[Elecraft] K3 in NAQP SSB

[Joe Subich, W4TV]

Alan writes: 

> The obvious solution is just to use the K3 +12VDC output 
> connector for all accessories.  It's rated for 0.5A which 
> should be plenty for most devices.  As a bonus it's 
> controlled by the K3 ON/OFF switch.

Unfortunately it's not that easy.  The +12VDC "output" is 
not +12V - it's somewhat less.  There are two series diodes, 
one in the +13.8V input and one on the +12V output plus the 
FET power switch that adds a variable voltage drop that 
depends primarily on the current drawn by the driver.  

In transmit the +12V output is often as low as +11.8V (or 
less if the primary supply is lower than 13.8V). If the 
accessory is designed with a series diode to protective 
against reverse voltage at the input, the supply voltage 
can be too low for reliable operation of 12V DC relays.  
Eliminating D34 in the K3 would be a big help with the low 
output voltage.   

73, 

   ... Joe, W4TV 
  



> On Mon, 2008-08-18 at 16:47, Joe Subich, W4TV wrote:
> > Tom writes:
> > 
> > > Be especially aware that rigs with external 12V supplies can
> > > develop what sounds exactly like RFI from "pin 1" problems.
> > > 
> > > This is because the power supply leads of the SSB radio are
> > > current modulated at an audio rate. If the 12V supply has a 
> > > ground connection common to the negative lead post on the 
> > > supply (or through other gear) the V- lead voltage drop can 
> > > cause a "pin 1" issue from dc potential that varies at an 
> > > audio rate.
> > 
> > This is a particular problem with transceivers that have a
> > single V- lead and only one "pin" for the V- connection (like 
> > the K3).  ...
> 
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