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...
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