Today's Message Index:
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1. 05:17 AM - Re: Re: Review request for RV-9 Electrical System (G3X, GTN, etc) (Robert L. Nuckolls, III)
2. 07:45 AM - If you really want a avionics switch - do it this way (JOHN TIPTON)
3. 11:34 AM - Re: If you really want a avionics switch - do it this way (user9253)
4. 12:56 PM - Re: If you really want a avionics switch - do it this way (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Review request for RV-9 Electrical System |
(G3X, GTN, etc)
At 03:46 PM 6/12/2016, you wrote:
>
>On avionics switch, would be interesting to have most
>avionics off while cranking the engine.
The idea behind the avionics master switch
was birthed about 1965 when transistors were
first starting to show up in the radios.
Compared to today's hardware, batteries were
relatively soggy 'soggy' devices. We were
'killing' radios in brand new airplanes
before they left the factory.
The real physics behind those failures was
poorly understood. See http://tinyurl.com/hye6mpe
Today's batteries are more robust as are the
radios themselves. All things potentially
hazardous to the community of silicon devices
are well known and easily managed. Today,
there is nothing that any vehicular dc
power system can throw at a properly
applied transistor that puts the device
at-risk.
>For the Trim breaker, Ray Allen indicates that a single 1A would be
>enough for both trim motors. This sounds odd considering Garmin
>recommended a 5A for each.
Breakers/fuses protect wires. You're
probably not going to wire with anything
smaller than 22AWG for which 5A is the
recommended protection. While 1A protection
for both is 'enough', 5A protection is not
'too much' based not on what's at the end
of the wire but based on the wire size alone.
The decisions for wire sizing/protection
are going to be simple . . . the more
difficult part is to craft an architecture
that produces the most failure tolerant
design based on the airplane's flight
handling qualities and how you intend to
use the airplane. In most instances, loss
of one or both trim systems does not
present an especially risky situation.
On the other hand, protecting each system
independently of each other is easy and
likelihood of ANY protective device being
tripped is exceedingly low.
>Yeah, I'm probably going to brake a few items out into fuse blocks.
>I'm thinking exterior lights get one, LRUs, Cabin Power, and maybe
>one other. Is it reasonable to have these fuse blocks have a pull
>breaker on the panel?
>i.e. Ext Lights panel breaker for 20A goes to a fuse block behind
>the panel with fuses for each light.
>Is this reasonable?
Why any breakers at all? They're expensive,
take up panel space, have no useful purpose
to the pilot in operating the aircraft. They
force you to route wires to relatively
unhandy places while increasing weight.
See http://tinyurl.com/hjkqsto
I proposed a shift from breakers back to
fuses in aircraft about 20 years ago. Since
that time, many failure tolerant airplanes
have been crafted with no breakers at a
substantial savings of weight, cost and
build time.
>The e-bus has a peak that exceeds the SD-8. This only happens if
>everything is on at once, including the boost pump running, the
>radio transmitting, both servos moving and trimming, and a dead ibbs
>battery charging. This will likely never happen, definitely not for
>any amount of time longer than a second or so. The normal draw for
>the bus should be about 6A with it maybe going up to 10-15A on
>landing. I'm looking at the SD-8 as more of a battery extender than
>a full redundant alternator.
You've done a load analysis . . . great!
The loads you describe do not suggest the need
for a relay . . . a toggle switch will handle
them nicely. If a relay is necessary, something
like this is entirely appropriate to the task
http://tinyurl.com/hdmkvvt
Bob . . .
Message 2
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| Subject: | If you really want a avionics switch - do it this way |
Hi Bob (et all)
I've been thinking of using this system (RV9a)
http://www.aeroelectric.com/articles/avmaster.pdf
for a avionics switch - would it be 'acceptable' to use a two pole, changeover
(2-10) switch, as neither circuit would/should be on together, so: centre all
off, switch up, avionics live, switch down, endurance bus feed live
Regards - John
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=457076#457076
Message 3
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| Subject: | Re: If you really want a avionics switch - do it this |
way
The 2-10 switch functions are:
Down: Both OFF
Center: One circuit ON
UP: Both circuits ON
The S700-2-1 switch functions are as you posted:
Down: One circuit ON
Center: Both OFF
UP: The other circuit ON
The S700-2-1 nomenclature is only used by B&C. If purchased elsewhere, it is called
a DPDT switch with center off.
If a switch description contains ( ) , that indicates a momentary position.
The disadvantages of having a switch in series with the diode are that the switch
adds complexity and can fail. If there is no switch, it can not fail.
There may be rare occasions when it is desired to shut off all avionics. But it
is better to shut off each one individually, because switches need exercising
to rub off oxidation. Aircraft switches are more likely to fail from corroded
contacts than from mechanical failure (wearing out).
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=457080#457080
Message 4
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| Subject: | Re: If you really want a avionics switch - do it |
this way
At 09:44 AM 6/13/2016, you wrote:
><jmtipton@btopenworld.com>
>
>Hi Bob (et all)
>
>I've been thinking of using this system (RV9a)
>http://www.aeroelectric.com/articles/avmaster.pdf
>for a avionics switch - would it be 'acceptable' to use a two pole,
>changeover (2-10) switch, as neither circuit would/should be on
>together, so: centre all off, switch up, avionics live, switch down,
>endurance bus feed live
I'd rather you use two separate switches.
No single point of failure. Having ALL switches
on at the same time poses no hazards.
Bob . . .
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