I checked the dc voltage on my phone line. The green is +48vdc and the red is ground. In my circuit I disconnect the phone line by disconnecting the green wire because it has positive voltage on it. What is going on here? Why is the green wire positive and the red wire negative? Is +48v the dc equivalent to an ac voltage? If so why is it biased so much to +48vdc?
What is the *correct* way to disconnect the phone service? Short the red, the green or both?
btw, shorting the green wire effectively disconnects the phone service ok, even if it is the wrong wire to short. When people try to call me all they get is a constant ringing, and I hear nothing. In one case an automated message from the pharmacy wouldn't disconnect no matter what. The message kept going even on only one wire. How did that happen??
Telecom has pretty much always been positive ground, but phone lines are balanced and should be floating so I'm not sure exactly what that means when you're talking. Tip = negative and Ring = positive.
In some cases grounding one of those 2 lines will answer a call or get a dial tone (or both ? Can't remember now)... That's how pay phones partially used to work years ago. So, maybe one of the lines were grounded and held the line ? I would think you would get a very loud hum though by unbalancing the line like that. They usual way to disconnect the phone line is to just open both tip and ring (green and red) wires, usually.
"Originally, the voltages on the wires were positive with respect to earth. This is called negative ground, since the negative side of the battery is grounded to earth. Then engineers discovered that with positive voltage on the copper wires, copper wires age quickly, due to electrolysis. With negative voltage on the wires, in respect to earth, (called positive ground) the copper is protected from corrosion. This is referred to as cathodic protection."
Try Googling 'POTS line' (POTS = "Plain Old Telephone Service"). There's lots - o - stuff there. POTS lines haven't changed much since the 1950s, and you could probably make a 1920 phone work with minimal modifications.
The normal standard is for the lines to be a balanced pair, grounded at the CO (or the local repeater), and positive ground (as noted). When your phone is on hook (hung up) your house should present an open circuit to the line. When your phone is off hook it should present a resistive load of about 600 ohms to the line. If it's a normal residential POTS line then you shouldn't connect one wire to anything other than the other wire, and you really (by law) should only do that with an FCC approved phone (although if you present it with something that looks like an FCC approved phone to the wires, then things should work OK).
If you're having issues with the phone line, you should try calling the phone company. Callers -- even machines -- who refuse to hang up is a business problem, not a technological problem, and will probably fall to a business solution before it falls to a technological one.
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
The phone operates at "-48VDC" (talk battery -- there really *is* a "battery" in the CO -- a really BIG battery!). This helps reduce corrosion in the wires (metal ions are drawn *to* the wire instead of sucked away from it... an important issue if your company has lots of money invested in *wire*!)
Ringing happens with 40-~130VAC (~90VAC nominal but depends on how many REN's are hanging off the line -- all that wire has some resistance to it!) applied to the pair.
What do you mean by "disconnect"? The "on-hook" condition is signalled by an "open" -- effectively no load (ring signal is capacitively coupled to the ringer (and mechanical ringers were designed to resonate at the ring frequency). When the phone goes "off-hook", the CO (or SLIC96, etc.) senses the sudden load (~600 ohms) and removes ring signal (loop-start) leaving talk battery (reverse battery). When you "hang up", the CO will "acknowledge" this by re-reversing or momentarily *dropping* battery.
[there have been different signaling schemes used over the years, e.g., ground-start was a common trick with payphones...]
If you want to *look* like your phone is "in use", put ~600 ohms across the pair. If you want to look like your phone is disconnected, DISCONNECT IT! (Gee, clever idea, that!)
[A heap "old button" can be fabricated out of an LED and bias network]
There are ways to remain connected yet present an even smaller profile.
You really don't want to short the pair. Nor let it reference earth, etc. TPC routinely checks lines for trouble and may elect (automated) to pt your line out of service if it finds such a problem. Perhaps even dispatching a service call (if the trouble is found to be *yours*, you might end up with the bill)
The reason they went to negative ground was in the days of single wire circuits using a ground return path they ran into corrosion problems with the ustomer end ground rods. Going around replacing these was getting costly so they reversed the loop votage moving the issue to the CO end, which then became a simple routine maintenance issue. Going to a loop system eleiminated the issue pretty much entirely with only a few special circuits such as PBX lines using ground path returns anymore.
