[Elecraft] Local Noise Problem

[Ron D'Eau Claire]

In the 24 volt aircraft systems I worked with power was typically routed
into the cockpit via 1/2" diameter copper rods that ran along behind the
breaker panels. Breakers for each circuit were tapped into the rod with
threaded holes and screw. 

That's what it takes to avoid excessive I^2R losses for a system that draws
significant current. 

And never lose sight of the danger of a low-voltage high-current system.
First, if you have an inductive load, the pulse that occurs when it's
disconnected can run into many hundreds of volts - enough to do real damage
if you're holding the two ends of the circuit as you disconnect it. 

But more commonly is the danger of a short circuit. Things turn red-hot
instantly and even thick bus-bars can melt. I saw a guy drop a screwdriver
into an aircraft 24-volt bus once. I was glad I was 50 feet away in the next
plane. It looked like a bomb went off as the shaft of the screwdriver melted
so fast it literally exploded with a huge bang, showering the poor guy with
molten steel and setting the plane on fire. 

I also knew techs missing fingers after getting a ring across a low-voltage
source. The rings heated so fast it welded to the terminals before he could
pull away and the hapless fellow had to suffer the melting metal burning his
finger off until he was able to jerk his hand away without the finger. I
heard of one fellow who wore a metal watch band and, yes, he reportedly lost
his whole hand.

To this day, I remove any jewelry when I'm working around a high-current
low-voltage source like an automobile battery.  

Ron AC7AC

 

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Edward R. Cole


Regarding high-current dc distribution:  I ran into that issue lots 
of years ago when I wanted to run some ham gear outside my travel 
trailer and use the 12v system as power source.  I quickly found that 
30-feet of #14 or #12 wire would not do.  I finally used a 12v 
battery and floated it from the trailer power.  And I was only 
running 150w on 2m.

At one time I was considering solar power for a two-story log-cabin 
in the bush, but the wiring gets expensive (so do the panels).  The 
cabin now has 120vac utility service so its a moot issue (and I sold 
it in 2003).

My present shack has 13.8v provided by  50A Astron which is connected 
via a 12-foot run of #6 welding wire to the main 30A buss fuse.  This 
connected to a heavy barrier terminal strip to provide power to all 
the amplifiers and high-current equipment.  I see about 13.5v at the 
main fuse when running loads; unloaded it shows 13.75v.

Two power leads run from there to two more barrier strips:  one is 
#12 solid house wire and the other #8.  Both about 3-foot long.  My 
new 144-28 50w transverter runs off the strip fed by #8 wire and 
voltage runs at 13.2 under load.  My K3/10 is fed from the #12 wired 
strip.  There are only two other items connected there: panel lights 
for my manual antenna tuner and the main station controller (fused at 5A).

Many of my new amps require 28vdc.  I have a 8A and two 50A supplies 
for 28vdc.  One 50A PS runs the 16-foot dish rotator-motors and will 
run 432-100w and 1296-300w PA's at the dish.  I run #12 romex out 
there for 120vac power (remotely switched).  The other 50A PS may go 
to the base of one tower where I may install some big amps for 900 
and 1296 MHz.  To place the amps at 50-feet will require #6 wiring up 
the tower.  Installing a switching PS at tower-top becomes attractive 
(not the 90-lb. 50A PS).

Ohms law rules dc-wiring!
Ed

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