Hi Peter, I see I have wrongly used the TVS abbreviation. Writing TVS I had in mind only semiconductor non-linear devices (TRANSILs). Comparing I/V curves of VDR and TRANSIL (both comparable sizes and for the same working voltage) I would expect smaller (DC) overvoltage needed to burn TRANSIL than VDR, and TRANSIL also makes shortage like VDR. I know, other reasons (1.5.9.2 Note 2) can damage VDR and not TRANSIL.
Having no experience I have one way of understanding the standards - take the text literally. I also expect that if any problems arise the text will be taken literally. I know, that even I ask any experienced agency the whole responsibility for the product is only mine. So when they tell me the intention was other should I follow it ? The intention isn't written in standard. I have asked about PTC, because I prefer them over fuses but recently I was told, that PTC can't be understand as interrupting device, as it makes no permanent interruption. >> If not the fuse with what approval should I look for ? > > I don't understand this question. Can you be more specific? Asking that I had in mind: If EN 60950-1 needs me to use here (or anywhere else) an adequate interrupting device, and I decide to use fuse, can it be any fuse which in my opinion is good, or I have to check the fuse manufacturer for some agency reports that the fuse comply with any particular component standard, and what standard ? May be it is the question everyone knows the answer, but I don't. Best Regards Piotr Galka ================== >> From: Piotr Galka >> Sent: Thursday, April 26, 2007 4:48 AM >> >> All 60950-1 Experts, >> >> I have in my device RS232 input line. >> >> I need to protect it against surge (because of EN 50130-4). >> >> Is it true that if I use the 33V varistor I will need to add >> a fuse in series with it (Cl. 1.5.9.2), but if I use the 33V >> TVS I need not to add the fuse. > > Hi, Piotr. > > That appears to be true, if the text is taken literally), but it doesn't > make sense, considering §1.5.9.2, Note 2, so I'm not certain that was the > intent. VDRs could be a concern for increased leakage current in TNV-2 > and TNV-3 circuits (remember that ringing voltages are applied during some > touch current tests), but I don't think the intent was to apply this to > all data bus circuits that typically do not contributing much to touch > currents and for which transients are not considered in touch current > testing. > >> Cl. 1.5.9.1 note 1 tells me that TVS is not VDR (in 60950-1), >> so 1.5.9.2 not apply to it. > > A VDR is still a TVS. §1.5.9.1, Note 1, only intends to differentiate > which are considered VDRs and which are not. > >> Reading Cl. 2.4 I understand that 12V/10mA (RS232 driver >> capability) is not the LIMITED CURRENT CIRCUIT so VDR needs fuse. > > Again, if literally taken, you'd be correct. > >> If the fuse is needed do the PTC resistor (for example >> Raychem miniSMDC020 - 0.2A/30VDC) can be considered as "an >> interrupting means having an adequate breaking capacity" ? > > Probably, but it may depend on how the RS-232 circuit is used, the > anticipated transients and the power supply capacity. > >> If not the fuse with what approval should I look for ? > > I don't understand this question. Can you be more specific? > >> In my opinion the same what can happen to the varistor during >> "-temporary overvoltages above the maximum continuous >> voltage." can happen to TVS. > > True, but other TVS devices have generally proven themselves more immune > to the kinds of problems VDRs exhibit, provided they are rated > appropriately for the application. > - 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 ______________________________________________________________________
