David, The back emf pulse from a relay occurs when the coil circuit is opened, and its polarity is opposite that used to operate the relay. A properly connected diode should 'swallow' the back emf, and a capacitor in parallel with the coil should reduce the amplitude of the pulse. Any delay introduced by the diode and capacitor will be on the trailing edge and will not normally introduce significant delay on the leading edge.
Anyone adopting this scheme would not be operating QSK, so any additional delays introduced will not be significant. Those desiring QSK operation should consider fast relays (vacuum relays) or solid state switching devices for their amplifier switching. 73, Don W3FPR David Cutter wrote: > That's a good thought and provides instant isolation, but have some thought > about the back emf off the coil - it could exceed the rating of the keying > transistor. I measured 800V on a very fast scope many years ago from a reed > relay. Just sticking a diode across the coil will slow it down a lot, so, > some experimentation with zeners and resistors will get the situation > optimised. I once used a separate keying transistor and relay and found > that a small cap from the base to the collector did the job quite well by > slowing the off rise time, so the transistor absorbed a bit more power but > gave good control. > > 73 > > David > G3UNA > > > ----- Original Message ----- > From: "Don Wilhelm" <w3...@embarqmail.com> > To: <elecraft@mailman.qth.net> > Sent: Thursday, February 12, 2009 4:00 PM > Subject: Re: [Elecraft] K3 external qro keying line > > > >> Terry, >> >> For those who prefer not to touch the K3 with a soldering iron, an >> external 12 volt relay is a good solution. Obtain the power from the >> K3 12VDC OUT jack and use KEYOUT to key the relay. It accomplishes >> the same thing and retains the ability to use QSK in the future without >> 'undoing' the internal relay. >> >> 73, >> Don W3FPR >> >> w...@comcast.net wrote: >> >>> I've seen numerous posts about the PTT output line on the K3 and using >>> older amps. Yaesu's, Kenwood's, Icom's all share the same issue. >>> >>> >>> >>> Unless you are running QSK and need instant keying, I would suggest >>> adding a small 12v relay inside the K3 that the solid state device >>> keys and presents the user with a set of dry, isolated contacts that >>> can handle positive, negative and up to 250v DC or AC. I have added >>> this to several K3's to protect the switching transistor from eventual >>> destruction. Once again, if you are using QSK, solid state due to it's >>> fast switching time is your only option. >>> >>> >>> >>> Yes, it means touching your K3 with a hot soldering iron, but it's a >>> "pay me now or pay me later" thing, sooner or later it will blow the >>> switch if you are hooking up older or HB amps that use other than low >>> current positive keying voltages. >>> >>> >>> >>> Terry >>> >>> >>> >> ______________________________________________________________ >> 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 > ------------------------------------------------------------------------ > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.0.237 / Virus Database: 270.10.23/1953 - Release Date: 02/14/09 > 18:01:00 > > ______________________________________________________________ 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
