Sounds like a wager for some beer money! :-)) Paul, W9AC
----- Original Message ----- From: "Scott Ellington" <[email protected]> To: "Elecraft Reflector" <[email protected]> Sent: Wednesday, February 16, 2011 5:10 PM Subject: Re: [Elecraft] K3 keying an amplifier > Sorry, I still don't think it will work reliably with the ICOM unpowered, > if it has a pull-up resistor, etc. However, the ICOM may in fact be open > collector/drain, in which case there's no problem. Might be worth > checking. > > Scott K9MA > > > > On Feb 16, 2011, at 3:48 PM, Paul Christensen wrote: > >>> I don't think the diode isolation will work in this case. >>> >>> Scott K9MA >> >> If the Icom used an amp key line from an NPN transistor with an open >> collector and a high value (e.g., 10K) pull-up resistor to a positive >> supply >> rail, then all one needs is a steering diode added to the Icom key line >> configured with the cathode to the junction of the Icom's collector and >> pull-up resistor. The diode's anode points to the Alpha 9500. The K3 >> with >> its MOSFET output simply parallels the 9500 key line. It doesn't care. >> >> If the Icom is unpowered or powered, a Schottky steering diode will >> isolate >> the pull-up resistor while still allowing the driving transistor's >> junction >> to pull low enough to ground. An unbiased or unpowered NPN >> open-collector >> switching transistor should not pull to ground potential if the rig is >> powered down. >> >> The only remaining obstacle is that when the Icom is powered down, the >> Alpha >> 9500 key line would see 10K resistance through the pull-up resistor to >> the >> power supply positive rail. Powered down, that rail may be close to >> ground >> potential. So, whether a steering diode alone will work is really a >> matter >> of: (1) the value of the pull-up resistor; and (2) whether the Alpha >> 9500's >> key line circuitry is sensitive to high-impedance switching. No problem >> if >> the Alpha uses a photo-transistor on the input key line, for example. >> But >> if say a pair of both pull-up and pull-down resistors are used ahead of a >> CMOS gate, then the value of the resistors need to be known. >> >> If a steering diode still doesn't work, then a separate switching >> transistor >> can be used, using an open-collector without a pull-up resistor -- or a >> MOSFET without a pull up on the drain. >> >> Paul, W9AC >> >> ______________________________________________________________ >> 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 > > Scott Ellington > Madison, Wisconsin > USA > > > > ______________________________________________________________ > 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
