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RV-3: Electrical Bus Structure

Ironflight

VAF Moderator / Line Boy
Mentor
I have had quite a few requests from other builders for the electrical layout of our RV-3 project. While the electrical system has been up and running for months now, it has taken awhile for me to get back to the computer to make updates to my fabrication drawings to reflect the “as built” configuration, and I didn’t want to put it out there until I had been able to show the “final” version (like anything is ever final in an experimental design….). Last night I had a chance to do a few revisions, so here is what I call the “Systems Level” drawing of the electrical bus structure. NOTE THAT IT IS NOT A WIRING DIAGRAM!! It does not show all of the circuit protection, wire sizes, or fabrication notes needed to actually assembly it – this is simplified to show how it works. I also want to point out that I am not an Electrical Engineer – but I sure do work with a lot of them! I describe my design goals below, and readily acknowledge that there are numerous ways to achieve them. Is this the “BEST” way? There’s no way I’d claim that, but it is the product of a lot of discussions and iterations with some smart folks. (A tip of the hat - Much of my philosophy comes from studies of the classics – Bob Knuckolls, Tony Bingelis, and the many GA airplanes I have worked on over the years.)

RV3%252520-%252520N13PL.jpg


So a few notes might help illuminate the design. My overall design philosophy was to build a system that was reliable, redundant, fault tolerant, and easy to operate. Reliability is (partly) inversely proportional to the number of parts that you have – keeping the parts count low, and using quality parts helps. I like redundancy, so that if the real-world quality doesn’t meet the expected standards, you still have the functionality that you need. Fault tolerance comes from looking at the types of faults you might expect (loss of power source, open circuits, and short circuits), and building in alternate ways of assuring that you can get power to the important boxes even if those faults occur. And simplicity 0f operation? My goal is that once the pilot turns it on, he or she shouldn’t have to touch it again until it is time to turn it off - or the actions that need to be taken are so simple and intuitive that it is virtually “set and forget”.

So how does this system work? Basically, it relies on all critical loads being fed from multiple sources (through diodes), and then having multiple paths (busses) to supply the power, and multiple power sources (main battery, Aux battery, Alternator) to supply power to those busses. Let’s look at buses – if any bus shorts to ground, it can be switched off, and power can still be supplied to the important loads from another bus. For critical loads (EFIS, electronic ignition, etc), there is no difference between busses. Non-critical loads are supplied from the main bus only – by definition, you can live without them (lighting, heaters, 12 volt power outlets, etc.). Each bus can be powered from multiple sources – the Main from either the battery or alternator, the Essential from the Main, the Battery, or the Aux battery, and the Avionics bus from the Main or Essential. (Turn the Avionics Master on, and it gets power from whichever is the better supply.)

Note that I make a distinction, with an all-glass cockpit airplane, between the EFIS and the Avionics. The EFIS consist of instrumentation that I simply don’t want to leave the ground without – not only the attitude platform and sir data computer, but engine instruments as well. I consider the Avionics to be the transponder, Comm system – and yes, the IFR GPS box. I consider these “peripheral” to survival so long as the EFIS has its own internal GPS that can be used as a backup (the G3X system has two of them). In this airplane, I can turn off the Avionics bus (if it has a short), and still find my way easily to a runway.

The Aux battery has more than enough power to keep the EFIS running until I can find a runway (assuming that I have killed both the alternator and the Main battery, which would power the EFIS long after I have run out of fuel). The other big benefit of the Aux battery is that it protects the EFIS from the big voltage sag during engine start – which allows booting up the system as soon as you get in the cockpit, expediting departure and minimizing ground run time on the engine.

The beauty of diodes is in my final requirement – “set it and forget it”. Turn on the Master and the Aux Battery before engine start, and unless you smell smoke or get a low voltage warning, leave everything on until you have finished the flight. If you have a low voltage, power off the big loads (lights, Avionics if it looks like the battery isn’t going to be up to the task…) and find a place to land. Although it might sound complex to say it is a “three-bus, three source, redundant system”, the truth is, it just isn’t very complicated. The normal master/starter contactors and their associated switches are the most complicated (looking) things on the drawing, and they are standard.

So now you know – one way to build a fairly robust system without a lot of moving parts.

Paul
 
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With the Aux battery switch always on, will the little aux battery and Ess bus diode tolerate the high current charge rate the aux battery will likely see?
 
