Power from phone line

[tjkolev]

Greetings!

Can I safely and reliably power a microcontroller from a (regular POTS) phone line? The device is based on an Arduino Pro Mini, which draws at most 5mA at 3.3V. There will be additional circuitry, but I don't think it will go over 10mA. The phone line provides about 48V DC. But then there's the ringing at 90V AC, and the off hook drop to 7 - 8V DC. I tried to search on the subject, but came only with simplistic "secret power from the phone line to charge your cell phone" schemes. I may not know what to search for. There are those buck type regulators, but none of them is rated for 90V.

What kind of circuitry do I need? Anything available already out there that would do the job?

Cheers!
tjk
(May be should've posted in the Power forum...)

[Ed.Kloonk]

A quick google search found this..

https://www.edn.com/get-power-from-a-telephone-line-without-disturbing-it/

[pcprogrammer]

That setup is not very good  You have to wait a couple of day's before the capacitor is charged and then the device can run for about 40 to 80 seconds 

You can try, but the moment you draw a bit to much current the system will see it as the phone off hook and when it lasts to long it might disconnect.

It is not setup to deliver free power. So why the need for this?

Edit: And yes it might get better answers in one of the other sections. Beginners might be the place. You can move the thread with the button at the left hand side on the bottom of the page.

[tjkolev]

@Ed.Kloonk,
Thanks for the page. Google doesn't like me - that site didn't come up on the first page of results.

@prcprogrammer,
I agree with you. I didn't know the spec for the phone line, but this:
> any device that connects to the phone line and is not actively communicating must present a resistance of at least 5 MΩ (Reference 1). To meet this requirement, a device's continuous-current drain must not exceed 10 µA.

This doesn't cut it for my application. I think I'll just use a battery.

Scratching the phone line powered idea.

Cheers!
tjk

[SL4P]

Maybe an interesting application for supercaps.

Everyone’s been trying to apply them to everything - from toasters to cars, but this type of problem seems like one of the better ideas (because I had it).

Of course, the micro design should be as power frugal as possible - but worth exploring.

[pcprogrammer]

The link Ed.Kloonk provides is about using a supercap, so check it out and you will find that it is somewhat pointless.

[darkspr1te]

@Ed.Kloonk,
Thanks for the page. Google doesn't like me - that site didn't come up on the first page of results.

@prcprogrammer,
I agree with you. I didn't know the spec for the phone line, but this:
> any device that connects to the phone line and is not actively communicating must present a resistance of at least 5 MΩ (Reference 1). To meet this requirement, a device's continuous-current drain must not exceed 10 µA.

This doesn't cut it for my application. I think I'll just use a battery.

Scratching the phone line powered idea.

Cheers!
tjk

Pulling power from POTS line is very limited, in fact all phone devices will have a REN' (Ring equivalent number)  with max being 4 on the subscribers end without special equipment. Most old usb robotics modems would be either a REN of 0.5 or 1, with some old phones being 1.5
this was to limit the current drawn from the telephone lines as the voltage/current are all part of the Ring circuit.
Some early phones with the very long cords would often have a 9v battery inside as and small boost circuit to assist in supplying power and current during drops often without the users knowing it as they would be no doubt changing the phone soon for a better model (wireless, answer machine with wireless, digital etc)


darkspr1te

[SeanB]

There are dedicated telecoms IC's that are there for use with on phone digital electronics, which are normally going to provide 5V at around 20mA, to power the digital logic in the phone off the line. They also include the ability to survive the ringing voltage, and as well will work with the phone off hook, though they are designed to do the hook switching themselves. Small DC-DC converter that is run off the nominal 48V line, and most of these are LT designs, now a part of Analog technology.

https://www.analog.com/media/en/news-marketing-collateral/press-releases/LT3010%20Final%20Rel%2010-xx-02.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/lt3010-3010-5.pdf

[Whales]

In Australia it's about 48V from the exchange.  You could theoretically use lots of protection (see below!), a full bridge rectifier and a buck converter to get what you want. 

Caveats:

* Very high impedance and leaky wiring under the streets.  You won't get 48V by the time it reaches you house, even with a high impedance DMM.
* About 100VAC when the line is "ringing", which will eventually happen regardless of how confident you are no one ever rings that line.
* Gets struck by lightning.  Kid you not.  You need lots of protection components, including isolation transformers and gas discharge tubes.
* Most countries have laws about only being approved to plug in approved devices in, specifically because of these safety issues.

Have a look inside a POTS phone or an ADSL router to see what I mean by protection components.  They will be using the minimum you can get away with, partly because they're cheap but partly also because they are insulated devices (humans don't touch metal bits).

