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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