With the Aux battery switch always on, will the little aux battery and Ess bus diode tolerate the high current charge rate the aux battery will likely see?

Good question Walt - there is a resitor in the charge line for the Aux battery that limits the current along that path. I've had that same exact circuit in my RV-8 for six years, and it seems to work pretty well.
 
Paul... what diodes did you use?

The big diodes are a Schottky "double pack" that I got from Stein - I'll have to go look up the part number when I have a chance. Large capacity and reasonable voltgae drop.

Paul
 
The reason for the resistor (as I'm sure you know) in the charge circuit is to act as a current limiter for the small back up battery, without some kind of current limiting you may "cook" the little back up battery when it's in a discharged state. The circuit as it stands has the potential to charge the BU battery at the full alternator capability (60A) if you leave the back up battery switch on after start.

Or you could just turn off the BU battery circuit after start (your rational was to leave everything on though I think).
 
The reason for the resistor (as I'm sure you know) in the charge circuit is to act as a current limiter for the small back up battery, without some kind of current limiting you may "cook" the little back up battery when it's in a discharged state. The circuit as it stands has the potential to charge the BU battery at the full alternator capability (60A) if you leave the back up battery switch on after start.

Or you could just turn off the BU battery circuit after start (your rational was to leave everything on though I think).

You are absolutely correct in that the resistor is there to limit the current flow through the diode and into the battery Walt. The battery is one of those little black alarm-system batteries (5 Ah) that are pretty hardy. It only charges if the input voltage is higher than the batttery voltage (up to it's limit), and then the current stops flowing in to it - in a fluid-flow analogy, it is a backwater in the stream. As I noted, I have used this same circuit now in a couple of airplanes for a number of years, and haven't had any problems with it.

Paul
 
Here are some specs I found on a typical high quality 5 ah alarm battery, note the max charge current:

MF VRLA Battery
1. Specifications:
-Model: YX12V5
-Voltage: 12V
-Capacity: 5.0AH/20HR
-L*W*H*(mm): 90*70*105
-Weight (KGS): 1.55
-Enclosure: ABS Plastic
-Maximum Charging Current: 1.50 A
-Charging Voltage Cycle Use: 14.4-15.0V
-Charging Voltage Standby Use: 13.5 -13.8 V
-Resistance (milliohms): 35
-Operating Temperature: -15º Cto 40º C

Also note the low internal resistance of 35 mili ohms, this means it can supply a high current output, and conversly will not present a high resistance to the charging circuit. Translation-without some kind of charge current limiting you have the potential of literally melting the battery with an excessive charge current.
 
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Translation-without some kind of charge current limiting you have the potential of literally melting the battery with an excessive charge current.

Correct - I agree with you - which is why I have the limiting resistor in there. Never had a problem with this circuit Walt, never melted or toasted a battery - what else am I missing?
 
"The beauty of diodes is in my final requirement ? ?set it and forget it?. Turn on the Master and the Aux Battery before engine start, and unless you smell smoke or get a low voltage warning, leave everything on until you have finished the flight. "


I'm basing this on the above statement that you plan on flipping all the switches on and leave them there, in which case the current limiting resitor/diode is out of the picture. The charge will be thru the ESS diode and aux battery switch.
 
"The beauty of diodes is in my final requirement ? ?set it and forget it?. Turn on the Master and the Aux Battery before engine start, and unless you smell smoke or get a low voltage warning, leave everything on until you have finished the flight. "


I'm basing this on the above statement that you plan on flipping all the switches on and leave them there, in which case the current limiting resitor/diode is out of the picture. The charge will be thru the ESS diode and aux battery switch.

OK - understand now! You're talking about it feeding directly from the bus.

The charge current that the Aux battery sees is proportional to the difference in voltage in the battery, and what it is seeing from the supply. The larger the voltage drop, the bigger the current. As long as the Aux battery hasn't been discharged completely, the voltage difference is pretty small, and the current flow is subsequently small. There is no real difference between the Aux and Main batteries in this regard - they both see full Alternator voltage when they are on line. Yes, if either battery is deeply discharged, you are going to have a greater in-rush current, in any type/size of battery.

Again, I've been using this circuit - in this way - for years, and a lot of hours without any issues, hot batteries, etc., and the design came from a couple of EE design guys I trust. I'm not selling it, simply letting folks see what I am using.
 
