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
From: [email protected] mailto:[email protected]] On Behalf Of Chris Maxwell Sent: Thursday, July 08, 2004 7:03 AM To: EMC-PSTC Subject: Resistors in ESD Test Setups http://www.ieee-pses.org/symposium http://www.emc2004.org/ -------------------------------------------------- All, The horse is dead, beaten, turned into glue (probably a few McDonald’s hamburgers) and buried :-) David Sterner and I have bounced a few emails back and forth (offline) on the subject. We discussed OHSA, EN 61000-4-2 and some individual test setups. Just to clarify for the benefit of the group. Most ESD setups require the 940Kohm resistance from the coupling planes (HCP or VCP) to ground. -The resistors complete the circuit from ESD gun to GRP through the coupling plane, allowing for potential equalization (charge bleeding) between discharges. This makes repeatable test results. -The high value of the resistors protects the test personnel in the following instance: if the test tech touches a hazardous voltage in the EUT with one hand and touch the coupling plane with the other hand; then the high resistance to ground will limit the current through the tech’s chest cavity (the worst possible area for hazardous current to flow). On the other hand, if the coupling plane was grounded, or bonded to ground with a low resistance, there would be a dangerous current through the tech’s chest cavity. This is OSHA’s peeve with grounded conductive tabletops. David was more enlightened regarding the OSHA requirements, so this is a tidbit that I didn’t have. -Having said all of that, if the tech touches a hazardous voltage or if the coupling plane becomes charged, the resistors will not protect the tech from the hazardous voltage going to ground through his feet or through his bottom to a metal chair… This is why you still have to use caution, even with the resistors installed. Although the initial email just asked about the power rating (which we mostly agree to be ¼ Watt, up to 15KV). Another point that I haven’t seen is with regard to the length of the resistors. A good rule of thumb is to require 1mm of air space to insulate 1KV of ESD. So, just make sure that your resistors are long enough from terminal to terminal to insulate the expected ESD voltage of your test setup. For example, if you plan on going up to 15KV, and you use two 470Kohm resistors; then each resistor is subjected to 7.5KV….which would imply that the resistor body should be 7.5mm (about ¼ inch) long in order to prevent the ESD from jumping over. Shorter resistors could be potted (RTV, varnish…whatever you have handy) to help out. This may also be part of the reason that Dave C. from Micron mentioned the 2W resistors. They would be plenty long enough to handle higher ESD voltages without flashover. Chris Maxwell Design Engineer Nettest ------------------------------------------- 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 ------------------------------------------- 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
