Hi Derek. A twopence worth perhaps. I'm sure I'm preaching to the holy.
EMC signatures of switches is a rats nest. There are so many factors, contact material, contact mass, spring pressure, local vapour pressure (especially with mercury wetted), mechanical vibration time constant of the contact/leaf assembly, snap action, cam action, arc suppression construction, contact spacing, impedance/inductance/capacitance of the switched circuits. You can expect a typical snap action (meaning contact closure/opening speed controlled by over-centre spring mechanism independent of operator speed) to bounce a dozen or so time over a period of up to 10mS. I've had relays and contactors bouncing for 50mS in a previous life! If this is a low voltage low current logic circuit, this corresponds to ten pushes of a button. If a high (50V) voltage high current (>100mA) circuit, the contacts will arc on each bounce if d.c., and cause significant emission during the arc period, but will be variable arcs on a.c. as the voltage swings through half a cycle of power frequencies while bouncing. The principle is that the transition from short circuit, to Arc voltage (50v???) is extremely fast and harmonics are generated up to hundreds of Mhz. Arcing on cam operated (contacts move at finger speed) contacts is a nightmare in it's own right. Slow release of d.c. switches drags out a significant arc which itself seems to generate wide spectrum noise for as long as the arc is present (typically 100mS) and occasionally as long as a second ( or until the contact and holder melt in the case of high current loads.) A.c. switching of cam operated contacts has arcs which rarely last for more than half a cycle of power frequency (10mS UK) due to the self extinguishing through zero. It goes without saying I guess, that inductive loads are bad on opening contacts, and capacitive loads are bad for closing contacts. But what the hell, I said it anyway. Trying to suppress switch bounce arcs with capacitors is a good way to cause huge GHz transients to rattle round equipment as the switch shorts out a very low impedance capacitor with prospective currents up to kA. ( And can in worst case cause contacts to weld!) That's why switch suppressor components have series resistors. If this is a switch that is operated often and quickly, like a trigger on a games amusement machine,or a Mic switch in an aircraft cockpit, the emissions can be significant. Immunity is not normally a problem, but at the far ends of the curve mercury wetted switches have been known to go short circuit in the presence of HV transients. Manual magnet operated Reed switches for logic level switching were prone to this many years ago on office machines. Gladly, RoHS has seen the end of these in service. Good luck. Chris Duprés Compliance Engineer Elekta Limited Linac House Fleming Way Crawley West Sussex RH10 9RR www.elekta.com tel: +44 (0) 1293 654311 fax: +44 (0) 1293 654260 - 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
