# Phone lines??? AC or DC problem

Discussion in 'General Electronics Chat' started by Mathematics!, Apr 28, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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I am confused are the phone lines (i.e green and red lines) DC or AC voltage.

Because when I measure the lines I have to set my multimeter on DC if I set it to AC mode I get 0 v.

In DC mode I get about 50 volts.

But then I saw this video at the bottom of the page of this link
http://www.engadget.com/2007/10/14/tap-into-the-phone-companys-current-if-you-dare/

They use a bridge retifier and a regulator? I thought bridge retifiers where used to convert AC to DC. So I should have my multimeter set to AC mode?

If it is not AC then why the retifier and regulator?
If it is AC then what is the frequency is it 60 HZ like the power lines?

2. ### Bernard AAC Fanatic!

Aug 7, 2008
4,170
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A bridge is also used to give a known polarity output regardless of input polarity, AC orDC. Draw a bridge rectifier and check it out.

3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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So a bridge retifier can be used with DC input to DC output.

I thought it was just for AC input DC output. (to convert AC to DC )

O, so do you mean it's just their to make sure the polarity +- are at the right places regardless of if you connected the green or red input wires the wrong way. (so bridge rectifiers can be use to make pluging a DC applaince into a DC power supply anyway without worrying which way the plug goes cool)

So is it DC ?

And if so I would think the electrical impulses travel both ways to make the analog voice. Because I would think pulses going in both directions would be need for a 2 way conversation.

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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It is indeed DC. Green is ground. Red is negative. Until the phone rings.

Ring voltage (in most US locations) is about 90VAC at 20Hz.

Note: Using power from the phone company this way might lead their big green box to think you have left the phone off the hook. This will happen if load resistance is down below 300Ω (give or take). The off-hook warning tone is a series of AC pulses - frequency and voltage vary by utility provider. Phone power will shut down after a bit if this is the case. The big green box will then check in from time to time to see if you've hung up yet.

Nov 25, 2008
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6. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Yes, indeed. It is a "grubby" little system. GRBY. Green,then Red, then Black, then Yellow.

7. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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Yes , I know how GR BY work.

Ok , DC but then how do they change the DC to AC.

They must do it at the switch but I was under the impression the POTS
circuit was just that each person had a green and red wire connected directly into the switch. And the switch just connected the red and green wires of 2 or more houses When a call was made. Just like they did when it first came out in 1910ish. Ofcourse now it is all computerize so no operator is required to physical plug wires directly into 2 or more peoples lines to make the connection.

I noticed I made mistake saying the lines are physically connected to each other. They do use multplexing at some of the big boxes before it reachs the switch. But local calls are like directly connecting the wires.

Anyway I am still wondering how they turn the DC generator into AC at the switch. And when do they use AC only for the ring or is it also the whole conversation as well?

The generator they use is DC generator or is it an AC generator?

8. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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Oh, good golly no! The big green box down the street is just chock full of sexy electronics. Near the end of this link you can find a picture of an older style card which would handle eight lines. The newer cards are smaller, denser, and more sophisticated. The green boxes are typically connected by optical fiber.

9. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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I guess we are not talking about the same thing.

The small green box's contain 25 to 50 wires maybe up to a hundred in some places that you can use to make free phone calls if you open them up. These box's are usually placed between 2 or 3 houses.

Then different neighborhoods boxes tie into a medium size green box. Which is 100 wires to 200 wires or so.
Up to this point everybody has a seperate wire

Then the medium size box runs to a big green or white box that has a power meter by it. This is where it is multplexed onto fiber optic lines and sent to the switch. Which the switch then demultiplex's the local calls and routes the long distance calls to the next switch station using fiber optic trucking ...etc

Main questions

But I wanted to know if their generators are DC or AC ?
And when do they use the AC part for the ring of the phone or for the conversation as well.
And how do the vary the DC to make it AC?

What happens if you put a 300 ohm or more resistor between the red and green wires. Will the power still shutdown?

Last edited: Apr 29, 2009
10. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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No phone company I know of has their own generators. They send DC down their lines. They buy from the power company like everyone else. With battery backup. Big big battery backup! 200 Amps (or more) at 48 Volts battery backup. (Ever see a screwdriver explode?)

The ring is about 90VAC. Voice runs 30-300 Hz AC (not a sine wave) superimposed on the DC.

Oscillators for the ring voltage. OPAMPS for voice.

What happens with resistances in parallel? http://www.allaboutcircuits.com/vol_1/chpt_5/3.html

11. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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So when power goes out how long will it take to drain one of those Big big batteries 200 amps. amp-hours?

I thought POTS used anolog voice? So they use digital???

So the microphone is not a regular microphone but has an ossilator in it. To make square waves?

And the phone speaker works with binary pulses instead of analog ???

Obviously you have the same voltage but the current drops.

guess I don't understand cutting the wire and connection a resistor between the green and red ends would be in series unless you extend the green and red wires beyond the resistor. But you are connecting green and red directly together with a resistor. In series will drop voltage and probably drop some current as well. So how are they detecting a phone off the hook?

12. ### Audioguru New Member

Dec 20, 2007
9,411
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When a 300 ohm resistor (or an off-hook telephone) draws DC current then the central office detects the current and either makes a dial tone or it answers ringing if the line was ringing. They have a timer for the dial tone that causes a beeping noise then they shut off the phone line if the current continues without a call.

13. ### Lmccrock New Member

Apr 29, 2009
1
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The big switches have a generator backup, just like some industrial settings, to replace AC if the mains go down. I am not sure if there is a standard for how long the batteries last, but certainly long enough for short duration outages.

POTS is analog, like a Plain Old Telephone, typical for home use. The phone company digitizes A-D to transport the call, then converts D-A at the other end.

The microphone in POTS is analog. The ring voltage is usually a sinusoid (some office systems use a square wave or modified square wave, so if you are building a ring detect circuit for a phone, you have to know this).

In the old days (pre-digital), there was a pair of wires which went from the premises (house, office, whatever), all the way to the phone company, maybe miles away. In a rural setting, maybe 10's of miles. There would be boxes to splice individual wire segments, but a DC path all the way.

Lee

14. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
POTS means "Plain Old Telephone Service." Today they call it "Category 3." It is a cable. It doesn't care whether it carries AC, DC, Analog, Digital, or all four simultaneously.

DC and AC can be on the same wire at the same time. There is a lot more in this universe than square and sine waves!

Microphones do not contain oscillators.

15. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok I get everything you guy's are saying
Thanks

16. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,039
287
Howdy:

Telco companies (central offices) still have MASSIVE 48 volt batteries to supply the operating current for landline phones. They probably will continue this until the last landline phone goes the way of the Pyramids.

The standard is POSITIVE ground for the simple reason that the early storage batteries (before good plastics were available) used a lead vat as the container and positive electrode for the storage cells. This was a natural ground point, of course.

Telephone circuts involve both DC problems and AC transmission line problems, which can be astonishingly complex. If you want to read some fascination history, look up the development of the long-line "loading coils" used for most of the past century. It was an astounding insight when they realized they could reduce transmission line losses by added lumps of inductance along the way.

I wish I was smart sometimes.

eric

17. ### 316204 New Member

Jul 13, 2008
2
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phone voltages to talk across is 48volt dc on hook.
off hook becomes about 6 volts dc.

ringing voltage is 90volts ac about 20 hertz/cycles.

thats newer phones have bridges/rectifiers.
hope this helps.