[nctnico]

IIRC you can load a POTS line with the equivalent of a 10k Ohm (or higher) resistor. So it should be doable to power a microcontroller circuit but you'd need to design is specifically for low power which an Arduino likely isn't.

[pcprogrammer]

But with a 10K resistor the system will see it as phone off the hook, start the dial tone and it depends on the system, but after a while it disconnects. Not sure if you can then still draw some power of it.

Another question is how long will standard phone lines be around. At least here in the section of France I live in, there will be no more maintenance to the lines starting from 2025. Simply because they are installing fiber optics cable everywhere.

[nctnico]

No. For a phone to be off-hook, the resistance is in the ball-park of 600 Ohms (the impedance of a phone line). Remember that phone lines needs to allow for some leakage current due to wet / broken wires.

[Geoff-AU]

Offhook phone is around 600 ohms, you might "get away with" a 10k load but it's an invalid load for the phone line.  Onhook phones are supposed to be in the Megaohms.

That EDN circuit is utter trash.  >99% of the energy is burned in the resistor, to drop 48V down to a maximum of 2.5V.  The maximum average power budget doing that is around 100uW.

You would be much better off with the correct number of Megaohms charging a regular electrolytic capacitor to 48V, and then running a very efficient buck converter to take 48V down to 3.3V.  Getting maximum power out involves keeping the input capacitor at around 25V.  You can then store as many Coulombs as you like in a supercap on the secondary.  Your average power budget is around 100mW though (1000 times higher than the EDN circuit).

You'll need an Arduino with very low sleep current, and only wake up for short periods of time.

[Peabody]

Maybe you could power up your circuit via the rectified ring voltage and a capacitor, or if it goes off hook.  Then there wouldn't be any on-hook leakage.

[peter-h]

I've designed this sort of stuff in the past. It can be done. I did some remote control devices which turned latching relays on/off. But isn't futureproof because the modern world is moving to FTTP (fibre to the premises) where you don't get any power from the exchange (obviously). UK's BT is threatening to remove POTS within the next few years.

[acourtois]

I had read from an old DMS200 telephone switch manual that the central office will get an alarm (Read Only Printer output) if there are more than 90mA pulled from the line. It doesn't mean it wouldn't provide more but the CO guys would probably send a tech to you home. Now as far as Multumedia Telephone Adapter that your cable operator install (Modem), I don't know about the maximum current you can pull from the RJ-11 connectors.

[coppice]

I had read from an old DMS200 telephone switch manual that the central office will get an alarm (Read Only Printer output) if there are more than 90mA pulled from the line. It doesn't mean it wouldn't provide more but the CO guys would probably send a tech to you home. Now as far as Multumedia Telephone Adapter that your cable operator install (Modem), I don't know about the maximum current you can pull from the RJ-11 connectors.

Digital telephone exchange line cards provide a current controlled loop, usually regulated to 35mA when the load impedance (i.e. the phone) is in its off hook state. You can't pull more current than that, unless the line is ringing. You see -48V + ~90VAC when ringing occurs, with a higher current limit.

[CatalinaWOW]

There are many reasons not to depend on POTS power, but the limits of the circuit shown in the EDN article don't exhaust the possibilities.  The section 48 rule basically says you can only draw about a half milliwatt of average power.  But for some duty cycles/ applications that is enough.  While a couple day charging time seems huge, there are many applications that only need once a week data or less.  And batteries for these long duration activities can be challenging.  The EDN circuit stops collecting power at roughly the two day point, and is thus collecting only about a third of the energy available for a weekly operation.

Of course there are many other sources of a half milliwatt of average power (solar, wind, etc.) and in many applications they would be a far better choice.  But there are always those corner cases where POTS might be the right answer.  At least while it is still with us.  The POTS line at my location has been disconnected for more than a decade now.  In principal it could still be reconnected, the service is still there about 50 meters away, but maintenance on it is somewhere between minimal and non-existent.  When the power company went through clearing intruding trees from their lines, the phone company declined to pay for clearing their lines on the shared poles.  Shows they have no long term plans for these wires.

[paulca]

I know here in the UK phones with LCD displays are very common.  Some even have momentary backlights when you lift the phone.

Also in the UK when you lift the handset, on hook, you get connected to a dial ton generator while you give them the DTMF.  If you don't do anything, but keep it on hook it will timeout after a few minutes.  Because phones left off the hook cost more money to run and consume dial tone generators it will time out.  However in the 1990s they added a warning.  The phone will start to generate a sweeping tone, up and down, like a siren.  It's meant to get the attention of the householder to put the phone back on the hook.  I'm sure you can harvest that power too.