This reminds me of years ago when I did board level repair. Occasionally we needed a resistor of some very specific value (bridge circuit) so we would take a carbon film resistor of a slightly lower value and shave some of the material off to get the exact value. A coat of epoxy finished the job. Bob Richards, NCT
[email protected] wrote: Well, there is laser trimming, and there is laser trimming. For commodity resistors with leads and cylindrical bodies, a spiral cut is fairly common. By varying the pitch of the spiral and number of turns, a large number of values can be had from a single base part. This also retains most of the resistive material so that usable area for heat dissipation is not reduced a lot. The disadvantage is that the voltage from turn to turn can be high enough (especially for impulsive waveforms) that there can be arc over between the turns. The other way of trimming is to take straight cuts along the axis of the resistor. The voltage gradient along the resistor is not affected. However, the base part needs to be close to the final value so that large amounts of resistive material are not removed, causing the heat dissipation to concentrate over a small area. This is more expensive to do, but yields a more robust component, appropriate for high voltage use. Donald Borowski Schweitzer Engineering Labs Pullman, WA, USA "John McAuley" .ie> To Sent by: "'Joe Randolph'" [email protected] , "'Ralph McDiarmid'" , 10/08/2008 03:55 AM cc Subject Please respond to RE: IEC60225-5 surge test .ie> High voltage resistors are becoming more readily available with thick film technology. See http://radionics.rs-online.com/web/sea ch/searchBrowseAction.html?method=getProduct&R=2509032292 A combination of these could meet the surge current requirement. You need to be careful with some of these though, they use laser trimming which results in affine line across the resistor that causes arcing. I understand that the one listed above does not use laser trimming. BR John McAuley www.cei.ie [email protected] ***************************************************************** DISCLAIMER: The information contained in this e-mail may be confidential and is intended solely for the use of the named addressee. Access, copying or re-use of the e-mail or any information contained therein by any other person is not authorised. If you are not the intended recipient please notify us immediately by returning the e-mail to the originator From: [email protected] [mailto:[email protected]] On Behalf Of Joe Randolph Sent: 08 October 2008 02:02 To: Ralph McDiarmid; [email protected] Subject: RE: IEC60225-5 surge test impedance? On 10/7/2008, Ralph McDiarmid wrote: Is it possible to insert a resistor between surge generator and CDN to provide the correct impedance? Here is one source of high-voltage, power resistors. http://www.caddock.com/Online_catalog/high_voltage/high_voltage.html Hi Ralph: It is common practice to insert an external resistor in series with the output of a surge generator to reduce the short-circuit current. In fact, some standards call out different values of external resistance in their tests. The resistor will influence the short-circuit current waveform, so you may need to pay attention to that. You must be very careful about the power rating and construction of any resistors you add. The resistors must be able to handle the surge power, and they must be non-inductive (not wire wound). Interestingly, one of the best types of construction for this application is the old carbon-composition type, but these are getting hard to find. As long as the resistor construction you use is non-inductive, you can achieve whatever surge tolerance you need by connecting several resistors in parallel. Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 (USA) [email protected] http://www.randolph-telecom.com - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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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