(& the statement that T1's are polarity sensitve is incorrect too)
One must also consider that when a phone goes 'off-hook' the exchange may reverse the polarity. This is done around here (not sure of the historical reason) with the telephone system.
shorting the wires should look like "OFF HOOK" and after the sequence of the phone company saying phone is off and beeping at you [which you can't hear since the wires are shorted] the phone company disconnects you from their network. Then, I don't know if the line appears BUSY or simply RINGS to an incoming call.
OFF HOOK is more like 100 to 120 ohms DC resistance and indeed is 600 ohm AC resistance.
And the phone compnay switches between the power supply that senses whether your phone is ON/OFF HOOK to another different supply that is used for conversations. You can actually see athe voltage drop to zero during that transition.
For operating ranges, like open circuit means leakage currents less than ?? uA [I know 1 uA is allowed.] see the IEEE spec, used to be FCC Part 68, but now it's an IEEE spec. There are all kinds of min/ max operating ranges listed in the spec.
I didn't know that any phone systems still did that. One reason for polarity reversal was for billing purposes. Depending on the old switching system, #1 crossbar, #5 crossbar, step, etc., the reversal would be for just a moment or for the whole call and sometimes not at all to the end user.
My experience with phone systems was from many many years ago and a lot has changed sine the 60s and 70s. But the loop from the CO to the end user is pretty much the same.
If your area has a telephone museum, I highly reccomend going to see it. Seattle has a very nice one called the communications museum now thatat least used to be run bypeople from the telephone pioneers of america and has #1, #5 crossbar, ESS, AND Panel all talking to each other as well as many more very cool exhibits. The volunteers can easily answer most of these questions too.
My recollection from dealing with system houses that design line powered gear is that you never trust the polarity of a phone line. They add a diode bridge to get the proper polarity.
OK, this is EDN, but still, I believe this is the general scheme for line powered design.
Tip - Green - wh/blu Ring - Red - blu/wh Ground - (Usually not used internally today)
There are a number of ways that other conductors could be used.
Ground start and loop start are two different ways to indicate you want a dial tone. Most modern COs provide primarily loop start circuits.
Usually ring voltage is about 20hz and around 90-100 volts AC. Most commonly this is over the same pair of wires as the voice, and dc power. However at one time two party party lines where common and they would send the ring voltage from tip to ground for one party and from ring to ground for the other party. This was a quick and dirty way to provide more phone services than they had pairs in the field.
Regarding party lines, another way it was done was to send different frequency ring power over the talk pair. This required multiple ring frequency generators in the CO, but were often implemented to provide service to upto 4 different parties on a party line. They had telco provided phones with ringers designed to ring at only a specific frequency. I have heard that some services attempted to implement ground ringing and multiple frequency ringing for upto 8 party lines, but that it did not work very well.
Most modern residential telephones are loop start and have "line voltage" ringers. They will ring at any frequency ring voltage received.
If you were on a party line in the past you were not allowed to provide your own telephone equipment. With modern analog and digital carriers capable of providing multiple true lines of service over limited pairs of wires I am not sure if there are many (if any) places where party lines are still in use.
I might add a short to ground may or may not cause the phone line to fail. Usually in normal circumstances a short to ground is due to a defect, is typically higher resistance, and causes loud obnoxious noise on the line. Similar, but not the same as an imbalance from one of the pair being broken in the field. A good phone man can hear the problem and identify it from the sound usually. Then its just a matter of determining the distance to the fault. A dead short on the line between the pairs is interpreted as a busy and will time out eventually. It may even be disconnected at the CO. It will depend on the CO and the actual management practices of the personnel there. An open an open on one conductor will simply not connect to the site. A call will ring until they give up or in some cases times out after so many rings (due to black box prevention timing).
Why not just buy a phone with a selectable ringer and turn off the ringer.
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