Common mode chokes work by having the flux-linkage of the first coil fighting the flux-linkage of the second coil. The third coil, if unused, should not be shorted in any way. Grounding ONE side only is OK, but may cause the choke to not be as effective at higher frequencies. Best is to leave it totally unconnected. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu