POTS Phone Line Power Capability

The phone company also tests for DC leakage while the phone is on-hook.

War story from the 1960s: I built my on smei-modern telephone on a bit of plywood screwed to the concrete wall of the basement. This phone had lots of 1930s era components. I needed a way to tell if someone was on the phone so I wouldn't barge into their conversation if I tried to use my phone, so I set up a voltage relay that detected when the line voltage dropped from 48 Vdc to maybe 10 Vdc. Presently, a phone man appeared at our door, looking for the current leak. He took one look at me and the contraption on the wall, and explained the current leak. And then he drove off. I changed to a current detection relay. No return visit.

Joe Gwinn

I think it depends what you're doing.

Some quick Googling suggests that a phone is nominally 20 mW while off hook.

I suspect it has more to do with the Telephone Company doesn't wanta bunch of people consuming for things /other/ than phone calls.

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Grant. . . . 
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No, unless you are on a really old system with a live operator making the connection. You don't have a DC connection between phones anymore. The test voltage used to be 500VDC, generated by the C.O., or later by a SLIC, in test mode.

ources, but I was wondering if it might be possible these days to power a p hone/answering machine entirely off the phone line. So many require AC pow er and fail to operate, might it be possible to vampire enough power from t he phone line so a cordless phone/answering machine might work from the pho ne line alone?

t power level draw is allowed on a phone line. I did find a Remote Subscri ber Terminal POTS equipment spec from Tellabs that specified it would detec t on hook at 9 kohms. It also specifies a 25 mA loop current at 600 ohms w hich I take to mean in combination with the internal current limiting resis tance in line with the 48 volt PSU. Doing the math I figure the in line re sistance is 1,320 ohms which gives a 10 kohm load up to 4 mA at around 42 v olts and 180 mW.

ot in real time operation.

The ringer is AC only and the AC is the ringing signal which is a relativel y rare signal. It is the DC load that is limited to 10 uA. That can be pr esent always. When the phone is off hook the load is about 1,000 times hig her current and approx 10% of the voltage, so 100x the power. A device can 't be off hook all the time, only briefly or during a call.

So energy would need to be stored but can be drawn in any of the three mode s, on hook, off hook or ringing. Together they can provide enough power to operate a cordless phone and answering machine if designed for low power.

A friend has one of these devices and every hand set has to be plugged into a power outlet. If used infrequently, they could also be charged from the phone line, but someone has mentioned that the 10 uA limit is the total lo ad on the phone line. So they might have to coordinate somehow or just cha rge when the line is off hook. Although with the number of robocalls these days ringing might be the more useful power source. 90 volts and some mA of current will provide a lot of charge very quickly actually. Maybe a com bination of the two is ideal. Get fast charges when the phone is used and trickle charges when it's not.

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  Rick C. 

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I think you may be confused with stealing power from the power company. The phone company can supply you with minute amounts of power unless you have incoming calls a large percentage of the day.

I did a simulation and I can charge a 10 mF cap in a small number of rings and staying within the limit of 1 REN while up to 5 REN is allowed on a phone line in the US.

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No, I'm very distinctly thinking of using the phone line to power things.

5 REN is different than what I remember. But I can see how that could easily be different in different parts of the country. I could also be misremembering. There's also a chance that the phone company said a lower REN (3) in the hopes of dissuading people of having up to 5 REN connected to the phone line.

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Grant. . . . 
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As usual Larkin doesn't understand what is being discussed. The issue is t hat in a power failure most phones won't work at all because people buy the fancy feature phones. So one that works off phone line power would still work when the power is out.

Someone mentioned using a UPS, but they are clunky and cost as much as the phone. I don't have a phone line, but if I did I would consider such a des ign to allow my standard phone to work in a power failure. In my other hou se I used to keep a simple phone by the demarcation box in the basement. I n a power failure it was there to use. I believe even a "dead" line can be used to call 911 or so I've heard.

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ok.

Where did you read that? From what I'm finding it is more like half a mill iwatt, 10 uA at 48 V.

