PSU surge protection - Metal Oxide Varistors

[ukrobotics]

Ok, so not strictly a robot related question but I figured there would be some people here who would know the answer....

Im building a linear power supply for another project and Ive been told that I should use metal oxide varistors to protect the transformer and capacitors from voltage spikes.

My limited understanding is that they should be put after the fuse but before the transformer and caps; positioned between live and neutral.

Under normal useage (At rated working voltage) the MOV has a very high resistance and does not effect the rest of the circuit. If the input voltage increases to a predefined level (clamping voltage?) then the MOV will gain a much lower resistance, effectivly shorting live and netutral and blowing the fuse much faster than it would without an MOV in place.

Without the MOV in place, the fuse wouldnt blow until every component in the PSU had recived a high enough voltage to increase the current to a level where the rating of the fuse was exceeded. Of course the danger here is that the components could be damaged by the over voltage before the fuse blows; even with a properly rated fuse.


First of all, to all the electronics experts out there, is my understanding correct and if not, where am I going wrong ?

Secondly, please could someone point me in the right direction of a suitable MOV device for my application? an RS (or similar) part no. would be great. I have had a look on the websites and catalouges but there are so many different ratings and types im not sure which one id need. Electrical spec of my PSU is as follows: input voltage 240vac , output voltage 160vdc (no load-unregulated), maximum continous output 10a.

Would this product be suitable for my application? http://uk.farnell.com/jsp/Passive+Componen...jsp?sku=1057204http://uk.farnell.com/jsp/Passive+Co...sp?sku=1057204

[kane]

http://en.wikipedia.org/wiki/Varistorhttp://en.wikipedia.org/wiki/Varistor

RS Part number http://rswww.com/cgi-bin/bv/rswww/searchBrowseAction.do?D=varistor&Nr=AND%28avl%3auk %2csearchDiscon_uk%3aN%29&Ntk=I18NAll&Nty=1&Ntt=va ristor&Dx=mode%20matchpartial&Ntx=mode%20matchpart ial&N=0&name=SiteStandard&forwardingPage=line&R=22 68040&callingPage=/jsp/search/search.jsp&BV_SessionID=@@@@0672139044.1184427126@ @@@&BV_EngineID=ccdeaddlhgkgdlhcefeceeldgkidhgi.0& cacheID=uknetscape226-8040 is the sort of thing you need I think.