Telephone Conferencing Equipment. What is proper telephone line current?

Edwards Finance > Phones

Q. Whall loop current is an important factor the actual voltage drop acrost the phone is also important. If the phone is not supplyed with sufficent current or voltage improper opperation can result. The fact that one phone fails to opperate when another is also off hook is typical of low loop voltage. Most "electronic" phones(as opposed to a traditional 2500 type set) require 5-6 volts minimum for proper opperation. If they dont get it they just "crap out" From your description I suspect that you are at the end of a very long loop or possably on a SLIC system and the circuit serving you phone may be acting marginaly.

A. How much current each requires to function is uniquely a matter of the design of that telephone set. There is no standard. Obviously if the current becomes too low there will not be enough voltage drop across the set to power the keypad, if it is line powered. Likewise the transmitter (microphone) is usually line powered and requires current to work, but how much depends on who manufactured it. The essense of it all is that using different styles of phones will make it difficult to use more than one at a time. There is nothing the telephone company is likely to do which will affect the problem. Note that even two similar phones do not work well together, as they each divide the incoming signal equally, thus making it half as loud for either compared to what it would be if one set only is used. In addition, the level from the other phone will be significantly louder than the level coming from the distant end. On a single telephone set the level you hear yourself at (sidetone) is reduced to make it reasonable, but you hear the extention at full level. I found a data sheet on Nortel's site for a POTS interface device, it's quite possible SaskTel's using this or one like it, I think they do use quite a bit of Nortel equipment. Anyway, it states that there are multiple settings for loop current limiting ranging from 23 mA to 75 mA, and that the 75 mA limit is the default. It does seem regulated, as it's always about 24 mA even when you short the line. Sounds like when more than one device is off hook, the voltage gets cut to keep the current at 24 mA which has the side effect of sometimes making one of the phones malfunction in the process because it's getting perhaps 3.5 volts. Definitely. There are variations, but yours sounds like it has a genuine current source. The original concept was just a high resistance battery tap, which more or less provides a degree of regulation (it sets the maximum current, and then the length of the loop can't be so long that it goes below 23 mA. But since yours was 24 mA with a single telephone set offhook and with a dead short, it most likely has a current regulator on board the line card.

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