Joe, Most "isolation" transformers have a non-magnetic, metal shield between the primary and secondary. The shield does not appreciably degrade the transformer's magnetic coupling at 60Hz; however, it does attenuate the high frequency differential noise passed from primary to secondary.
The ground scheme that you mention (Earth carried through and tied to secondary nuetral) is very common (no pun intended) in isolation transformers. The theory here is that the secondary will present a power outlet to the load that looks just like a normal outlet. In the US, 120VAC neutral and Earth are bonded together at the building electrical entrance. So, in most 120VAC sockets, you would have continuity from Neutral to Earth, just like the output of your isolation transformer. Of course, if you are using the isolation transformer to isolate and "step up" to 230VAC; then the nuetral/Earth connection in your isolation transformer may not simulate the real world. This is due to the fact that many 220/230/240VAC circuits do not have a "nuetral" bonded to ground. Chris Maxwell | Design Engineer - Optical Division email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA web www.nettest.com | tel +1 315 797 4449 | > -----Original Message----- > From: j...@aol.com [SMTP:j...@aol.com] > Sent: Thursday, August 29, 2002 10:21 PM > To: emc-p...@majordomo.ieee.org > Subject: Curious about Tripp-Lite "isolation transformer" > > > Hello All: > > Some of the laboratory tests that I perform require a source of AC mains > power that is fully isolated from earth. To provide this function, I made a > catalog purchase of a Tripp-Lite model IS-1000 "isolation transformer." This > unit is equipped with a grounded (3-prong) 120 VRMS plug on the AC mains cord > used on the input side, and has a 3-prong AC mains socket on the isolated > side. > > When the unit arrived, I was surprised to find that the green-wire ground on > the input side was carried across to the ground pin on the AC socket of the > "isolated" side. Furthermore, one side of the 120 VRMS socket on the output > side was also tied to the green wire ground. In other words, the output of > the unit is not isolated from ground, even if I use a "cheater adapter" to > break the connection of the ground pin on the output socket. > > I also noted that the unit has capacitors and what appears to be a large MOV > placed across the "isolated" output. > > My theory is that this type of isolation transformer is intended to provide > protection from transients or noise on the AC mains. However, I can't see > what protection the transformer provides, except maybe from common mode > transients on the AC mains pair. Even so, I am having difficulty > visualizing any scenario where a common mode surge would appear on the AC > mains pair but not on the ground wire (given that in North America, one side > of the AC mains pair is tied to ground at the service entrance). > > Is it possible that the transformer is designed with frequency-dependent > coupling so that frequencies above 60 Hz do not couple well? Such a scheme > would provide some degree of protection from differential noise. > > I am wondering if any of you are familiar with this type of "isolation > transformer" and the applications where someone might need such a device. On > the surface, I can't see what "isolation" benefit the transformer is > providing when it is wired this way. > > It is relatively easy for me to modify the wiring to provide the fully > floating, balanced AC output that I was seeking in the first place. The only > thing I haven't decided yet is whether to leave the capacitors and MOV > connected across the isolated output. > > If any of you are familiar with this type of "isolation transformer" and can > > > explain the intended function, I would be interested to learn about it. > Also, if you see any flaws in my rewiring plan, please let me know. I'm a > little bit uneasy modifying a design that I don't fully understand. > > > > Joe Randolph > Telecom Design Consultant > Randolph Telecom, Inc. > 781-721-2848 > http://www.randolph-telecom.com > > > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
