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
