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Avionics Bus/ Radio Master

RV74ME

Well Known Member
I am about to start ?pre-wiring? 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 (CB?s 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.
 
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.
 
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
 
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
 
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
 
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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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 Ausmann?s 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.

I?m an airline pilot, but dont want to be doing any hard ifr in a single engine plane. Just wanna be able to punch thru 1000? deck on occasion. VFR for most part.
 
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)
 
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
 
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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switch rating

If you only have avionics on the bus, then one of the standard switches will handle that light load. Bob does not recommend an avionics bus, since then you'll never turn off the avionics, and working the power switch on each device is a good idea to keep the contacts "clean". My two GRT devices don't have an on/off switch, so I'll let have a switch linked to the IBBS to power them.

Lots of good advice can be found here: http://forum.matronics.com/viewforum.php?f=3
 
ah, another never-ending debate.

NO avionics master switch here. Avionics are divided, appropriately per failure analyses, between Main and Essential busses, with the E-buss wired per Nuckolls' diagrams.
 
No avionics master here.

Air start engine - power "spikes" on start irrelevant. As far as "convenience" goes, there is an EFIS, a com radio, and a transponder to switch on and off at the beginning and end of each flight. A task which I have to say I don't find all that arduous. Even if I had another com radio and another EFIS display, I'd have to say the task of running from left to right or right to left and switching equipment on once, and off once each flight, would not exactly be the hardest procedure I've done in my aviation career. I have to admit to getting a wry smile when I read this as being a compelling reason to wire in yet another switch!
 
I have to admit to getting a wry smile when I read this as being a compelling reason to wire in yet another switch!

Trust me, I want to keep it as simple as possible...the fewer switches, the better. My first plan was to not have one, but the avionics guy building my harnesses said I should definitely have one.

Again, electrics/wiring is NOT my forte!
 
Legend has it, waaay back when solid state avionics 1st came on the scene, they used germanium semiconductors (preceded silicon), which *were* susceptible to brownout/surge failures. The avionics master was born, and the legend lives on....
 
Legend has it, waaay back when solid state avionics 1st came on the scene, they used germanium semiconductors (preceded silicon), which *were* susceptible to brownout/surge failures. The avionics master was born, and the legend lives on....
Yeah I've flown some aeroplanes with some complex avionics which go completely bananas for a few seconds with power transients during engine start or other power/voltage related processes, but never in my life have I seen one of those bits of avionics fried as a direct consequence.

I have to conclude that if it really does happen, then GA type avionics really is produced to a significantly lower quality standard than the stuff used in the heavy metal types. Yet the manufacturers involved in marketing their avionics to the experimental community all seem to say that their gear is every bit as good as the certified stuff. Doesn't make much sense to me. They can't have it both ways.
 
Not to mention that if your panel is all glass, at least some if it will have to be on during engine start to give you RPM, oil pressure, etc., readings.

I just leave everything on all the time...I don't even switch it off after shutdown.

600 hours, zero issues.
 
Yeah I've flown some aeroplanes with some complex avionics which go completely bananas for a few seconds with power transients during engine start or other power/voltage related processes, but never in my life have I seen one of those bits of avionics fried as a direct consequence.

I have to conclude that if it really does happen, then GA type avionics really is produced to a significantly lower quality standard than the stuff used in the heavy metal types. Yet the manufacturers involved in marketing their avionics to the experimental community all seem to say that their gear is every bit as good as the certified stuff. Doesn't make much sense to me. They can't have it both ways.
The avionics go bananas due to the voltage drop while the starter is hogging all the electrons. :eek:
The heavy iron has a different scenario electrically so they don?t have that issue.
I think the GA avionics is very high quality. If the heavy iron avionics saw the same voltage drop you?d see the same reaction.
I have an electrically dependent engine, so I have dual batteries (one alternator), and during start I use one bat for starting and the other to run the avionics, so no bananas during start for me :cool:
 
I use a DigiKey single throw 2, 3, or 4 pole (or more) switch as an avionics master. Each device is individually fused. Problems with one radio, pull the fuse. Starting, avionics off.
 
The avionics go bananas due to the voltage drop while the starter is hogging all the electrons. :eek:

Well, mine don't really "go bananas"...the devices without a backup battery directly attached to them (i.e., Comm panel, 430W and SL40) simply reboot.

Any avionics worth its salt can handle having power abruptly pulled without any dire consequence.
 
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