[Elecraft] Elecraft Digest, Vol 114, Issue 11

[Edward R Cole]

Another thing that states this is current ratings on switches (ever 
look at the specs?).

Typical toggle switch may be rated 20A at 125vac and 10A at 
250vac.  Why?  If the load resistance is the same then twice the 
voltage will double the current.  Heat dissipation of the switch is 
the same for either voltage thus the current rating is halved at 
higher voltage.

For my station my Astron 50A PS outputs 14.2 vdc thru about 18-feet 
of 6awg welding wire to the main station fuse, a 30A BUSS 
fuse.  Under max load of nearly 30amps the voltage drop is about 0.4v 
to 13.8v.  From my engineering pocket handbook awg6 is rated at 
0.3952 ohms/1000-feet.  18/1000*0.3952 = .007 ohms.  IR = E:  30*.007 
= 0.21 volts

Of course there is smaller wiring from the distribution terminal 
strip to individual equipment, so additional voltage drop will exist 
depending on the wire size and length (resistance).  One reason to 
keep high power amp power cords short.

Ron' point to measure internal voltage and current is that what you 
have at the terminal of the RF transistor is how much work it will do 
(making RF).  In general having slightly more voltage will run the 
transistor cooler as less current is needed for a given output power.

Transistors all run at below 100% efficiency and the amount of power 
not making RF makes heat (per I^2*R law).  60% is typical efficiency 
of a HF transmitting device.  So if that equals 100w RF, then 67w is 
being dissipated as heat.  My 8877 running at 660ma at 3700v = 2442w 
dc input.  With 1500w RF output 2442-1500= 942w of heat to 
dissipate!  Luckily the 8877 is rated at 1500w dissipation.  The 
cooling system is really tested when I key down for roughly a minute in JT65.

73, Ed - KL7UW
http://www.kl7uw.com
dubususa at gmail.com
"Kits made by KL7UW"