OK - understand now! You're talking about it feeding directly from the bus.

The charge current that the Aux battery sees is proportional to the difference in voltage in the battery, and what it is seeing from the supply. The larger the voltage drop, the bigger the current. As long as the Aux battery hasn't been discharged completely, the voltage difference is pretty small, and the current flow is subsequently small. There is no real difference between the Aux and Main batteries in this regard - they both see full Alternator voltage when they are on line. Yes, if either battery is deeply discharged, you are going to have a greater in-rush current, in any type/size of battery.

Again, I've been using this circuit - in this way - for years, and a lot of hours without any issues, hot batteries, etc., and the design came from a couple of EE design guys I trust. I'm not selling it, simply letting folks see what I am using.

It only takes a small voltage difference to create a very high charge current because the internal battery resistance is very low. Just a 1 volt differnence into 35 miliohms will result in 28 amp charge :eek:

That's why it's not unusal to see full alternator current (40-60A) charging a battery after start. For those of you that have a current meter hooked up to monitor battery charging current you know what I'm talking about.

I'm not saying it hasn't been working for you and if your aux battery is always fully charged it probably won't present much of an issue as the battery will top-off quickly before it overheats. But that doesn't change the fact that the potential for a battery melt down/ESS diode overheat is still present with no current limiting to that little battery.

If you really do leave the aux battery switch on all the time then you might as well ditch the current limiting resistor/diode/5A breaker as they serve no purpose.
 
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I'm not saying it hasn't been working for you and if your aux battery is always fully charged it probably won't present much of an issue as the battery will top-off quickly before it overheats. But that doesn't change the fact that the potential for a battery melt down/ESS diode overheat is still present with no current limiting to that little battery.
.

Please don't get me wrong Walt - I appreciate the comments, and this will lead me to do a little extra test - I can drain that battery down a bit and hit it with the load pretty easily with the test equipment. Far better than finding out in the field.

Of course, thinking about it, there might be a very simple fix - the addition of another diode between the Aux battery feed and the ESS bus should prevent a big back current. As a matter of fact, I have two of those twin-pack diodes in the power box right now, for a total of four diodes - but am only using three at this time. Pretty easy to jumper in the fourth to do that job.

Paul
 
Of course, thinking about it, there might be a very simple fix - the addition of another diode between the Aux battery feed and the ESS bus should prevent a big back current. As a matter of fact, I have two of those twin-pack diodes in the power box right now, for a total of four diodes - but am only using three at this time. Pretty easy to jumper in the fourth to do that job.

Paul

Perfect solution Paul, that will keep the original aux battery charging circuit performing it's intended function.
 
Similar but different

Thought I'd share my design here too. Conceptually not a million miles from Paul's architecture but combines Paul's AV & ESS busses in my single AV Buss. My objectives were also similar to Paul's - pretty much set and forget. I've elected not to have an Alternate Feed for my Aux Battery. I believe I have addressed the charging question that Walt raised. My Aux Battery is a 9Ah device. I'm using similar dual packaged diodes to those described by Paul plus a couple of the Perihelion Design diodes. A/C mission is VFR (no night or IMC flight allowed in UK 'permit to fly' A/C). I like the idea of being able to power up my AV buss from the Aux Battery alone and have the AV buss totally disconnected from the Main buss, e.g. during engine start. Just have to switch my AV Master Switch ON after engine start.

Like Paul, I'm not looking to sell folks on my design and I have yet to build it so there is still time for me to make adjustments.

I would appreciate any constructive comment. I'm no eletrical engineer either. This is the first A/C electrical system I've sought to design. Have adapted ideas from Aeroelectric Connection.

Steve

95pp9g.jpg
 
Thought I'd share my design here too. Conceptually not a million miles from Paul's architecture but combines Paul's AV & ESS busses in my single AV Buss. My objectives were also similar to Paul's - pretty much set and forget. I've elected not to have an Alternate Feed for my Aux Battery. I believe I have addressed the charging question that Walt raised. My Aux Battery is a 9Ah device. I'm using similar dual packaged diodes to those described by Paul plus a couple of the Perihelion Design diodes. A/C mission is VFR (no night or IMC flight allowed in UK 'permit to fly' A/C). I like the idea of being able to power up my AV buss from the Aux Battery alone and have the AV buss totally disconnected from the Main buss, e.g. during engine start. Just have to switch my AV Master Switch ON after engine start.

