[Elecraft] K3: pwr out - does this sound right?

[Joe Subich, W4TV]

> You are testing the regulation capability of your power
> supply. 

Sorry, Don.  Using the internal voltmeter in the K3 does not 
test the regulation of the power supply.  The pick-off point 
for the voltmeter is after Q2 (FDS6375) the FET power switch 
and D6 (SB530) reverse voltage protection diode.  The drop 
Doug is measuring is primarily the drop through Q2.  

If you don't believe me, check the voltage readings at 2W, 
5W and 10W then compare them to 20W, 50W and 100W.  Then 
compare those same voltages to a calibrated DVM directly 
across the power supply. 

Note, the voltage read by the K3 voltmeter will be .3V less 
(at no load) than the voltage at the PowerPole due to D6 and 
the drop will increase with current through Q2.  Power for 
the KPA3 100 Watt amplifier is picked off before D6/Q2. 

In my case (an Astron RS-35A that drops 0.01V from no load 
to > 20A measured directly at the output), I see 13.7V on 
the K3 with no load (power supply set to 14.0V), 13.3V at 
10.0W and 13.1V at 100W.  The 0.2V difference between 10W 
(3.7A) to 100W (18.0A) represents 0.014 Ohms [0.2V / 
(18.0-3.7A)] of power cable and connector resistance.  

My concern is the relatively large change in voltage with  
current through the FDS6375.  The current through Q2 should 
be less than 4 A (no current for KPA3).  The Fairchild data 
sheet (www.fairchildsemi.com/ds/FD/FDS6375.pdf) shows 
Rds(on) should be less than 0.03 Ohms for -Vgs > 2V.  

I am concerned that the .4V swing on the unregulated +12A 
line to the LPA (driver) might be responsible for some of 
the IMD seen in the K3 output at higher power levels.     

73, 

   ... Joe, W4TV 
 





> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Don Wilhelm
> Sent: Sunday, December 21, 2008 9:50 AM
> To: DOUGLAS ZWIEBEL
> Cc: Elecraft Reflector
> Subject: Re: [Elecraft] K3: pwr out - does this sound right?
> 
> 
> Doug,
> 
> You are testing the regulation capability of your power
> supply. I would say that is fairly normal - most of the drop 
> will occur in the 
> initial part of the I-V output curve for the PS, then will 
> flatten out 
> as the current becomes greater.
> You are indicating a regulation of 3.7% over that current draw range 
> which is quite good.
> 
> 73,
> Don W3FPR
> 
> DOUGLAS ZWIEBEL wrote:
> > After reading the recent comments on POWER OUT, I decided to check
> > mine.
> >
> > Using the voltmeter on the K3....
> >
> > Zero power out:  13.6v
> > 5 watts out:  13.3v
> > 100 watts out:  13.1v
> >
> > The drop from 13.6 to 13.1 for 100w doesn't seem so bad.
> But why the
> > drop when running only 5 watts out...seems out of line
> compared to the
> > 100w level drop.
> >
> > Does the 5 watt output V drop seem right to you?  Comments?
> >
> > PS is 25-30 year old Astron RS35A
> >
> > Thanks,
> > Doug KR2Q
> >