The flux can be very considerable! As the flux collapses it can induce a very high voltage transient across the coil far exceeding the voltage produced to energize the relay. There is a certain amount of energy stored there in many instances. The 'back EMF" pulse can wreak havoc with solid state gear and shock the hell out of someone who happens to be across the circuit when the field collapses! Seems like I was havinga discussion of neon tubes (NE-2?) across relay coils in the old ARC-27 UHF aircraft transceiver (a true boat anchor piece of gear!). These were probably primarily to limit the pulse to whatever the ionization potential of the NE-2 is. (probably around 60-65 volts?) Silicon diodes didn't exist in those days or they probably would have been used instead.
73, Sandy W5TVW ----- Original Message ----- From: "Paul Christensen" <w...@arrl.net> To: "'Elecraft'" <elecraft@mailman.qth.net> Sent: Wednesday, April 08, 2009 4:11 PM Subject: Re: [Elecraft] Can a 1N4007 Be an Effective Relay Snubber? >>> To answer, I hope definitively, the question "is a 1N4007 power diode >>> too slow to be used as a relay coil snubber," > > Just curious -- over the years, I've seen references to the main purpose > of > the snubber diode being used; (1) to decrease the action time of the > relay, > since the decaying flux of the relay coil tends to keep the contacts > engaged > (NO), or disengaged (NC) longer than necessary after voltage is removed > from > the coil; and (2) to primarily protect a solid-state switching device > (e.g., > an open NPN collector) from damage, also being caused by the snap of the > decaying flux of the relay coil. I realize both uses of the diode are > valid, but it seems that unless there's an extremely high flux developed > by > coil, the primary purpose of the diode is for relay speed, rather than > switching device protection. > > Paul, W9AC > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html -------------------------------------------------------------------------------- No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.285 / Virus Database: 270.11.47/2047 - Release Date: 04/08/09 05:53:00 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