Like Paul, I'm not looking to sell folks on my design and I have yet to build it so there is still time for me to make adjustments.

I would appreciate any constructive comment. I'm no eletrical engineer either. This is the first A/C electrical system I've sought to design. Have adapted ideas from Aeroelectric Connection.

Steve

Looks good to me, the only thing I might suggest is using a switch in place of the aux battery relay like Paul did, there is not really enough load on this circuit to require a relay for switching and the relay/contactor uses juice from the aux battery.
 
Walt,
Thanks very much for spending the time to give this a once over. It is much appreciated.

Regarding the Aux Battery Relay, I did think about this quite a lot, and could still switch to a switch :) Although the diagram presents the Aux Bat Relay indentically to the Master Relay, the Aux Bat relay is infact a lower current draw 40A automotive-style relay.

Thanks again,
Steve
 
Walt,
Thanks very much for spending the time to give this a once over. It is much appreciated.

Regarding the Aux Battery Relay, I did think about this quite a lot, and could still switch to a switch :) Although the diagram presents the Aux Bat Relay indentically to the Master Relay, the Aux Bat relay is infact a lower current draw 40A automotive-style relay.

Thanks again,
Steve

I just think switches are more reliable than relays, so if I can use a switch I will. I also just noticed the 60 amp ANL, if you're using a 60A alternator I would suggest stepping that up to an 80 ANL to prevent nuisance blowing of the ANL.
 
Steve - I am with Walt (obviously) on using a switch alone when you can instead of a switch and a relay. I've just seen enough relay failures, and fewer switch failures. You need to make sure that your switch is adequate for the job, and if it is - get rid of one failure point!

Paul
 
Nice work Paul. I like your "set it and forget it" ideology. That's pretty much the way Boeing did the 777 (except we've got like 16 levels of redundancy). And like the Triple, your critical buss gets fed automatically if either of the feeds fail. Boeing calls those things "transfer busses".

Well done!
 
Steve - I am with Walt (obviously) on using a switch alone when you can instead of a switch and a relay. I've just seen enough relay failures, and fewer switch failures. You need to make sure that your switch is adequate for the job, and if it is - get rid of one failure point!

Paul

I've had several switch failures in my RV. Switches that carry incandescent lighting loads (landing, taxi) or strobe power supplies are highly stressed, and I will be using relays for these types of loads in my Rocket. When/if the relays fail, they are easy to replace... even in flight.

This may be the fault of the switches (search the aeroelectric list for my failure analysis).

The only conclusion... design for maintenance. If you have to fix something (and you will), make it accessible. Everyting breaks eventually. I don't even know if you can crawl into an RV-3!

V
 
I just think switches are more reliable than relays, so if I can use a switch I will.

Steve - I am with Walt (obviously) on using a switch alone when you can instead of a switch and a relay. I've just seen enough relay failures, and fewer switch failures. You need to make sure that your switch is adequate for the job, and if it is - get rid of one failure point!

Paul

Paul/Walt, Thanks. I'll review that decision of mine once more. The switch is already man enough to take the load. I'm using Eaton toggle switches.

I also just noticed the 60 amp ANL, if you're using a 60A alternator I would suggest stepping that up to an 80 ANL to prevent nuisance blowing of the ANL.

Walt,
I'm not so sure that 60A ANLs with a 60A alternator are such an issue. My understanding is that the ANL current limiters are very slow blow that can take a good deal more than the rated current indefinitely. According to the Bussman ANL spec sheet even a 50A rated ANL will take 100 seconds to blow when exposed to about 110A. See the chart below. If anyone understands the logic behind the rating scale for ANLs I'd be keen to learn.

I'd also be interested in hearing from folks that have had nuisance trips on 60A ANL/60A Alternator combos and solved it by uprating to an 80A ANL.


10x4s9v.jpg


The above is cut from http://www.cooperindustries.com/con...Resources/Data Sheets/Bus_Ele_DS_2024_ANL.pdf

Thanks again,
Steve
 
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Paul/Walt, Thanks. I'll review that decision of mine once more. The switch is already man enough to take the load. I'm using Eaton toggle switched.



