John, The susceptible components in a filter are the "Y" and "X" capacitors.
Y are for line to earth connections. These are typically rated and certified as Y1, Y2, etc. This rating is specific to the surge withstand capability in the IEC component test standard. Similarly the X capacitors are for across the line. As you might expect, these are rated X1, X2, etc. and also have a specified surge withstand capability. Some Y caps may dual rated as X and Y since the surges from line to earth are generally more severe the from line to line. The difference is usually ceramic (Y) vs film (X). The latter being able to provide large capacitance values and heavier filtering across the line. The former being smaller values and will limit ground leakage current if kept small. The certified EMI filter you purchase should have all these designations marked. In short, the answer is yes, MOVs can be downstream of the EMI filter given the correct capacitor (X, Y) surge ratings for the specified over voltage category of the product. In fact, this is typical design when using AC input modules with integral EMI filters. The best practice comment I made is based my experience of the possibility of long term degradation of the MOVs (years). This puts a little design edge in the product to make them last just a bit longer. Any number of companies who make EMI suppression caps have good app notes on the topic. Doug Douglas E Powell http://www.linkedin.com/in/dougp01 http://www.linkedin.com/in/dougp01 -----Original Message----- From: John Woodgate <[email protected]> Date: Sun, 16 Dec 2012 20:57:20 To: <[email protected]> Reply-To: John Woodgate <[email protected]> Subject: Re: [PSES] Why use an MOV? In message <1454929815-1355690903-cardhu_decombobulator_blackberry.rim.net-192443596 [email protected]>, dated Sun, 16 Dec 2012, [email protected] writes: > Good design practice says install the MOVs "downstream" of the EMI >filter to buffer the surge with a small bit or impedance. Does the EMC filter survive the surge? -- OOO - Own Opinions Only. See www.jmwa.demon.co.uk The longer it takes to make a point, the more obtuse it proves to be. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ 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]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ 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]>
