I agree that Y caps are good since they can handle the surge/tranisent exposure. I believe a Y2 can go to 5KV and a Y1 could go as high as 8KV The voltage rating for a Y cap is the sustained voltage it can handle. What value are they asking for?
Note that the Y2 caps are a rather large and Y1 even larger. Most practical sizes will be 4.7-2.2 nF from a mechanical point of view. There are also som SMT Y2 caps you could consider - I believe Murrata has them. If you put them on the bottom side of the board you should avoid wave solder. They are so big that they can crack from the thermal shock. I have done well with 10-4.7nF with other shielded interfaces.....but there are confounding issues to deal with on Ethernet. The down side of this decoupling of the shield jack is that you loose effective bonding to the housing. You'll have a pig tail effect back to the chassis that will radiate noise into the housing. ESD will also take a bad path... One way to address this is to bond DC direct to the housing for local Cat5 connections on the same ground and then use fiber optics to go the distance. Chris ----- Original Message ----- From: "Scott B. Lacey" <[email protected]> To: "Jim Eichner" <[email protected]> Cc: <[email protected]> Sent: Wednesday, March 07, 2007 7:34 PM Subject: Re: Ethernet connector shield decoupling capacitor > Jim, > > I have used a similar technique to get good high-frequency performance > from > an ethernet shield without the effects of dc and low frequency loops. The > capacitor > ratings are likely based on the maximum voltage that may be impressed on > the > shield. > > Scott B. Lacey > > On 7 Mar 2007 at 12:05, Jim Eichner wrote: > >> We're being hit with a requirement that I don't understand. >> >> We are using a shielded Ethernet connector that contains the typical 2kV >> cap from the common point of the 75 ohm resistors to the shield. This >> is used on the output of an off-the-shelf Ethernet transceiver module, >> and the mfr of that module is telling us we need to connect the >> connector shield to their digital ground through a 1nF 250V Y-cap, not >> directly. >> >> 1. Why not directly connect the shield to digital ground? >> >> 2. Why would a Y-cap be required? The cap would be joining two parts of >> the same circuit, so it would not be across an isolation boundary. I >> can't think of a reason to require either the 250V rating or the Y >> rating. >> >> Any thoughts anyone? >> >> Thanks, >> >> Jim Eichner, P.Eng. >> Manager - Compliance Engineering >> Xantrex Technology Inc. >> e-mail: [email protected] >> web: www.xantrex.com >> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
