[Elecraft] Requirements for keying a KXPA100

[John Marvin]

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

On 1/14/2014 5:28 AM, Lyle Johnson wrote:
> John,
>
> The diode drop is likely the primary culprit.  If you must use a 
> series diode, use a Shottky diode (like a 1N5711) and not a silicon 
> diode (like a 1N4148).
>
> The key input goes to a microprocessor digital I/O pin inside the 
> KXPA100, and a diode drop - especially with a significant series 
> resistance -- puts the detected voltage in the indeterminate region.
>
> 73,
>
> Lyle KK7P
>
>> So, I got my KXPA100 kit and have assembled it. But I'm having 
>> trouble keying it with my transceiver ...There is a 560 ohm resistor 
>> in series internally, along with  diodes...
>