nuckollsr(at)cox.net
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 Posted: Sat Jan 13, 2007 1:23 pm Post subject: Wgt complexity and parts count vs running (+) wires |
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At 02:19 PM 1/13/2007 -0600, you wrote:
| Quote: |
<terrence_miles(at)hotmail.com>
Bob,
I am looking at your example in the switch chapter on how I could use a 700
2-10 and wire Nav and Strobe lights. As I do this, I am mindful of keeping
things simple, but not at the risk of long power runs in the event of a post
accident short or loose wire or related risks.
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Length of wire runs becomes a non-issue if the circuit is either
fused very lightly (5A or less) or is downstream of a DC power master
relay where it becomes "cold" as a result of preparation for landing
be it on or off-field.
| Quote: | I have an overhead switch panel in the Velocity I am building. When
possible I have tried to have those switches supply grounds (pin 85) to some
40 amp relays I got on Ebay from a Car Audio outlet that I will mount behind
the panel area.
The distance from the power fuse block to this o'head switch panel is about
8 feet one way. The load in question is a 1 amp (2 amp fuse) for the nav
light LEDs. (Not the strobe. It is fused at the control box.) Here is my
question.
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We run power and control leads for distances as much as 50-60 feet
in some of our aircraft. As long as the wire gage is sufficiently
sized to offset voltage drop, there are no good reasons to add
relays EXCEPT to avoid having a LARGE switch in a row of otherwise
small switches or to avoid a LONG run of relatively fat wire (Larger
than 16 or 14 AWG).
| Quote: | Is there a good practices recommendation you might have given my choices are
an 8 foot run (+) power into my overhead panel over ground wire into the 700
2-10 switch that then requires two separate relays in nose.
Follow on question--if 1 amp loads (20awg) are OK, is there an amperage
break point where you would opt for remoted relays?
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Do your math. 20 AWG wire is 10 milliohms per foot and drops
10 millivolts per foot per amp. Assume total wire in
2A nav light circuit from bus to lamp and back to ground is
say 25 feet. 20AWG will drop 10 x 2 x 25 or 500 millivolts.
As a percentage of system voltage, this works out to 500/14000
or 3.5% . . . generally considered quite tolerable. 5% max is the
rule of thumb. I'll suggest your lowest cost of ownership and
highest reliability will happen when relays are used sparingly and
the S700 series switches perform happily at 7A continuous and
satisfactorily at 10A.
Bob . . .
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< What is so wonderful about scientific truth...is that >
< the authority which determines whether there can be >
< debate or not does not reside in some fraternity of >
< scientists; nor is it divine. The authority rests >
< with experiment. >
< --Lawrence M. Krauss >
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