[Elecraft] Requirements for keying a KXPA100

[John Marvin]

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