Today's Message Index:
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1. 09:31 AM - Re: Re: Minimum ANL size of 35A vs. power feed AWG (Robert L. Nuckolls, III)
2. 03:38 PM - Re: Re: Minimum ANL size of 35A vs. power feed AWG (Robert L. Nuckolls, III)
3. 04:36 PM - Re: Re: New role for the E-Bus? (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Minimum ANL size of 35A vs. power feed AWG |
>It's about an 8' run over to the center CB, but even at that
>imaginary 67 amp load, on this 28V airplane, I could in theory
>handle it with an 8 AWG wire from the side CB panel to the center CB
>panel. ...but that would burn through before the ANL interrupted.
>
>So you are saying, don't worry about the ANL. Can I also not worry
>about maintaining 6 AWG all the way to the center CB panel?
You're over-worrying it. Drop the wire gage for the
last leg of the trip to center panel. Ditch the
ANL.
>While we're at it, I have a 27 amp "E-bus". (Yes I know that is
>extreme but really that's my whole g3xtouch system with all its
>canbus friends and relatives includeing the eis and auto
>pilot) Today I broke one of the diodes that feeds it and am
>looking for some replqcements. How do I identify those at digikey
>or wherever?
Is this a bridge rectifier type? Just
about anything that LOOKS like it
will do. Voltage rating is not
terribly significant in the 28v
system. Current rating is more significant.
HOWEVER, with this much
current through the device, I'd
plan on some heat sinking . . . say
36 square inches of aluminum sheet
or equivalent.
You can 'kill' a 50A rated diode
with 30A flowing if it's not
heat sinked.
Here's a likely candidate:
https://tinyurl.com/yahl74yy
Bob . . .
Message 2
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| Subject: | Re: Minimum ANL size of 35A vs. power feed AWG |
>It's about an 8' run over to the center CB, but even at that
>imaginary 67 amp load, on this 28V airplane
Really? Have you conducted a load analysis?
This isn't a 'conjuring' activity, it's
a numerical analysis of your architecture,
component sizing, failure modes and crafting
of plan-B etc.
You can do this on a spread-sheet . . . there
are several examples in Excel to be had
at
https://tinyurl.com/9rt6ymn
but my personal favorite is the hand-entered
and edited chart
https://tinyurl.com/7jqypwj
One sheet per bus. Each line depicts
a accessory feeder. You pick the protection
size (either fuse or breaker . . . don't
fuss over the symbol, just get a number
on it). Pick the wire size. Then fill in
the boxes as to the current draw for
each accessory in the flight conditions
called out across the top.
When I do a wirebook, the architecture
drawing comes first then these pages
follow. I had a column to call out the
page on which that accessory's wiring
details are called out.
Hence, the architecture and loads
pages become the foundation and index
to the whole system which gets added
on subsequent pages.
I'm a bit perplexed by that 67A assertion
above . . . I'd like to know how
that was arrived at . . . you really
NEED to know that the number(s)
are golden.
Bob . . .
Message 3
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| Subject: | Re: New role for the E-Bus? |
At 12:46 AM 4/17/2020, you wrote:
>
>Hi Bob,
>
>I just had a good look at Z101. I like it. Five comments, certainly
>things you have thought of.
>
>1) There is no overvoltage protection for the aux alternator. I
>assume that is because having an overvoltage event immediately after
>having a main alternator failure is too unlikely to justify it ?
EXACTLY. In an ideal world, the AUX alernator
will never be operated in 'distress'. In a practical
world, it would be unusual to have it see more than
a hand-full of hours operation over the lifetime
of the airplane.
It gets pre-flight tested. OV events are also rare.
Adding that protection just doesn't seem justified.
Hence simple, light, inexpensive. But easily 'adjusted'
with a crowbar ovm if the builder so desires.
>2) Is the intention that the Engine Bus Alternate Feed be on during
>normal flight ? If not, switching off the Master or a battery
>contactor failure would remove power from the electronic ignition.
Yes. What is the probability of these events?
Battery contactors get fussy before they become
a failed-open device. If it DID fail open in
flight, you probably wouldn't know it . . . the
alternator would continue to run things nicely.
You might not know it was bad until you shut
down and happened to notice that everything
went dark on first motion of the master switch.
If not noticed, you'd sure pick it up during
next pre-flight.
Inadvertent opening of the master switch
in flight might be a concern . . .which
is why I recommend ergonomic grouping of
switches to separate DC power and engine
controls from things that get routinely
switched in flight.
https://tinyurl.com/ybaw83gd
But if one wished to fly with the alternate
power path to any bus closed, it's no big
deal.
>3) During normal flight, the brown out booster is unpowered, but the
>power from the Clearance Delivery bus back feeds into its output. Are
>these devices typically okay with this ?
The ones I've fiddled with so far don't
mind. Backfeed currents observed are on
the order of 3 milliamps . . . typical
of resistance in the voltage setpoint
divider. But it's certainly something
to be verified. We could add a diode
to the output path. Since the device
is loaded for only seconds per flight
cycle, the diode could probably be an
axial lead, plastic device even if
'slightly' overloaded. I'll keep that
in the ponder-pot.
>4) Is there a risk that the brown out booster could generate a
>significant overvoltage event ? Given their switching method to boost
>voltage, I think not ?
Yeah, an open resistor in the setpoint
divider could cause it to 'regulate' at
some higher voltage. These resistors
are surface mounted and lightly stressed.
Pretty bullet-proof I suspect.
>5) The green color used for the engine bus circuitry is too light a
>green, making it hard to read. I suggest a shade darker.
Check the 'fix' . . .
Thank's for the time/attention to this exercise!
Bob . . .
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