----- 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
