Hi Treg In Europe, we are used to dissipating a few Watts. The UK spec used to be 50v 400 Ohm with a max feed of 125 mA (i.e. a load line from 50 v 0mA to 0v, 50/400). If we go back to the days of pulse dialling, power could peak at over 6 W during the s/c part of the dial pulse. A modern dc sink circuit (some call it a gyrator) will have no problem handling this and in fact there are some good SM power transistors for this. The idea of using a seperate current sink is as follows:
1. High ac impedance (has little effect on the transmission path impedances). 2. Slope of the sink and max I can be set by simple diode / resistor network. 3. Sink can be made to remain active during short breaks in line feed (by adding a small value cap) Have a look at Texas (and others) data books for modem chips. Some of these have reasonable sink circuits. Regards: Bill Ellingford Approvals, Motion Media Technology ---------- From: Bob Lee[SMTP:[email protected]] Sent: 12 March 1999 13:26 To: [email protected] Cc: [email protected] Subject: TBR 21- 60 mA max. loop current Subject: TBR 21- 60 mA max. loop current Hello all, I am brand new to this list.....so please be gentle 8-) My concern is the possible increased power dissipation re- quired by the terminal equipment (TE) when connecting to the EU PSTN. Assuming the worst case of: the test model 50.0V (could even be higher in practice) battery and 230 Ohm battery feed resistance, and short (co-located) loops, this equates to 50.0 -.06A x 230 = 36.2V across the TE T,R port. Now, since Ohms (not to mention Kirchhoff's) law has yet to be repealed, results in a maximum per port power dissipation of P(T,R) = 36.2V x .06A = 2.172 Watts! This is about (in our case) 3x the heat dissipation that we ob- serve when interfacing the North American 48V/460 Ohm battery feed model. Has anybody out there also "taken the heat" 8-) and if so, been able to defy Mr. Ohm and/or Mr. Kirchoff and somehow dissipate less heat (and still meet TBR 21, of course)? Thank you...cool it...and.. cheers, bob lee
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