http://www.ieee-pses.org/symposium http://www.emc2004.org/
From: [email protected] [mailto:[email protected]] Sent: Thursday, July 08, 2004 6:40 AM To: [email protected]; [email protected] Subject: RE: Resistors in ESD Test Setups http://www.ieee-pses.org/symposium http://www.emc2004.org/ Although 1/4-watt resistors have sufficient length to not flash over at 7.5 kV, they can still fail. A carbon film or metal film 470 k ohm resistor often has a spiral resistive structure. This can arc turn-to-turn and the resistor will open. This usually occurs during fast dv/dt events. The carbon composition resistor is a solid cylinder of resistive material and does not suffer this fate. An alternative would be any 470 k ohm high voltage resistor rated for 7.5 kV. No need to worry about the series inductance of a resistor of such a high resistance. More on resistors and HV. Many precision resistors have been adjusted by laser trimming. The narrow gap created by the laser is often the site of failure under high voltage conditions. It is hard to beat a carbon comp resistor for HV. They have quite a bit of mass and can absorb a good amount of energy before failure. A 2 watt carbon comp can take 40 joules before exploding. The downside to these parts, besides being scare, is the initial accuracy combined with long term drift in the value. Dave Cuthbert I have long used 2-Watt carbon composition resistors for dummy loads and to fiddle with source impedance on pulse generators, and the comments so far have been in line with my experience. Those carbon comp resistors are cheap, rugged, and can handle a big zap before explosively shattering. But........ Continued zapping may not cause a catastrophic failure (bang!), but it exacerbates drift. And I wonder what's actually happening inside that "composition" that forms the resistance. Is there a series of little particle-to-particle arcs with every pulse? Does this tend to burn away path after path on successive pulses, or do the particles tend to "weld" together? My experience has been that carbon comp resistors tend to drift upward in resistance, making me assume that the "wear" mechanism is one of destroying successive paths. Anyway, when I want to make a really good load device, I try to find a surplus silicon carbide resistor (the old Carborundum / Cesiwid Globar, tube or hockey puck style). These don't drift, either because they are simply massive bulk resistances, or maybe because their structure is better. Getting back to the topic of resistors in an ESD test setup, I would say that the 2-Watt carbon comps will last so long that you will likely physically bust them long before they become electrically bad. Anyway, you should be periodically checking the resistors in the test station, and the resistors are so cheap that you should replace them at the first sign of drift. Regards, Ed Ed Price [email protected] WB6WSN 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. IEEE PSES Main Website: http://www.ieee-pses.org/ To post a message send your e-mail to [email protected] Instructions for use of the list server: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
