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Aux Bus Circuit breaker


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Welcome aboard Daniel...

What you need to know about, is how the design for the wiring circuits works...

There are two relays, the main buss and the avionics buss.

The avionics buss is normally on (closed), until the main buss is turned on.  Main buss on, the other buss relay opens, until the avionics buss is intentionally closed, ( turned on)...

What this does is it supplies power to the avionics buss when the main power switch fails unintentionally.... you intentionally failed a CB and the system adjusted accordingly...

This is a series of normally closed and normally open 

sorry I am missing the proper terms for these things tonight.  But hopefully you understand this brilliant wiring safety theory...

PP advice only. The POH probably explains how/why this works.  

How is that? Clear enough?

Best regards,

-a-

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  • 1 year later...

Bump...

I'm aware of the phenomenon, but is there a reference which explains it for the average pilot other than a study of the full electrical system and knowledge passed down from an instructor? It's weird that what the AUX BUSS breaker does is not explained in the manual.

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I used to win beers with this one, “Bet you I can turn on the radios by pulling a CB.” Even long time Mooney check pilots don’t always know about this. The avionics relay is designed to be fail safe - if it fails or loses power, the avionics bus (not buss, that’s a kiss - look it up) will fail ON. The avionics switch powers the avionics bus relay when it is in the OFF position and de-powers it when ON. Pulling the AUX breaker cuts power to the relay and voila, the radios go on.

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Skip, Ya mean I could have been getting beer for this knowledge? :)

For Mark.... none of my knowledge comes from deep electrical theory, it has got to be in the POH...

There has to be a sentence or two regarding electrical safety and how a failed avionics relay avoids being a single point of failure.

As you go further up the chain of complexity, adding the essential bus is specifically designed to not have these types of added failures, and still provide protection for the devices...

The hardest thing to understand is the relays... which ones are normally open, and which ones are normally closed... when they fail (the normal way) they go to their normal state... when power gets applied, they go to the other state...

For more detail, like the wiring diagrams of a modern Mooney... the MM manual is the place to go ...

1) Master switch on, aka master relay on... flops the avionics relay open...

2) Avionics switch on... flops the avionics relay, again, but to the closed/on position this time...

one relay is normally open, the other, normally closed... the master relay powers the avionics relay to open...

3) fail the power to the avionics relay...

it closes, allowing power to pass to the avionics bus...

Follow Skip’s beer buying theory... find the CB that fails the power to the avionics relay... it stays (normally) closed and keeps the power flowing...

For a better understanding... find the electrical diagram... highLight the switches and relays... set up a logic chart...

Where this theory goes awry... is something mechanically breaks the avionics relay in a non-closed manner... highly unlikely as far as failures go... it will most likely fail right after start-up and become known immediately... all the opening and closing happens early on... a dirty relay can happen...

Sounds terribly complex, but it is quite simple... 

PP thoughts only, not a mechanic...

-a-

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And fun facts like this is why the current art in my hangar is a printout of the entire wiring diagram from the maintenance manual. Doing a cursory check there's no way to figure out from the POH(1994 M20J at least)  that it's actually hooked up this way. They do mention the avionics relay is fail active, but the wiring diagram does not appear to show that relay is on the Aux bus.

 

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Normally closed.   Normally Open   or in diagrams NC or NO.    Normally close is power is off circuit is complete.  

 

image.png.b63b2b0c98976ffceb036b43e7abee8c.png

Pin 85 and 86 are the coil ie a switch.   

Pin 30 and 87a are NC.   Power will flow as long as there is no voltage on 85 and 87

When power is applied to 85 and 87,  Power will flow across pin 30 and pin 87 which are NO

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So the radios are wired to 30 and 87a.     When the radio master is on.  No power is flowing through the radio master switch.  radios work.

When the radio master is turned off power flows.  pin 30 and 87a are open.  The radios don't work

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16 minutes ago, Yetti said:

Normally closed.   Normally Open   or in diagrams NC or NO.    Normally close is power is off circuit is complete.  

 

image.png.b63b2b0c98976ffceb036b43e7abee8c.png

Pin 85 and 86 are the coil ie a switch.   

Pin 30 and 87a are NC.   Power will flow as long as there is no voltage on 85 and 87

When power is applied to 85 and 87,  Power will flow across pin 30 and pin 87 which are NO

Wait, that diagram makes it look like 30--87 is NC and 30-87a is NO.  The coil should pull the contact towards it when powered (e.g. radio master "OFF"), and the contact should move away from the coil when unpowered (radio master "ON").

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