1N4007 diodes have a notoriously long switchover time. That's what makes them useful as a poor-man's PIN diode. They aren't quite so useful in back EMF suppression, though.
If you use a fast diode, you should be able to keep the back EMF voltage down to the forward-bias voltage of the diode. On Apr 4, 2009, at 9:52 PM, Carl, WCØV wrote: > > ----- Original Message ----- > From: "Carl, WCØV" <[email protected]> > To: <[email protected]> > Sent: Saturday, April 04, 2009 8:50 PM > Subject: Re: [Elecraft] Inductive relay "kick" > > >> Good point. The diode will conduct the back EMF minus internal >> voltage >> when the supply voltage to the coil is removed. You do need a diode >> with a >> high enough PIV rating. But even Radio Shack has silicon diodes >> with 1000 >> PIV rating, 2 amps forward current rating at five for a dollar. >> >> We make electric actuators where I work, motors with internal >> rollerscrews >> and often they get equipped with electric safety brakes (apply when >> power >> is removed, release when power is applied). We normally supply 1N4007 >> (1000 PIV rated forward bias) diodes with brake equipped motors. >> >> 50% of those you ask would do that. The other 50% would use a very >> fast >> switching diode as you suggest. Both work just fine since the only >> job is >> to conduct the EMF coming out of the coil when the actuating >> voltage is >> removed. BUT you do need a diode with a high enough PIV rating. >> Some of >> our bigger unsuppressed motor brake coils can spit out over 600 vdc >> when >> the supply is removed. >> >> And yes some of our customers hook up the diode or the brake supply >> backwards and blow the diode (if they don't blow the supply first). >> Protection is gone, and the back EMF from the relay coil is free to >> blast >> wherever it wants to go. >> >> I am sure the Elecraft relays are way below that. My point to Don >> was that >> simply doubling the voltage rating of the coil to get the back EMF >> is way >> off. A scope will tell the real tale. >> >> BTW I got yelled at by a medical tech who pointed out that the pain >> (real - I tried the relay wire experiment myself) is the voltage >> going >> right across your heart. To some, 450 vdc may be a very bad day. >> >> 73, Carl WC0V >> >> ----- Original Message ----- >> From: "Bob" <[email protected]> >> To: "WCØV" <[email protected]> >> Sent: Saturday, April 04, 2009 2:07 PM >> Subject: Re: [Elecraft] Inductive relay "kick" >> >> >>> Hi Carl, >>> >>> What am I missing here? Using a regular silicon diode that >>> has a fast switching time, or even better a Schottky diode >>> wouldn't they >>> clamp at the forward bias of the junction .7 volts of less. If >>> not how >>> long could you expect the diode to survive that abuse of 75v >>> across the >>> forward biased junction. If the diode then failed open you would >>> never >>> know your protection was gone. >>> 73, >>> Bob >>> K2TK >>> >>> Carl wrote: >>> >>>> Don, seriously disagree about the magnitude of the "kick". For >>>> years I >>>> taught a course on industrial installations and demo'ed the >>>> "kick" issue >>>> by soldering two bare wire to the coil of a small Potter and >>>> Brumfield >>>> "ice cube" 12 vdc relay. I would pick the biggest guy in the >>>> class to >>>> hold the wires while I touched them to a nine-volt transistor radio >>>> battery. And then watch as he would throw the relay across the >>>> room. A >>>> reverse diode would vastly reduce the effect. >>>> >>>> Well, I got yelled at and was told "Hey, Stupid (and stupid >>>> wasn't the >>>> word they used)! Put that on a storage scope to see what you're >>>> messing >>>> with." The scope showed a spike of 450 vdc! The diode reduced it >>>> to less >>>> than 75 vdc. >>>> The magnitude is due mainly to the large inductance of the coil, >>>> but >>>> until some measurements are taken, some caution is advised. The >>>> inductance of any coil inside a K2 or K3 is bound to be much >>>> smaller. But >>>> the phenomenon is the same. >>>> >>>> 73, Carl WC0V >>>> ______________________________________________________________ >>>> Elecraft mailing list >>>> Home: http://mailman.qth.net/mailman/listinfo/elecraft >>>> Help: http://mailman.qth.net/mmfaq.htm >>>> Post: mailto:[email protected] >>>> >>>> This list hosted by: http://www.qsl.net >>>> Please help support this email list: http://www.qsl.net/donate.html >>>> >>>> >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html George William Coleman mail: [email protected] Senior Software Developer phone: 770 978-0196 Bill Coleman, AA4LR, PP-ASEL Mail: [email protected] Web: http://boringhamradiopart.blogspot.com Quote: "Not within a thousand years will man ever fly!" -- Wilbur Wright, 1901 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
