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
