Telephone On-Hook Current

Don't know about the US, (so why am I replying etc :-), but in the uk, the off hook current could be anywhere between 10 and 50mA, depending on how far you were from the exchange, using a resistor (?) at the exchange to limit current from the 50v bus. Modern systems use a constant current at around 25mA, fwir.

The project I worked on in '93 was a line powered emergency comms device for the disabled etc, using a cmos processor and no relays, hence the description in the previous thread about mosfet switching and optocouplers...

Regards,

Chris

still not figured out how to use google? on first page of a simple search on 'on hook current draw max fcc':

-Lasse

Engage brain etc... You actually said on hook, right ?. Not much, as if the us systems are anything like the uk, the exchange detects when you forget to hang up and sends a warble down the line as a reminder. They do this to minimise load. ie: 10,000 lines off hook at 25 mA = 250 amps !. Any significant amount would probably log a leaky line fault on modern systems, so would guess 5mA or less, though you could experiment with a trivial amount of hardware.

Of course, you could ask the phone company, but depends what the project is for :-)...

Regards,

Chris

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

I seem to recall that 100K ohms might even trigger their automated loop testing system to declare a fault. However, each central office switch will have its own limit.

I'd just try it - whatever "it" might be - and see what happens. If you see a truck outside your house with a bunch of guys carrying M16's then decrease your load a bit.

Bob

--
== All google group posts are automatically deleted due to spam ==

Google found this:

So it seems (according to unknown authorities posting on the Internets) that the threshold is 18 mA. I didn't read through the entire thread. Someone might have actually cited an authoritative source. A true first for the Usenet, I'm sure. ;-)

--
Paul Hovnanian  paul@hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.

And then there's this:

--
Paul Hovnanian  paul@hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.

"Dc on-hook resistance is required to be greater than 5 megohms for an applied voltage of up to 100 V and greater than 30 kilohms for an applied voltage of up to 200 V (see Figure 3). The telephone companies use an automated insulation test system as part of their maintenance routine. If the dc on-hook resistance of an attached piece of equipment is too low, the line is interpreted as being damaged, and repair procedures are initiated. The dc resistance requirement prevents false service alarms."

CFR 47, part 68.312.b.1.i (1993 version -- you can look up the latest yourself):

"The DC resistance between tip and ring conductors, and between each of the tip and ring conductors and earth ground, shall be greater than 5 megohms for all DC voltages up to and including 100 volts."

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

No WAY! You can get a couple mA, but anything more than that will prevent a line from "hanging up" at the least, and more will put the line in an off hook state. You might be able to get by with as low as

10 K Ohms, but that will likely get a call from the CO people after a while. 10 K at 50 V is about 5 mA.

Jon

Which is more in agreement with the less than 10uA *ON*hook current that I recalled seeing in my errant youth when phone phreaking...

Which reminds me - check any of the alt.2600 usenet groups (lairs of the phone phreaks)

Why not ask Ma Bell?

Good Luck! Rich

--
Michelle?

Nah, she's not interested in how you load the phone line - she just wants to make sure your lunch burger doesn't have any real food in it.

Cheers! Rich

Yeah, I noticed that.

:this implies that current draw of less than 4.8mA :must be seen as on-hook

In the distant past, a device with which I had contact had a figure of 3mA as a guaranteed on-hook condition (as determined by the old "A" relay).

:If you are just doing a home project and not selling equipment, :then I would say you could get away with drawing 1or 2 mA :before anyone or anything would notice or care. : ISTM, that was what Thompson was after.

Sont des mots qui vont très bien ensemble?

I gets all excited when you speaks dirty to me ;-)

This is what I'm after...

Call blocker disconnects telephone from line during the first ring while it examines CID, then reconnects.

I want to detect loss of the ~45VDC when the connection is first broken... resetting everything so I can count to "dump-em" time ;-)

To avoid drawing power that's Ma Bell detectable I'm thinking of charging a cap, dumping it into the LED of a photo-coupler when the ~45VDC goes away.

Any ideas on how to do that? "Loss" is an open connection. It'd be easier if it was a hard zero volts, but it isn't :-( ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed

Install a 20ma relay in series with one leg of the phone line. It will close on off-hook.

Use the contacts anyway you'd like. Used to use this for recording. it starts the recorder when off-hook and stops on-hook. Audio derived across relay coil.

BTW: happy birthday!

On hook?

Not much *ON* hook since 20mA would trigger off hook detection, and that's the 'guaranteed' value. It's less in reality. Up to about 4mA would probably be ok.

Don't forget the ring voltage. You're stuff has to survive that.

I'm not sure there's a regulatory maximum for off hook but, rather, it's determined by the line impedances. I.E. There's only so much you can pull through the line and still have enough voltage for the line to operate (something like 9V minimum if I recall).

50mA is probably a safe off hook value but I've seen app notes that claim you can get up to 150mW.

Note that those numbers are for the *line* and all devices on it so if one, like a line powered phone, pulls half of it there's only half left for the rest. That also includes the load when a (non powered) phone, or phones, go off hook.