[Elecraft] Requirements for keying a KXPA100

[Don Wilhelm]

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 
resistor in the amp.

You may want to redesign your keying interface to eliminate the diodes.  
If I can suggest using a switching FET such as the 2N7000. If you need 
isolation between 2 sinks, use 2 2N7000s to accomplish that isolation.  
If you need more current or voltage handling than the 2N7000 will do, 
look at a HEXFET.

73,
Don W3FPR

On 1/14/2014 4:08 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