In a message dated 7/1/99, Peter Merguerian writes: > By the way, a transformer is not always needed on a modem. The > new design of modems have capacitors (not required to be > Approved) between the TNV-3 and SELV.
Hi Peter: It is certainly true that some of the new modem DAA devices use capacitors instead of a transformer (Silicon Labs, Krypton, Conexant, Siemens, Analog Devices, etc). However, I do not agree that the capacitors somehow do not have to be approved. On what basis have you reached this conclusion? There certainly does seem to a debate about what requirements might apply to capacitors used in this application. For basic insulation, some people have argued that as long as the physical size of the cap meets the creepage and clearance, and the cap passes the hipot test, you are all set. My impression is that some safety labs accept this interpretation, while others do not. Sometimes the design goal is to provide supplementary insulation, to comply with requirements in Norway and Sweden. In this case, the isolation requirement includes a minimum distance throgh solid insulation of 0.4 mm. I believe that most multilayer ceramic caps violate this requirement. It is still possible to use such a capacitor by using an approved Y2 cap (per IEC 384-14), but these caps tend to be physically large and somewhat expensive in surface mount. I am aware of a couple of vendors who are trying to get smaller caps approved for bridging supplementary insulation, but I have not heard yet that these efforts have been successful. Do you have more information on this subject? Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (voice) 781-721-0582 (fax) --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
