Paul Sutter <sutter@apple.com> (05/15/91)
Three DAA questions: 1) FCC Part 68 says the dc on-hook impedance should be 5 megohms. EIA-470 says at least 25 megohms. Which should I follow? iI is much easier for me to exceed 5 megohms than 25. 2) Many DAA circuits I have seen include the following surge protection: (tip) ----/\/\/\/\/----+-------- | (varistor) | (ring) ----/\/\/\/\/----+-------- With 5 ohm resistors, how are the wattage ratings determined? I have seen anything from 1 watt to quarter watt resistors used. likewise I have seen variation in the varistor used. Since these are for surges, how do you calculate the necessary ratings? 3) Can anyone suggest a cheapo transformer with 1.5kv isolation that does not have to be beefy enough to pull the DC offset? The cheapest we have found was a Midcom for $.80 (in moderate quantity), but since it was designed to pull the DC offset, I suspect a cheaper/smaller one may be found. Any suggestions? Thanks. Paul Sutter (not writing on behalf of my employer)
Jon Sreekanth <jon_sree@world.std.com> (05/27/91)
In article <telecom11.372.8@eecs.nwu.edu> Paul Sutter <sutter@apple. com> writes: > 1) FCC Part 68 says the dc on-hook impedance should be 5 megohms. > EIA-470 says at least 25 megohms. Which should I follow? iI is much > easier for me to exceed 5 megohms than 25. The way it was explained to me, for a "DC REN" of 1.0, the DC on-hook impedance below 100V should be 25 Meg. Since a max of REN 5.0 is permitted, the minimum DC impedance is 5 Meg. I've seen the EIA spec, but it can't be mandatory, because I have a Rat Shack gadget which has detailed specs, and lists its resistance as 10M on-hook, 300 ohm off-hook. The only issue is whether you expect other parallel connected devices (extension phones, etc) sharing the line with your device, in which case you don't want to use up the REN 5.0 budget. > 2) Many DAA circuits I have seen include the following surge > protection: > (tip) ----/\/\/\/\/----+-------- | (varistor) | > (ring) ----/\/\/\/\/----+-------- > With 5 ohm resistors, how are the wattage ratings determined? I have > seen anything from 1 watt to quarter watt resistors used. likewise I > have seen variation in the varistor used. Since these are for surges, > how do you calculate the necessary ratings? The way I've done it is figure out the maximum "regular" voltage on the phone line (48V + 1.4*150 ... FCC ring B has a max of 150V). Varistors seem to have a broad range of breakdown voltage, not a crisp zener like voltage, so I've ended up using a 360V nominal varistor. The resistors should have enough wattage not to blow under the FCC simulated lightning strike (25A of 10*560 uS surge at 800V). I actually use a slow blow fuse in place of the resistors; that gives me UL1459. Regards, Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com