Bill: Are you sure about this? A snubbing diode has to have fast turn on, but slower turn off is not as much of a concern. A while ago I looked at the turn on and turn off times for several diodes including the standard 1N4007 and found all sufficiently fast to work well as a relay snubbing device. Details are at http://www.cliftonlaboratories.com/diode_turn-on_time.htm
Jack K8ZOA www.cliftonlaboratories.com Bill Coleman wrote: > 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" <wc0...@embarqmail.com> >> To: <k...@att.net> >> 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" <k...@att.net> >>> To: "WCØV" <wc0...@embarqmail.com> >>> 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:Elecraft@mailman.qth.net >>>>> >>>>> 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:Elecraft@mailman.qth.net >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> > > George William Coleman mail: aa...@arrl.net > Senior Software Developer phone: 770 978-0196 > > Bill Coleman, AA4LR, PP-ASEL Mail: aa...@arrl.net > 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:Elecraft@mailman.qth.net > > 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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
