Joe, The advise not to worry about current limiting in countries that don't required could be a problem down the road. If approvals are required later in countries that require limiting, product will not pass. This will require a redesign or at least re-approval and limits its marketibility.
In my opinion one size fits all is the most economical way to go. i.e. depending on what country your in select or de-select current limiting via firmware. Duane >In a message dated 98-07-15 17:11:36 EDT, Peter Halford writes: > >>I am more than a little worried about my Gyrator. The 50V thru 230R >>test is going to waste my existing design, even though as a matter of >>course I have almost always included an I-limit of 60mA where I was >>able to, however, 2.2W is not going to do a BCX38 any good... Can >>anyone please suggest a suitable SMD transistor which they have already >>used to good effect? >> >>Is there any kind soul who could give some advice regarding the holding >>circuit ( re: Caveats? ). > > >Peter: > >I am out of my office right now, so the part numbers that I mention below are >from memory. Please pardon me if they are incorrect. > >Your concerns about the TBR 21 holding circuit are justified. There are three >issues that must be addressed: > >1) Approximately 2 watt power dissipation in the 50V/230 ohm feed condition. >2) Requirement to limit the loop current to between 49.5 and 60 mA. >3) Stringent requirements for the off-hook transient response of the loop >current. > >For requirement #1, you can use the Zetex PZT604 darlington in a SOT223 >package. You could also consider using a Motorola MJD243 NPN in a DPAK >package, with a SOT 23 NPN such as the Motorola BC846 connected to the base- >collector of the MJD243 in a darlington arrangement. I have used both of >these configurations successfully. I particularly like the MJD243 solution >for applications where the required board area is available (it runs cooler >than the PZT604). > >You should also pay careful attention to items #2 and #3 above. Since these >parameters are easy to test on the bench, you should evaluate your circuit >yourself before you waste time and money at a test lab. > >I note that you do not mention the country (countries?) where you intend to >seek approval. Many countries which base their national requirements on TBR >21 (e.g., UK and Switzerland) do not impose the current limiter requirement. >So, depending on your target market, you may be able to avoid the current >limiter requirement for the time being. > >I hope the above is helpful. > > >Joe Randolph >Telecom Design Consultant >Randolph Telecom, Inc. >781-721-2848 (voice) >781-721-0582 (fax) >[email protected]
