did find a Remote Subscr iber Terminal POTS equipment spec from Tellabs that specified it would dete ct on hook at 9 kohms. It also specifies a 25 mA loop current at 600 ohms which I take to mean in combination with the internal current limiting resi stance in line with the 48 volt PSU. Doing the math I figure the in line r esistance is 1,320 ohms which gives a 10 kohm load up to 4 mA at around 42 volts and 180 mW.

not in real time operation.

So no concern about the difference in reliability? Here in the US the syst ems that carry IP are rather much more prone to interruption by the very so rts of events that require emergency communications. The land line based p hone system in comparison is a rock.

--
  Rick C. 

  ++ Get 1,000 miles of free Supercharging 
 Click to see the full signature
Reply to
Ricketty C

As a load, POTS receivers are RENs (ringer equivalent networks).

You could play with that.

RL

Reply to
legg

Not likely.

First, there is an exceptionally good chance that the nuisance caller is on the other side of a digital divide that is the PSTN and your ... technique ... won't make it to them.

Second, your ... technique ... will quite likely damage something in the telephone equipment and the TelCo will be less than pleased the first time it happens. The second time ... well that could get difficult to explain.

--
Grant. . . . 
unix || die
Reply to
Grant Taylor

It used to be the same way here in the U.S. I don't know if the rules / laws are still on the books or not. (Probably b/c nothing is ever removed.) But I don't think anybody enforces anything. At least not without provocation to do so.

I've seen POTS transitioning from complete network to last mile to last

100 feet by TelCos. Many people are even down to POTS for the last 10 feet (or less) with personal ATAs inside next to the phone (maybe even less than a foot).
--
Grant. . . . 
unix || die
Reply to
Grant Taylor

I thought it was "Ringer Equivalency Number" as in how comparable / equivalent is the device to a traditional ringer.

I vaguely recall that different TelCos allowed houses to have a range of REN, between 2 and 3.

I remember hearing stories of a mans shaver being powered off of the phone line.

Stealing power from the phone company used to be a thing and there used to be a lot of trouble with it.

--
Grant. . . . 
unix || die
Reply to
Grant Taylor

We still have a couple of real telephones at home, and our old number, but it's through the cable provider now, and it's free. AT&T land-line was unreliable and expensive. Of course, we can't profitably steal power from the phone pair; it's our power now.

At work, all the phones are internet. People working from home now just took their work phones home.

--
John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 
 Click to see the full signature
Reply to
jlarkin

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.