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  #1  
Old 09-13-2018, 05:26 AM
RV74ME RV74ME is offline
 
Join Date: Dec 2015
Location: Auburn, AL
Posts: 105
Default Avionics Bus/ Radio Master

I am about to start ďpre-wiringĒ and planning the power distribution. I have no experience with electrical work and have been reading everything I can find to educate myself.

I plan to use fuses for most everything (CBís for alt fld, elec trim, and autopilot).
A main bus and a separate Avionics bus (so two separate fuse blocks).

My question is, do i simply run the appropriate size wire from main bus to a switch (AV master) and then to the avionics bus? Or will i need some kind of relay, and why?

Btw, the avionics bus will only have GTR 200, GTN 625, GTX45R, and maybe G5).

Thanks.
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  #2  
Old 09-13-2018, 06:15 AM
Deweyclawson Deweyclawson is offline
 
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Location: Enon Valley
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Default Avionics buss

I would, did, put all the avionics on the avionics buss. It is a royal pain to turn each one on and off. You do not want any of those expensive electrons exposed to the power surges happening during eng start. The only avionics that is on during start in my RV6A is the eng gauges. Also..Use a sw that locks on, like a lift lock. You really do not want to inadvertently turn it off in flt, at night, in the wx.

As much as feasible, use the same size wire, probably 20ga. If 22ga will work for an item, it is not worth messing with it. Then use the same size fuses for all on the same size wire. Remember..The fuse/CB protects the wire, not the electronics. Especially with fuses, you can reduce the number of spares you have to carry. Also much simpler/easier to build.

Since losing the Av buss due to a sw or CB fail is a real possibility, been there, done that, I put in an alternate (guarded) AV master and AV CB in parallel to the main.

I did not use any kind of relay.
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  #3  
Old 09-13-2018, 06:35 AM
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uk_figs uk_figs is offline
 
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Default Similar setup

I also used fuses except for the alternator field circuit. Did not use a relay but similar to Dewey I have a avionics buss switch (which is off at engine start) and I have an avionics alternate essential buss circuit and switch which is a guarded switch and powered directly from the battery per the aeroelecric book.
No relays.
Figs
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  #4  
Old 09-13-2018, 06:42 AM
rv7charlie rv7charlie is offline
 
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Location: Pocahontas MS
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Default

Steve, have you obtained and read The Aeroelectric Connection book? That's step zero, if you're wiring an a/c.

I agree about fuses.

The question about wire size is yes; the questions about switch/relay/architecture don't have simple answers. The right answer for your installation is driven by your installation. :-)

I differ a little bit from Dewey on the reason for an avionics switch. Convenience is fine, but I don't worry about power surges; pretty much any piece of expensive avionics these days will be designed to tolerate surges and spikes far greater (think low level lightning strikes) than anything a start sequence can generate.

A bigger driver for switchology is 'resource management', if you lose your primary alternator. Questions like, do I have a backup alternator? If so, can the backup handle the full electrical load of the plane? If not, am I IFR, or can I just shed all my electrical loads and survive? Do I have an electrically dependent engine? If so, do I need to shed electrical loads to survive?

See what I mean?

Aeroelectric Connection. Get the book; join the Aeroelectric email list. You will get far more reliable advice there, than here. The author of the book participates in discussions on the list, answering questions and offering guidance.

Charlie
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  #5  
Old 09-13-2018, 07:17 AM
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Carl Froehlich Carl Froehlich is online now
 
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Default

If you really must use an Avionics Master Switch, then use two. Put Nav #1, Comm #1, etc. on one and then Nav #2, Comm #2, etc. on the other.

If you plan this plane for IFR, PM me your email address and I can provide some power distribtion ideas.

Carl
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  #6  
Old 09-13-2018, 07:59 AM
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RONSIM RONSIM is offline
 
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Location: Largo, FL
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Default Dual avionics master switches

Everything on the avionics buss have pullable breakers, with two avionics master switches wired in parallel.

