In a message dated 96-08-27, Lars Poulsen writes: > >If a DAA were to limit the current to 20 mA, would that violate the >French requirement ? (Since if you are at 20 mA, you are still limiting >the current to 60 mA or less.)
Lars: Your point is a good one. There is some benefit from reducing the current, but not as much as you might first expect. This is because as the current is reduced, the voltage across the TE increases. Since power dissipation is the product of the current and voltage, the power does not decrease proportionately with current. Some benefit is available, though. Using the 54 volt, 300 ohm feed condition in France's B1123A specification (prTBR 21 uses 50 volts and 230 ohms), the calculated power at 20 mA is 1 watt, at 40 mA it is 1.7 watts, and at 60 mA it is 2.2 watts. In the French B1123A spec, there are some considerations that might make it undesirable to use a current limit setting as low as 20 mA. These have to do with other requirements that specify minimum currents that must be drawn under other feeding conditions. The specifics depend on the type of current limiter that you implement (flat-top of fold-back). In prTBR 21 (at least the draft that I have), there is a requirement to draw a minimum of about 49 mA with a 50 volt, 230 ohm feed. Combined with the upper limit of 60 mA, this gives us a relatively narrow "window" for the current limiter. It is difficult to stay within this window over all operating conditions and production tolerances. Of course, the 49 mA lower limit also guarantees that the DAA will have to dissipate about 2 watts in the worst case feed condition. I can't see any technical justification for setting the lower limit at 49 mA. I think that 20 mA or 25 mA would be fine for ensuring that the central office continues to recognize the off-hook condition. As I said in my earlier posting, I am hoping that the requirements for current limiting are either relaxed or deleted before they show up in CTR 21. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
