I have heard that carbon composition resistors are no longer allowed to be used in commercial products because they burn when mistreated. I can personally attest to that, but I keep a bunch around because they are excellent resistors at high frequencies.
From: "Price, Ed" <[email protected]> List-Post: [email protected] Date: Wed, 14 Mar 2007 06:08:37 -0800 To: [email protected] Subject: RE: OT: standard component values From: John Woodgate [mailto:[email protected]] Sent: Wednesday, March 14, 2007 1:37 AM To: [email protected] Subject: Re: OT: standard component values In message <[email protected]>, dated Wed, 14 Mar 2007, Fred Townsend <[email protected]> writes: >Gentlemen what you say may apply to some components but with regard to >5, 10, and 20% composition resistors it is dead wrong. The question was where the 'preferred values' came from. Not how carbon composition resistors are made and marketed now. I decided to see just what the pricing differential really was for 1%, 2%, 5%, 10% & 20% resistors, so I grabbed my Mouser catalog (the Newark and Allied catalogs were too heavy to lift) and started looking for a plain old 1/2 Watt carbon comp resistor. In ten minutes, I'm feeling like a dinosaur that can't find his swamp. Looks like component manufacturing has come a long way in 35 years or so. <g> Manufacturing variations must be capable of being held much tighter, and pricing by value tolerance is difficult to find. (So are carbon comp resistors.) I did find some examples; for instance, Kemet ceramic chip capacitors, 1000 uF, 100 VDC, pricing each, 2% @ $2.11 / 5% @ $0.18 / 2% @ $0.12. Since the manufacturing process has obviously improved, maybe we need to tighten our definition of "high accuracy." Instead of trying to find a price differential between 2% and 10% parts, I wonder what the price would be for 0.1% versus 0.5% parts? Apparently, existing circuit design practices don't typically require these tight tolerances, so the parts dealers are not featuring them. Back during my time of manufacturing components, I readily admit that our process control was primitive, and a bell-curve distribution would often have been welcome. I often had production runs where the overall yield was 30% (after physical, hipot, DF, IR, ESR and tolerance tests had each taken their bite). To get back to the original question, the component value system was set up in a time of relatively poor process control which produced a wide distribution. The progressive value system made a wide value selection available to the user, and allowed the manufacturer to sell all he made (assuming all other parameters were OK). If the user needed tighter tolerance, he could either pay a premium per part cost, or he could buy a bunch of wide tolerance parts and add his own labor to sort and/or adjust the parts into the needed tolerance. Today (aside from some small capacitors that are rated -20% / +80%) you almost can't find 20% parts; from what I saw in the catalog, 5% or even 2% is the new "plain vanilla" tolerance bracket. BTW, does anyone know when component manufacturers started using this tolerance progression? From my own observation, the system was in place for 1930's era design. Was it codified under some old EIA standard, or is it even older? Ed Price [email protected] NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (FAX) Military & Avionics EMC Is Our Specialty - ---------------------------------------------------------------- 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 ______________________________________________________________________ - ---------------------------------------------------------------- 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