Ron
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Last edited by RONSIM : 09-13-2018 at 08:00 AM. Reason: clarification
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  #7  
Old 09-13-2018, 12:50 PM
RV74ME RV74ME is offline
 
Join Date: Dec 2015
Location: Auburn, AL
Posts: 105
Default

Quote:
Originally Posted by rv7charlie View Post
Steve, have you obtained and read The Aeroelectric Connection book? That's step zero, if you're wiring an a/c.

I agree about fuses.

The question about wire size is yes; the questions about switch/relay/architecture don't have simple answers. The right answer for your installation is driven by your installation. :-)

I differ a little bit from Dewey on the reason for an avionics switch. Convenience is fine, but I don't worry about power surges; pretty much any piece of expensive avionics these days will be designed to tolerate surges and spikes far greater (think low level lightning strikes) than anything a start sequence can generate.

A bigger driver for switchology is 'resource management', if you lose your primary alternator. Questions like, do I have a backup alternator? If so, can the backup handle the full electrical load of the plane? If not, am I IFR, or can I just shed all my electrical loads and survive? Do I have an electrically dependent engine? If so, do I need to shed electrical loads to survive?

See what I mean?

Aeroelectric Connection. Get the book; join the Aeroelectric email list. You will get far more reliable advice there, than here. The author of the book participates in discussions on the list, answering questions and offering guidance.

Charlie

Yes, I have read thru Bob Nuckolls book several times, and also Mark Ausmannís Aircraft wiring guide. Starting to all make some sense now. I am trying to keep it simple as possible...1battery, 1alternator, and IBBS for the G3x. And the G5 will have back up battery too.

Iím an airline pilot, but dont want to be doing any hard ifr in a single engine plane. Just wanna be able to punch thru 1000í deck on occasion. VFR for most part.
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  #8  
Old 09-13-2018, 12:51 PM
RV74ME RV74ME is offline
 
Join Date: Dec 2015
Location: Auburn, AL
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Quote:
Originally Posted by Carl Froehlich View Post
If you really must use an Avionics Master Switch, then use two. Put Nav #1, Comm #1, etc. on one and then Nav #2, Comm #2, etc. on the other.

If you plan this plane for IFR, PM me your email address and I can provide some power distribtion ideas.

Carl
I will only have one comm (gtr 200), an a gtn 625 for ifr nav (no vor/loc)
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  #9  
Old 09-13-2018, 12:53 PM
RV74ME RV74ME is offline
 
Join Date: Dec 2015
Location: Auburn, AL
Posts: 105
Default

Quote:
Originally Posted by uk_figs View Post
I also used fuses except for the alternator field circuit. Did not use a relay but similar to Dewey I have a avionics buss switch (which is off at engine start) and I have an avionics alternate essential buss circuit and switch which is a guarded switch and powered directly from the battery per the aeroelecric book.
No relays.
Figs
I remember now reading that in aeroelectric connection
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  #10  
Old 09-13-2018, 01:26 PM
DRMA DRMA is offline
 
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Location: Sugar Land, TX
Posts: 234
Default

One reason for using a relay is the max current draw of all equipment connected to your avionics bus. If you intend to power that bus directly through a switch, you need to make sure the switch is rated for that maximum DC current (not the AC current rating of the switch, but the DC current rating). If your switch is capable of handling the fully current, then no need for a relay. But if not, then a relay should be used which is rated for the full load current of the avionics bus.

If you do use a relay, you might consider a SPDT or DPDT relay, wired such that the avionics bus is energized when the relay is not powered (not picked up), and the relay is open when it is powered (e.g. connected to the NC contacts). Then turn the switch on your panel upside down, and wire it so that it powers the relay when the switch is in the down (off) position. This way the relay will pick up with the battery contactor on and the avionics switch off. Once the engine has started, turning the avionics switch on (removing power to the avionics relay) will send power to the avionics bus. If the relay or avionics switch fail, the avionics bus will default to being energized, rather than defaulting to unenergized.
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Last edited by DRMA : 09-13-2018 at 01:30 PM. Reason: fix typos
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