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Well, let me contribute what I can from
some of the things I've learned. An electrical circuit requires two conductors;
there is no such thing as "ground" or "earth", although I've heard they found
the absolute reference ground at 0:00:00 Lat, 0:00:00 Long. If
you throw "ground" out of your vocabulary and instead substitute "signal
return", you'll have a much better time in wiring things, especially on
dielectric (plastic), as well as metallic airplanes. Every morning when you
arise, repeat ten times "There is no such thing as ground in electrical
circuits". That being said, when a signal goes from a source to a load,
there must be a way for that signal current to get back to the source. A
shielded wire, or a twisted pair, is a perfect medium to do that, providing that
the return is connected to common at the source end.
If it is not connected at the source end, then the current must find its way
back by some other path, often bringing with it other undesirable currents,
giving rise to common-mode coupling, so-called ground-loops. These currents
arise from the difference in potential, AC and DC, between the source
common and the load common. Obviously if the load is tied to its common, the
return conductor must be tied at the receiving end also! This is where opto
isolator circuits find good use in keeping out unwanted signals; they only
require being tied to the opto circuit at each end without a common return
connection between them.
One of the best things you can do to
minimize interference is to have a single point at which all power returns are
connected. Keep the main and common buss close together and run twisted pair or
shielded wires to each load. It also helps to have capacitors connected between
the main and return buss. Parallel a 4700 uF 35V with a 1uF 35V film and a .01
ceramic disc across them. This keeps both the main and common buss at the same
AC potential. Under no circumstance should loads share a return! Please
don't "daisy-chain" returns from one circuit to another! Down that
path lies destruction, Alice! On my Lancair, I have two terminal boards that are
mounted horizontal and parallel, one below the other, with a copper return
buss below the bottom terminal board. The return buss is made from flattened
copper tubing with brass screws through it at the same spacing as the terminal
board terminals. The upper terminal board is for the main, aux, and
avionics buss. The one below it is for load distribution. I run twisted pair
from the main and distribution terminals to each circuit breaker. Current flows
from the main, to the circuit breaker, then back to the distribution terminal.
That provides cancellation of the magnetic field in these conductors. Then
each load has a twisted pair from the distribution terminal and the common. Here
again, the currents go out and back over these wires causing the magnetic fields
to cancel, and all circuits are tied together at one
common point. An advantage of this scheme is that each load's
source wiring is easily found on the distribution board for connecting,
dis-connecting, and trouble shooting. Another thing you might try is to mount an
LED with each circuit breaker and wire it directly across the breaker
terminals with a series resistor, 1.2k 1/2W for 14V systems, and 2.7k, 1/2W
for 28V systems. Then if a circuit breaker is open under a load, the LED will be
on, showing that it's open.
Radio Shack and JameCo have snap-on
ferrite interference suppressors that can be placed over coax and wire bundles
to cut down on interference between circuits. They are tubular ferrites, about
3/4" OD, 1/4" ID, and 1" long, sliced in half down their long dimension,
and secured in a hinged plastic cover that can be snapped in place over
wiring. They cost less than $2 each.
One more consideration is where to put
the field circuit breaker if an alternator circuit breaker is used. If your
battery is run down and you have to jumper it to start, and then shortly after
you take off, the charging current on the depleted battery could be very high,
tripping the alternator breaker. If your field breaker is connected to the main
buss, the regulator will sense the low voltage on the buss and feed maximum
current to the alternator field. With the alternator breaker tripped and no
load, the alternator will put out well over 100V! Then when you push the
tripped-breaker back on, the sudden inrush current and high voltage could
fry your electronics. A better place to connect your field breaker is on the
alternator-side of the alternator. That way, if the alternator breaker trips,
the alternator field will still be connected to the alternator output and keep
its voltage under regulation!
For a much better look at
the subject of shielding and grounding, try "Interference Handbook" by W.R.
Nelson, WA6FQG and "Grounding and Shielding Techniques in Instrumentation" by R.
Morrison. Morrison has good illustrations of how interference currents get
coupled in, electromagnetically, electrostatically, and common-mode! Nelson's
book has interesting tales of the interference problems he analyzed
and solved!
Whew! This started out short and got long-winded!
Oh, well! Back to CA form 540A!
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