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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