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Avionics Master Relay

Rallylancer122

Well Known Member
Hi All,

I'd like to install a relay to activate my avionics buss. I have about 40 amps on the buss, and that's too much to run through the rocker switch I have. (I'm rehabbing my Dad's -8, and it already has nice engrave rocker switches, otherwise I'd just replace the switch.)

Can anyone recommend a suitable, aviation grade relay?

Thanks,
DEM
 
Check with B and C.

IIRC, that is where I got mine.

40A seems pretty high.............
 
You could just use an additional standard relay such as most of us have as our master. I wonder what you have on that bus: 40a seems like a bunch for even older avionics!
 
Check your load calcs, 40 amps seems way high, like by a factor of 4 or more. If so you can simplify and add lightness with a simple quality switch and dump the relay.
Tim Andres
 
Entire panel is only 12.5 amps

AFS5600T, Dynon D10A, Dynon 120, GTN 650, GNS 430, TT 385, TXP 327, PM800BT, iFly 740, NavworxEXP..

Using heavy duty toggle switch(s).

Ron
 
AFS5600T, Dynon D10A, Dynon 120, GTN 650, GNS 430, TT 385, TXP 327, PM800BT, iFly 740, NavworxEXP..

Using heavy duty toggle switch(s).

Ron

You have about 20 amps max here steady state while in flight and everything running on this bus. Stein's avionics bus will work, as will just using another master relay from Van's, Spruce, or B&C. Don't forget to put the diodes on the relay to protect the equipment.

Vic
 
There is a plethora of options

There are many relays that fit the bill. I use four of these:https://www.alliedelec.com/schneider-electric-magnecraft-92s7d22d-12/70185034/

I look for silver contacts and coil rating. I run #10 wires from each battery to each relay, the relay?s contacts wired in parallel. There is a primary Avionics #1 relay on the left battery as well as a primary Avionics #2 relay on right battery. There is also an ?Alternate? feed relay on each battery (Avionics #1 on the right battery and Aviionics #2 on the left battery). These relays operate independent of each battery?s master solenoid.

I strongly recommend you not use a single relay to feed your avionics. This is, but definition, designing in a single failure point that will leave you with a dark panel. See if you can split your panel into Nav #1, Comm #1, Nav #2, Comm #2 and so forth to divide the loads between Avionics #1 and Avionics #2.

Carl
 
Relays

In the cert world many planes use a normally closed relay for the Avionics buss, (spring loaded to the closed position). A failure of the avionics master switch or its circuit results in this relay closing or staying shut keeping the avionics buss powered.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Although I can see the advantage of a normally closed relay I worry about a situation where you would Not be able to turn off that equipment.
I much prefer the simplicity of splitting the avionic loads between two robust simple switches as suggested by Carl.
 
In the cert world many planes use a normally closed relay for the Avionics buss, (spring loaded to the closed position). A failure of the avionics master switch or its circuit results in this relay closing or staying shut keeping the avionics buss powered.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

This would be hard to do unless the avionics relay is connected to the output side of the master solenoid, not the battery. So - this just shifts the single failure point from the avionics relay to the master solenoid (or more likely the associated connections).

This would also addresses just the one failure mode of the avionics relay, the coil fails. Other common failures are the contacts as well as the terminals.

Carl
 
I'm with Mike on this one... 40A seems ridiculously high for a GA airplane, even a fully IFR airplane. You might want to reassess your loads to see what they really are. Remember, intermittent loads like VHF COMM transmit current aren't typically used for rating things like an avionics master contactor. For that purpose one would use the steady-state or turn-on current of the avionics attached to the bus.

Also keep in mind that you are better off if you configure the avionics master contactor in a fail-safe configuration. That is to say, wire it so the power goes through the contactor when the contactor is in the relaxed, unpowered position.

Typically a 40amp automotive relay is plenty stout enough for the purposes of an avionics master, but you can up that pretty easily to a 50amp relay. Again, if you're drawing 40A continuous than you might want to think about replacing some of those vacuum-tube radios!
 
You have about 20 amps max here steady state while in flight and everything running on this bus.
IMHO if you have 20A max STEADY STATE, then you should have about 25% additional capability for surge. That means you should have a "switch" rated for 25A. If you can't find an appropriate size switch for your particular installation, you can use a smaller switch controlling a larger capacity relay.

FWIW: In my case I have a max of 25A steady state on my avionics buss but I wanted to use a specific LED lit switch rated for only 15A. So I put two of these switches each controlling a separate RayExElec 40A car relay (LD1A-12F) in parallel. This way either switch/relay combination can easily handle the max surge load. It also provides 100% backup and a simple method of pre-flight testing of each switch/relay combination. YMMV

282b.jpg


Avionics%20Switch%202.jpg

:cool:
 
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with my panel all lit up on the ground, I rarely see more than 12 AMPs draw. Turn everything on and watch your ammeter to get a better feel for what you need.

do not archive.
 
Sorry, it's not 40 amps continuous, that's max. (Actually I think it adds up to 37.5 or something like that, and I was just rounding up.) I would expect the steady state to be in the 20-25 amp range. That's still a little more than I'm comfortable pumping through the mystery switches my Dad installed.

And yeah, I could just change the switch, and have a nice row of matching engraved toggle switches with one oddball in the middle. But it turns out I'm kind of vain when it comes to my planes. Oh well.... ;)

Thanks all for the advice!

DEM
 
I'm using an ice cube relay for my avionics bus. It's a high-quality P&B/TE VF4 which is used in the automotive world for airbag and ABS actuation, among other things. One nice thing about using a relay is it can be located near where the majority of the avionics loads are, and just have a small-gauge wire going to the av switch which grounds the relay coil.

I also have a 30A switch-breaker on the panel with a switch guard to ensure the avionics work IF the relay goes TU, wired directly to the battery so that I can get avionics power even if the master switch is off. There is also an inline fuse near the battery to protect the 12ga wire upstream of the switch-breaker.

Heinrich
 
Or....you could ditch the avionics master entirely :)

See Nuckolls' Aeroelectric write-ups.

No Avionics Master here...
 
Those of you that are using the automotive relays, do you add noise isolation diodes or do these relays have the internal diode like the one that Stein sells?
 
Those of you that are using the automotive relays, do you add noise isolation diodes or do these relays have the internal diode like the one that Stein sells?
FWIW I fly a fiberglass (non RV) airplane, don't have the noise isolation diode and have had no issue whatsoever with any kind of noise. I have 4ea LD1A-12F relay (about $2.50/ea) for controlling different things in my airplane. YMMV

:cool:
 
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The primary purpose of diodes across inductive coils is to protect switches from arcing when they are shut off. Arcing will shorten the life of switches. If diodes happen to reduce the clicking noise when a switch is shut off, that is a minor side benefit.
 
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