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Well OK... I thought this was to run an answering machine ? The phone will always ring before it needs to answer. I just didn't understand why the micro has to be running in-between calls ?

Sure you can do energy scavenging or harnessing. You could also harness a small amount of PV energy from ambient light too or if there are are string radio signals around, harness and store that.

It will be interesting to see how you end up doing this.

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

en that might happen. A device can always go off hook itself to get some c harge as needed to prevent going inoperative although I don't think that is needed. The ratio of power available when off hook to on hook is huge. B ut even so, I don't think it would be difficult to maintain a device charge state even while on hook. 10 uA at 48 volts is more than enough to keep a processor running at a low level. These days this is no trivial amount of power.

d have to be protected against that energy and a diode bridge is needed to allow either polarity to work. That pretty much provides energy from the r ing for free.

When do you listen to messages???

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The available off-hook current is a function of the loop resistance. That is a function of the length and gauge of the loop. You may be served by a CO up to ~40kft away, a SLC or other digital remote mux, or others.

The single phone usually draws about 23ma but you can take whatever is there; WECO 2500's DTMF would work well with as little as 3-4V across them.

My sister had a business less than 100 ft. from the CO. The usual "busy" light on a phone never lit; it was a voltage detector set at ~10VDC; less it was on, more off. I fixed that with ISTM 470ohm resistors in both tip and ring sides.

AND If in Verizontal land, you may have zero ma. available. The license fee for 5ESS generics is not chump change, and if they can force you onto fiber, they will: BTDTGnodialtone last fall.

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Not sure of what issue you are trying to address. The only thing clear is that the maximum power drawn from the phone line when off hook requires an MPPT circuit. But that's not terribly important. I did some simulations u sing a circuit that emulated a fixed resistor of about 7 kohms. It would c harge up 1 mF to 5 volts in about two ring times.

A 10 volt detector for off hook is within the expected range of off hook op erating voltage. So clearly that was a bad design.

Sorry, I don't know what that term means. Are you saying they give you a p hone line via a VOIP box which plugs into your home? That may be. But the y can't give you that if you aren't on a fiber. Where my one house is they would need to run fiber for some 12 miles or more and they ain't gonna do that. My other house could never use a 56 kbps modem because it's on a pai r gain amplifier. Know what that is? When you figure it out you may under stand that the phone company doesn't update old equipment if they don't hav e to.

Oh yeah, the pair gain amplifier predates the 5ESS. I see in Wikipedia the y don't call it an amplifier. That was the term the phone guy used at the time. Seems it was installed in the 70s when people were getting second li nes and they didn't have the copper for it. I was told it was a digital mu x using three phone lines to provide some dozen or so connections. I would guess that's one pair each for data in each direction and one for clocking and sync or just clocking and the sync was in the data.

They aren't going to run fiber up the main street through town, so this is what we have until something drastic happens like hover boards (not the eba y kind, the Back to the Future kind).

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Why not take the phone off-hook say once an hour, but never dial anybody and enjoy the large current ? Listen for the dial tone and when it stops, put the phone back on-hook.

The problem with devices connected to both phone line and mains is what happens during a thunderstorm. Both these networks are grounded but grounded at different places, having a different potential during a lightning strike.There can be huge potential differences between the grounds of these networks (kilovolts) causing flash over in the device and can cause property fires.

Powering the device from only the phone network might cause a potential jump inside the phone, compared to the house potential during a lightning strike, but using good enclosure insulation and keeping a large distance between the headphone to the user ear should solve the potential problem.

Alternatively when fiber is used and the device is mains powered, there is also just one grounded network.

r 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 power and fail to operate, might it be possible to vampire enough power fro m the phone line so a cordless phone/answering machine might work from the phone line alone?

what power level draw is allowed on a phone line. I did find a Remote Subs criber Terminal POTS equipment spec from Tellabs that specified it would de tect on hook at 9 kohms. It also specifies a 25 mA loop current at 600 ohm s which I take to mean in combination with the internal current limiting re sistance in line with the 48 volt PSU. Doing the math I figure the in line resistance is 1,320 ohms which gives a 10 kohm load up to 4 mA at around 4

2 volts and 180 mW.

f not in real time operation.

vely rare signal. It is the DC load that is limited to 10 uA. That can be present always. When the phone is off hook the load is about 1,000 times higher current and approx 10% of the voltage, so 100x the power. A device can't be off hook all the time, only briefly or during a call.

