In a message dated 5/2/2003, you write:
For cordless phone, in most cases, the charging circuits are touchable. So based on 6.2.2.2, we need apply 1000Vac between the charging circuits and TNV-3. Usually it'll fail if there is no isolation transformer after Tip and Ring. Most of the manufacturers don't want to add an isolation transformer to avoid increasing the cost. So does anybody there have any idea to solve this problem without an additional transformer? Hi Carl: There are several ways to solve this problem, but the most straightforward options are the ones that you have already identified: 1) Make the charging contacts inaccessible. Note that you can probably call these contacts "connectors" and use the blunt probe. Another approach would be a mechanical interlock that only connects the charger when the handset is in the cradle. 2) Use a transformer in the phone line interface. Suitable transformers only cost 30 to 50 cents in high volume. The total cost impact might be more or perhaps less, depending on how the related circuits are affected. It is worth noting that transformer interfaces simplify some other design considerations, such as RF immunity and lightning surge tolerance. In some designs I have done, the transformer was optional because it was not needed for isolation, but I ended up using it anyway because the total cost impact was small and there were performance advantages. 3) Isolate the charger, using either transformer or capacitor isolation. Since the caps would have to stand off 1000 VRMS, a transformer might be the best choice here. Also, don't overlook the requirement for basic insulation in clause 2.3.2 (UL 60950-1). If this product will be sold in Europe, watch out for the additional requirements in Annex ZB of EN 60950-1. There are some other compliance schemes you could use, but they get complicated. It is probably best to stick with one of the simple approaches listed above. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 [email protected] http://www.randolph-telecom.com
