[Elecraft] 120V vs 240V

[Ron D'Eau Claire]

I certainly agree with running 240V when it is available. However 120V will
work just fine with the KPA500. I am testing a KPA500 on a 120V, 20 amp
circuit that is fully 50 feet from the breaker panel. 

Yes, there is significant droop in the mains voltage when I key down at 500
watts output, but the KPA500 has internal transformer taps that can be
chosen to provide the correct voltage under such conditions. There is a
limit to the droop allowed as internal voltages in the KPA500 rise when not
transmitting (or running at reduced power). The tap is chosen to stay within
the upper limit "key up". 

I submit that my situation is just about worst case - considering that it's
only a 20 A (instead of a 30A) circuit and the 50-foot distance from the
breaker panel involved, yet I easily found a tap combination that provided
the voltage for full output without exceeding the upper limit key up. 

I have a home built about 30 years ago, and one thing I have noticed that
applies here. Commonly used power outlets wear out. Long ago I noticed that
I could hear that our vacuum cleaner (which draws 12 amps at 120V) ran more
slowly on some outlets than others. Investigation showed that the springs in
the outlet itself were weak after many years of having plugs pushed in and
pulled out. As they weakened, the contact with the plug got loose, causing
heating and further weakening of the springs (there's a reason outlets are
mounted in fire-resistant enclosures!). 

Replacing the outlets cured the problem. It's a simple job for most. Just be
certain you have the circuit off at the breaker panel, and everyone in the
house knows what you are doing so no one flips it on while you are working
(professionals lock the main breaker panel shut and put a big sign on it
while working on a circuit elsewhere).

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Edward R. Cole
Sent: Wednesday, February 16, 2011 10:47 AM
To: Elecraft at mailman.qth.net
Subject: [Elecraft] 120V vs 240V

Important reasons for running HP amps with 240vac.

I used to run a smaller PS that provided 2500v at 400mA = 1000w dc 
input with 600w RF out on my 2m-8877.  That PS ran on 120vac and drew 
8.3 amps on ordinary house wiring and I saw some flicker in the 
lights on CW.  I don't recall what the voltage sag was.

Now I run 3700v at 720 mA = 2664w dc (1400w RF) which represents a 
10.9 amp load on 244vac (my line voltage).  I see about 2-3 volts 
line -voltage sag so that is 33w dissipation in the 8-4 wiring. If I 
had tried running with 120vac the load would be 21.8 amps which would 
exceed the rating of the wiring and likely trip breakers.  The power 
supply is capable or 1.5amps dc load so represents a max load of 5kW 
and that hits over 20A which is my current breaker rating.  Of course 
this would produce 2500w RF which exceeds legal level by a lot.

Important stuff to consider when running significant RF power.  The 
KPA-500 will likely run on 120vac with no problem but better if run on 240v.