Telephone CO Question

See if this will help, Jim.

John

\ Hi jim, I don't know what they do today, but in the olden days there was a coil that was activated by that current, so was limited by the coil (and line) resistance.

In modern CO equipment, they would basically have a active sense circuit to detect and measure that current. Too little current, and they determine a line fault condition. Probably too much causes an alert as well. Since there are multiple customer circuits on a single line card, I would assume that they basically have a resistor in there to measure current across, and maybe a relay to cut the line power if it is out of service...

Charlie

I have sort of a weird line... almost textbook... 50VDC open circuit,

50mA DC when shorted, so a 1K source impedance (or an active circuit).

I going to switch on a "noise maker"... at first jolt, it's simply just a small valued resistor in series with a zener and a diode.

I don't have any current probes so I was just trying to estimate the initial surge. I guess just try it while keeping my face covered ?:-) ...Jim Thompson

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| 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

'The Phone Company always has a steady source of DC power available. Huge banks of storage batteries backed up by generators, supplying from

23 to 80ma of power on every phone line... for you to use whenever you want!"

Nice product list, including:

TELCO POWERED? VIBRATOR

There's a lot of stress when the power is out!

Use our soothing Vibrator to relax your muscles after dealing with this serious problem. Click on the Vibrator to see it Glow in the Dark.

Use it as long as you need to... it's powered by the phone company!

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

An old Strowger CO (aka step-by-step) had -52V (or so) in series with a 200 ohm resistor (iirc) and also in series with a sense relay and a coupling transformer. The net source resistance was about 400 ohms so you could get about 120mA if you shorted the loop AND were close to the CO (i.e. minimal loop resistance). I believe that crossbar switches had similar line circuit characteristics.

When I was designing stuff for a digital PBX, I was using silicon devices that were connected to the same -52V battery supply but had a psuedo current source that provide a maximum of 40mA into a close-to-switch shorted loop, but the current would tail off if the total loop resistance got bigger than

52V/40mA (ohms). Modern COs have similar electronics and might also provide less open-circuit dc volts.

Keep in mind that you might have a decent amount of capacitance from tip to ring due to the twisted pair, but there's always some series R. The point is that your initial short circuit current will be somewhat higher than the dc steady state values. Also, with old switches, there's a ton of inductance in series (several Henries), so when you open the loop you will see many volts due to Ldi/dt.

Bob

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

How far are you from the CO ? Don't forget to add the miles of copper resistance of the loop.

boB

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Try values of R between 0 and infinity. I did once and calculated my line source resistance, but I forgot what it was.

--

Reply in group, but if emailing add one more
zero, and remove the last word.

Across the park from the CO... maybe 1/2 mile as the wire goes ;-) ...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

Good point. I just did open and short measurements. A variable R would detect if it's resistive or an active circuit (like a current limiter). ...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

And all the inductance/capacitance of that loop cable! But I wouldn't worry too much, as it is pretty benign. However, consider lightning surges from nearby strikes!

Charlie

All underground here. ...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

[snip]

What sort of "pop filter" could be placed across the line without upsetting Ma Bell?

4.3K in series with 10nF ?? ...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

| 1962 |

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

Can still couple to the cable, as the strike goes to ground...

Charlie

Famous last words. Lightning can (and seems to prefer) striking high value underground equipment to some depth. And then there are plain old power system faults that cause nearby, mutually grounded conductors (cables in the same trench) to jump in voltage.

Invest in a couple of surge arresters (even spark gaps are better then nothing).

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
"Nothing Important Happened Today"
        -- King George III, diary entry July 4, 1776

I wonder how long it takes after the telephone goes off hook (or someone plugs one of these in) without placing a call until the telco assumes a fault condition and cuts the line off.

After about a minute, they start playing that annoying warbling tone.

That explains the wife spending so much time on the phone. I thought she was getting some somewhere else. Little did I know it was the phone company itself.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
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Write a wise saying and your name will live forever. -- Anonymous

I have a whole-house arrester at the service entrance. Ever since some years ago we had a substation 3 miles away hit by lightning... trashed a couple of TV sets and all the light dimmers in the house ;-) ...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

Has Larkin been sneaking into your house and using your phone vibrator ?:-) ...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

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Add an appropriate one on each telephone line. And cable TV if installed separately. Try an Internet search for 'subscriber line interface circuit'. There used to be a hot market in ICs to use for that.