[Elecraft] Requirements for keying a KXPA100

[Don Wilhelm]

John,

Rather than rely on the particular switching point of a sample of 1 
KXPA100, why not measure the voltage across the variable resistor - when 
it is 1.3 volts or lower (1.0 volt if you are adding the voltage drop of 
a schottky diode), measure the value of the resistor - that is the 
maximum resistance that can be in the circuit.

73,
Don W3FPR

On 1/14/2014 4:21 PM, John Marvin wrote:
> Oops, sorry for the prior mistaken posting.
>
> I just realized I can try an experiment directly shorting the key 
> input of the KXPA100 through a variable resistor, in order to find 
> what is required to trigger it. I can then do the math and add another 
> .1v safety margin in order to determine whether I can go with a simple 
> change or something a little more involved.
>
> John
> AC0ZG
>
> On 1/14/2014 2:08 PM, John Marvin wrote:
>> Lyle,
>>
>> Thanks. I would have thought that a transistor driver would have been 
>> used to drive the I/O pin. Anyway, I measured the voltage drop across 
>> the key input of the KXPA100 when the transceiver was attempting to 
>> key the amp. The drop was 2.1 volts. Since the voltage for the 
>> circuit is internal to the KXPA100, this is the voltage drop within 
>> my transceiver (i.e. the switching diode, the 560 ohm current 
>> limiting resistor and the keying circuit on the Hermes board), 
>> leaving 2.9 volts for the KXPA100. I don't know what else is in the 
>> circuit inside the KXPA100, so I don't know if the microprocessor is 
>> seeing that entire 2.9v. Do you know whether or not that is the case?
>>
>> I could get another .3 volts or so by switching to a schottky diode, 
>> and possibly another .15 volts if I cut the resistor value in half 
>> (I'd like to guarantee a maximum current of about 50ma (when keying a 
>> 12-13.8V circuit), since the Hermes keying circuit is rated for up to 
>> 100ma). That would give about 3.35 volts, which still is under the 
>> 3.5v normally required to trigger a high state for cmos. That might 
>> work, but I'm not sure I want to live with something that might be 
>> flaky.
>>
>> I want to preserve the capability of triggering an amp with a relay 
>> coil in the keying circuit, so I don't want to drop the diode. I 
>> fried the tiny IC switch on the Hermes board once, and although I'm 
>> reasonably competent when it comes to surface mount soldering, I 
>> don't relish doing that repair again. I haven't had another failure 
>> (not entirely sure of the cause of the first failure) since adding 
>> the additional protection in the circuit (the diode, resistor and a 
>> bypass capacitor). I may have to consider a different design where 
>> the Hermes keys a transistor switch which then provides the path to 
>> ground for the tx preamp and the external amp. That way the 
>> transistor can be my "fuse" and I can then reduce the circuit 
>> elements in the keying path.
>>
>> John
>> AC0ZG
>
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