Since this seems to be the day for rummaging around into oddball thoughts:
I have been running new phone line at the fire station. Some of it may have been as old as 1947 when the brick part was built. The goal was to get better internet to the modem which happens to be upstairs at the opposite end of the station. I have been told that polarity doesn't much matter on these newer phone systems but I wanted it done right anyway.
After some research, I found out that the green wire (tip) is less than one volt and the red wire (ring) is -48v DC. The tip and ring referring to the old switchboard where the operator actually plugged you into a line to complete a call. Think of a mono or stereo phone plug with the tip at the end and the ring being right below the tip.
Checking the wires with a volt meter, red lead to the green wire (tip) and black lead to the red wire (ring), I get +48 volts. Red lead to the green wire (tip) and black lead to a good ground I get less than a volt. Red lead to the red wire (ring) and black lead to a good ground I get negative 48 volts.
I just don't understand the concept of having a negative 48 volts DC. I'm hoping someone can explain it to me.
Anyone?
James
I had zero clue but was curious so take the following for what it is worth.
Yep, that's it. Negative voltages repel negative ions, which tend to
be the ones that corrode metals. Same thing is used to protect
buildings, pipelines and underground tanks. It's called cathodic
protection, and the posative voltage is applied to a sacrifical anode.
The anode by itself is often used to protect smaller tanks.
BTW, phone lines are AC ballenced, with about 60 dB of CMRR, which is called
longitudional ballance in the industry. It can even be measured by
meters such as the sidekick. Poor CMRR causes power line harmonics
(espically 540 Hz) to be audible on the phone line.
Pat Turner FAA Telecommunications
Voltage measurements are always relative to something--you are measuring a difference, not an absolute.
If A to B is positive then B to A is negative. Nothing magical.
Just like measuring heights.?ÿ
Back in the day the positive anode of the wet cell batteries in the central office were grounded as opposed to the negative (cathode) being ground.?ÿ For some reason most old telephone circuits were built this way.
Corrosion protection, as mentioned in the quote above.
@bill93 If I put the red lead on a good ground and the black lead on the red wire (ring) I would have a positive 48 volts DC but ground doesn't have a charge.
I just don't understand the concept of creating a negative DC charge when compared to ground.
So you could compare it to an old 6 volt positive ground car?
@jaro?ÿ
I had (at different times) a 1951 Chevy sedan and a 1951 Chevy PU and both were 6v systems.?ÿ The car was a negative ground and the truck was a positive ground.?ÿ If I needed a jump for the PU I had to make sure I was the one attaching the cables.?ÿ
