I got a variety of suggestions, and decided to go the easiest route
(since it worked) of removing the current limiting resistor and
switching the diode to a schottky diode. The extra diode is required in
my case because the PTT also keys an internal TX preamp whose keying
circuit is run on
So, I got my KXPA100 kit and have assembled it. But I'm having trouble
keying it with my transceiver (HPSDR Hermes with alterations to support
driving the KXPA100). My circuit didn't have any trouble keying my
previous amp (a HFPacker V4). If I manually short the KXPA100 key input
things
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
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
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.
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
John,
For reliable operation, the input to a CMOS device must see a low level
of 1.5 volts or less. Your 2.9 volts is in the indeterminate region.
As a side note, a high level should be at least 3.5 volts (up to 5
volts), but in this case, the high level is determined by the pullup
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
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