[Elecraft] K3 keying an amplifier

[Paul Christensen]

Sounds like a wager for some beer money!  :-))

Paul, W9AC

----- Original Message ----- 
From: "Scott Ellington" <sdelling at facstaff.wisc.edu>
To: "Elecraft Reflector" <Elecraft at mailman.qth.net>
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
>>
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>
> Scott Ellington
> Madison, Wisconsin
> USA
>
>
>
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