atcomsystems.ca/forum Forums General General Telephone & Systems Information loop current

Skip, It was sometime ago when I was still trying to convince the lec, so had nothing to do with you. He is a very interesting person to talk to. I don't remember where he said he got the actual information, but he did say he also couldn't get his hands on actual standards to support it. When I was checking it out I got the same info and I also couldn't tell you now from where.

Remember that the loop current you're measuring is OFF HOOK, and that's NOT 48V. It's going to be more around 6 to 12 volts. So actually terminate the line with a 600 ohm load, then measure voltage and current, then do your calculations to arrive at the resistance you want to stick in the circuit.

Then, divide the resistance by half, and put a resistor in EACH side of the circuit - one on the Tip, and one on the Ring. Otherwise, you're going to have an unbalanced circut.

When calculating the Wattage, keep in mind that you also have to pass Ringing current as well.

According to my source, the off-hook voltage should be between 3 V and 9 V.

Also according to my source, a typical telephone will have a resistance of about 200 ohms, although it may be between 100 ohms and 1000 ohms.

The off-hook current will vary depending on the resistance of the telephone or test set used. My Lil Buttie Pro test set displays off-hook current and has an "apparent" resistance of 275 ohms, according to the manual.

In order to test out the line, fist measure the on-hook voltage in parallel to your telephone or PBX. Make sure that it is around 48 V. If not, it must be corrected because there is probably a a break in the line. Then measure the off-hook current in series with your telephone or PBX. This should be between 20 mA and 75 mA. The current is very much a function of the resistance of your particular telephone or PBX.

(Remember not to measure current in parallel with your telephone, or you will blow out your meter.)

If you think 75 mA or 65 mA is too high, then you can lower it with resistors or the Sandman's device.

The idea of using balanced resistors instead of one resistor has a lot of aesthetic appeal, but I am not sure it is sound. Can anyone explain this?

MP:

Does your "source" work for Verizon? If so, I will pay you a lot of money for their phone number (joke).

To answer your question about balanced resistors, since tip/ring is a balanced pair that sometimes travels for miles, it's very important that the individaual conductors be approximately the same length door to door. If this isn't maintained within reasonable tolerances, the pair becomes unbalanced and subject to AC induction (hum).

Load coils are placed every several thousand feet of cable to equalize imbalances that occur over long distances of twisted pair; these bring the pair "back in balance" so to speak. You can't miss them on poles, they are about the size of a trash can, usually bolted parallel to the pole but sometimes attached to the strand (the little ones anyway).

Adding a resistor to only one side of the line could introduce enough of an imbalance to cause hum.

thats a lot of feedback. thank everyone so much. i need to print it out and get to work. thanks again. dave

If you click on the word "source" you can see my source.

I still don't get this balanced line thing. What does "balanced pair" mean? The current is the same everywhere in a loop.

I mean, what is the difference between

------------600 ohm-200--ohm--600 ohm------------

and

--------------1200 ohm--200 ohm------------------

Few things to keep in mind ya’ll… There’s a lot of different signals (both DC and AC) coming down them there phone wires. Most of ya know stuff but as I read down through here seems like some ya’ll get mixed up a bit.

DC current will ONLY be seen when off-hook and should range between 20mA minimum and 65mA. The first is almost a thing of the past now days with cordless phones and key systems out there, is to power the teleset when in use and two is to let the switch know that the teleset has gone off-hook and it now needs to do something about that.

The battery source is located at the C.O. (or remote) is 48 V DC. The only variance to change the DC loop current from the Telco side is the inherent resistance of the conductors that make up the pair (total round trip.) Then there’s also the DC resistance* of the equipment attached to factor in the complete circuit. You always here folks say NEGATIVE 48 volts when referring to the voltage. That’s the way the C.O. batteries are set up as a reference. 48 volts is 48 volts the negative is just seen when you measure it cross tip and ring.
* Don’t confuse the impedance rating expressed in Ohms as the DC resistance… two different things.

Placing resistors in series (with conductor) on both tip and ring will reduce the amount of loop current. To all signals (the AC signals too) these resistors will electrically appear to be a longer loop. Adding a 300 Ohm resistor to each side (600 total) would be ‘bout the DC equivalent of adding 5, 000 feet of cable to the loop. Just fine if you’re ONLY concerned with DC current.

There also a couple different types of AC current superimposed (DO NOT FORGET THE SUPERIMPOSED PART) over the 48 VDC.

Everybody’s favorite AC voltage RINGING! Yaaaaa! 104 V AC supply with 30Hz on for 2 seconds then off for 4 seconds. If enough gets there to initiate the teleset to ring, you’re good. When talking about adding resistors to the circuit as long as the telephone rings and the resistor is capable of handling the current, who cares.

Then the other and most important AC component you’ll see on the telephone line is the one were all here for right. The signal! The voice traffic! 300 to 3000 Hz AC signal of varying voltage levels… We (the telephone company) will not ever measure this in terms of voltage or current. The will me measured and referred to as a dB (decibel) level. Decibels is a ratio and is in reference to a 0 dB.

For frame if reference over 0 dB voice level is a hot signal and at risk of overdriving equipment. 13-14 dB (negative or below the zero refernce) is usable but low. 6-8 dB negative (say that as “6 to 8 dB down”) is right round perfect.

Adding a 300 Ohm resistor to each side (600 total) would be knock you’re voice signal down roughly 4 dB. Is it was low to begin with you’re going to be WAY in the dirt… If you were a bit on the hot side, well… it will talk just fine then too.

at a basic level A balanced pair means the tip is the same length as the ring...when a pair is unbalanced you basically are building a long antenna which will cause noise and transmission issue

88 micro farad load coils are placed in a standard loading scheme 3000 ft from the C.O. and then every 6000 ft after that. They have little to no effect on the DC component being passed down the pairs. They are there to off-set the capacitance that is inherent to two conductors being twisted together for thousands of feet.

They are used to ensure the voice level rolls off (goes down) flat over greater distances. Meaning that a 250 Hz signal losses roughly the same amount as the 2500 Hz as the signal travels through the copper. If those frequencies didn’t stay close to the same the voice may make it to the other end but the characteristics of the voice of the person speaking would be lost.

BTW, Gee whiz stuff, the smaller silver upside down looking trash cans on the poles are usually repeater housings for Digital Data services.

Good info Bryan!