First of all, that would be a royal PITA, not to mention unneeded. I think I've analyzed it enough to realize it will work. There are three modes, r inging, on hook and off hook. Two of those modes provide more power than n eeded to use the phone. The other is a large percentage of the time so eve n small amounts of power make it useful. Unless the phone is designed so i t uses too much power to be line powered while off hook, this really should be very viable.

odes, on hook, off hook or ringing. Together they can provide enough power to operate a cordless phone and answering machine if designed for low powe r.

nto a power outlet. If used infrequently, they could also be charged from the phone line, but someone has mentioned that the 10 uA limit is the total load on the phone line. So they might have to coordinate somehow or just charge when the line is off hook. Although with the number of robocalls th ese days ringing might be the more useful power source. 90 volts and some mA of current will provide a lot of charge very quickly actually. Maybe a combination of the two is ideal. Get fast charges when the phone is used a nd trickle charges when it's not.

There is a level of isolation so that's not an issue. Grounds are not conn ected. Typically the ring detect is done through opto isolators and the vo ice is done through capacitive isolation since transformers are so 20th cen tury.

Or use the standard isolation techniques.

You mean fiber to the home? Not happening here, not for a few decades at l east. Hell, I'm lucky to have Internet other than dial up.

I think many don't understand how the rest of us live.

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It needs to have a dialtone... if they run out of lines in your street an unsubsribed phone service may be disconnected to provide service to a paying customer.

Same with a mobile phone, you only need "bars" to call enercency services.

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  Jasen.

What is the creapage distance between those separate grounds ? A few millimeters ? I have seen PCBs with much greater (>10 mm) creapage distance and still flash over damage between the circuits.

Unfortunately, it seems that reliability in anything seems to be an out-of-date concept in this throwaway age. There have been many concerns over the lack of emergency cover in a wide-ranging power cut, as the POTS has its own power and calls to the emergency services (999 here - equivalent to your 911) can still be made from simple, wired, non-powered telephones. I think the feeling is that most people have cellphones nowadays anyway, and provided not all the cellphone towers went down, an emergency call should still be possible.

Until that one day it is needed, of course... :-(

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Jeff

d into a power outlet. If used infrequently, they could also be charged fr om the phone line, but someone has mentioned that the 10 uA limit is the to tal load on the phone line. So they might have to coordinate somehow or ju st charge when the line is off hook. Although with the number of robocalls these days ringing might be the more useful power source. 90 volts and so me mA of current will provide a lot of charge very quickly actually. Maybe a combination of the two is ideal. Get fast charges when the phone is use d and trickle charges when it's not.

onnected. Typically the ring detect is done through opto isolators and the voice is done through capacitive isolation since transformers are so 20th century.

Sure, there is a spec devices are designed to but that has nothing to do wi th actual real-world possibilities. I worked on an alarm system once where the perimeter wire ran through rafters in the same hole as power lines. L ightning was close enough that the induced current in the line caused an ar c THROUGH the insulation of both wires. There would be a bit of char and a tiny split in the insulation. The AC wires were fine, but the alarm wires were melted in half.

But equipment is designed to a 5,000 volt spec I think. That's what sticks in my mind anyway. That will stop the vast majority of power/telephone li ne surges. Still, who cares if the phone is wireless? Then you have a six foot air gap.

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systems that carry IP are rather much more prone to interruption by the ver y sorts of events that require emergency communications. The land line bas ed phone system in comparison is a rock.

Cell towers are still very vulnerable. They can go down in bad weather. I 'm in an area where everything is vulnerable. AC, Cell, Internet. The onl y thing that might still work is a land line. If I really need it, I know where the box is at the end of the lane and can clip onto it. I don't thin k they support pulse dialing anymore, so I couldn't dial by touching the le ads with a jumper.

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