Walt,
I'm not so sure that 60A ANLs with a 60A alternator are such an issue. My understanding is that the ANL current limiters are very slow blow that can take a good deal more than the rated current idefinitely. According to the Bussman ANL spec sheet even a 50A rated ANL will take 100 seconds to blow when exposed to about 110A. See the chart below. If anyone understands the logic behind the rating scale for ANLs I'd be keen to learn.

I'd also be interested in hearing from folks that have had nuisance trips on 60A ANL/60A Alternator combos and solved it by uprating to an 80A ANL.


Thanks again,
Steve

Steve,
After a review of the ANL chart above I will agree with you, stay with the 60A. My hanger mate blew an ANL after installing a plane power alternator, but I'm not sure what size it was but I'll find out (I was thinking it was a 60 but I'll double check).
 
use a switch alone when you can instead of a switch and a relay. I've just seen enough relay failures, and fewer switch failures.

Paul, I am trying to address my new electrical design and agree it would bve nice (and easier) to use a switch rather than a switch+relay.

I want to have my 60A feed from the alternator/battery spit to essential and non-essential buses.

What source and what switch have you found for 20A or 40A circuits ?

Also, you use "twin-pack diodes" - could you share the MFG and part # ? I have not been able to figure out how to size these diodes.
 
Paul, I am trying to address my new electrical design and agree it would bve nice (and easier) to use a switch rather than a switch+relay.

I want to have my 60A feed from the alternator/battery spit to essential and non-essential buses.

What source and what switch have you found for 20A or 40A circuits ?

Also, you use "twin-pack diodes" - could you share the MFG and part # ? I have not been able to figure out how to size these diodes.

I'm traveling right now Glen, so don't have the part numbers for the diodes - but I know that Christer at Steinair does - give him a call!

As for large-capacity switches, well - if you're trying to switch anything 20 Amps or larger a relay is probably more appropriate. I prefer not to use relays any more than I have to, as I said, but for large loads, you are kind of stuck. I really prefer not to have relays in my critical path to get power to essential loads - the fewer active components between your source and critical loads, the better.

Paul
 
Thanks Paul.

I agree to the desire for switches over relays. What was confusing me in the RV-3B diagrams and photos was I did not see any relays for the buses and I did not see any large breakers. What I did see was one or two primary switches and lots and lots of individual component breakers.

I must have missed the point of protection for your essential bus. That is what has been confusing me on most of the threads.

Once I have the fundamentals down, I will write it all up with diagrams and share it back on VAF.
 
I must have missed the point of protection for your essential bus. That is what has been confusing me on most of the threads.
.

True - that was a system's level architecture drawing - not a wiring diagram that showed all components.
 
So, for the actual implementation, did you find a hi current switch or did you use a relay ?

I've found a suitable relay but not a suitable switch at the bus current loads.
 
Given the expertise here, here is our proposed more "minimal" RV-3 Elec layout:
rv3_elec.jpg

No Master (Electrical) Relay, may include a mechanical battery isolation sw. Start Master sw is to remove a single point failure leading to activation of the Starter. Comments welcome ;)
 
No Master (Electrical) Relay, may include a mechanical battery isolation sw.

Andy, have you found a switch rated for the total bus current load? If so, I'd be interested. This has been a challenge for me. I have not found a source for a switch capable of 12V+ and 60A (my alternator rating) - not even a switch at 30A (my auxiliary load).
 
Hi Glen...
...have you found a switch rated for the total bus current load?
Doubt an issue:

1. Alternator pushes out 8A max at 2700 RPM
2. Only load of note is the Fuel Pump ~7A (?)

So a 15A switch (CB) should be fine I think...
 
30 Amps is an awful lot of Aux load given what you really need to fly Glen. I used a heavy-duty 20 Amp switch (protected by a fuse inside my distibution panel to answer a previous question), and realistically, it just doesn't see that. Of course, we're talking modern EFIS and radio gear - plus not that much is "essential" on these airplanes. I rarely see more than 10 Amps in steady state flight with lights on.

Paul
 
Thanks Paul. I agree with the logic. What I was looking at - in both my design and your actual breaker panel, was the sum of all possible loads. I guess it's a bit like having a 200 Amp service into a house. If I were to add up all the breakers in the box, they total a lot more than 200amps.

Given that, I can go the simple[r] route of putting a safety fuse or breaker upstream of my switches.

BTW: I've also sourced my diodes so I can add my plan of a second battery to my design.

With everyone's 2¢, I've collected some real valuable change ;)
 
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