[Elecraft] K3 in NAQP SSB

[Alan Bloom]

Or replace the series diodes with power schottkys.



On Mon, 2008-08-18 at 21:29, Joe Subich, W4TV wrote:
> 
> 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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