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]
