Power Distribution and Buses

[Spaceman]

I am continuing to fumble my way through designing my electrical system, but I'm curious how you guys are designing your buses and handling power distribution.

I'm only barely not totally electronically illiterate so I've been figuring this out pretty gradually, but after reading the AeroElectric Connection book and listening to some podcasts and youtube videos, I think I have a decent start.

I have also significantly plagiarized the following examples so far:
Bob Nuckolls figure Z-11 (pretty much this: https://bandc.com/wp-content/uploads/20 ... le_alt.pdf)
Noel Wade's Diagram: http://sonexbuilders.net/viewtopic.php? ... ric#p26716
Frank Alvarez's Diagram: http://sonexbuilders.net/viewtopic.php?f=6&t=1564

Background:
-I'm using an Aerovee Turbo, so 20A alternator, with coils & mags for ignition
-I'm going to build a Garmin G3X Touch system. So one display, engine interface box, ADAHRS box, magnetometer box, radio box, transponder box, and a GPS position source box is pretty much the avionics system.

Here's a list of all the electrical components I'm planning and their associated loads:

Master Contactor - 1A
3ohm Coils x2 - 9.4A (or maybe 5ohm coils - 5.6A)
GDU 450 display - 1.25A
GEA 24 engine interface - 0.43A
Sonex fuel probe transducer - 0.1A
GSU 25C ADAHRS - 0.2A
GMU 11 magnetometer - 0.1A
GTR 20 radio - 0.6A (3A when transmitting)
GTX 45R transponder - 0.72A (1.3A max)
GPS 20A position source - 0.2A
ACK E-04 ELT - 0.16A (I think)
Aveo strobe lights - 2A peak
Aveo nav/pos lights - 0.51A
Duckworks landing lights x2 - 1.3A total
GSA 28 A/P servos x2 - 0.72A total (max 3.6A total)
iPad dock charger - 0.75A
12v Power outlet - ?
Interior LED lights - ?
Seat heat - ?
Sonex flap motor - ?
Turbo Cooler Fan & Water Pump - ?

Anyway here are the questions I'm pondering right now:

1. I want to protect my avionics during engine start but still be able to monitor the engine while I start it. That means I will have to have at least the display and engine interface box powered up during the start. If I understand correctly, the Bob Nuckolls alternator control relay setup will allow me to keep the alternator offline until I switch it on, so there'd be no danger of alternator induced voltage spikes during startup. So the only remaining hazards would be voltage dips. Garmin recommends using a TCW power stabilizer to catch any dips during engine start (https://www.tcwtech.com/IPS-12v.htm).

The cheapest model supplies up to 4 amps, so if I went with one of those I could power the display (1.25A), engine interface (0.43A), GPS (0.2A), and radio (0.6A) for a total of only about 2.5 amps during start, and that should do it. Does that all sound right or is there something else I'm missing?

OK next:

2. Not really a specific question, just looking for some thoughts on buses. Seems like the basic and typical way to do it is to have a "main" bus powered whenever the battery contactor is on, then an "avionics" bus for the instruments. Then there are many Bob Nuckolls examples that forego the avionics bus but add an "endurance bus" for essential things in case of alternator failure. Would it not make more sense to just have the essential things on the primary bus, and all the less-than-essential stuff on a downstream bus? That way if you need to shed some loads you just switch that bus off and you're down to your essentials. Something like the attached picture is what I'm thinking.

Does that make any sense? Am I missing anything obvious, or any ways to make it more simple?

[Spaceman]

As soon as I hit send on that last version I had more ideas.

The lights and flaps draw no power unless they're on/moving, so might as well just put them on the main bus since they can be easily switched off. Also no reason to power the utility bus off the avionics bus, so I branched it off from the main bus instead.

Now the utility bus is just things that don't have a built in off switch, except the seat heat, but I think it can stay there. Maybe I should call the utility bus the "convenience bus."

Still lots of details to fill in of course, this is just a rough visualization of power flow. Critiques welcome!!

[Rynoth]

It seems to me that you're on the right track, and I don't see any glaring issues with your plan so far. I also followed the Bob Nuckolls book as best I could.

Just to add to your library of electrical systems, here's mine: https://drive.google.com/file/d/0B4ApjY ... ZuYkU/view
It's about 95% accurate to what's in the plane, though it's missing my turbo cooling solution that I added later on.

I wired my turbo cooling a bit differently, allowing me to have the "auto" mode closed only when my master switch was "off", while adding a second circuit to it that allows me to run it manually and independently of the thermo-switch. This helped for testing the system and allows me to run it whenever I want, otherwise it runs off the thermo-switch anytime my master is off and I never have to flip a turbo-specific switch. PM me if you want more details on that.

Also, just to throw another term into the mix for your utility bus, the Swiss would call it a "non-essential" bus. =D

[Spaceman]

Thanks! I printed your diagrams out and added them to the mess on my desk. So far I don't see anything drastically different between yours and what I'm shooting for.

For anyone else's reference, I found Jeff Schultz has another nice diagram here: http://www.sonex604.com/misc/Sonex1374E ... 5Sep13.pdf

I've made a few more adjustments after working through things in a little more detail.

First, I moved the avionics and utility buses to be in parallel with the main bus... no reason to put them in series really.

Next, I switched around some stuff with the power stabilizer. My initial thought was I wanted the radio available prior to engine start for ATIS, clearance, etc, but it doesn't need to be on WHILE the engine is starting. Plus I think the power required to transmit would bust the 4A limit of the stabilizer anyway.

So, what I came up with was to put the radio back on the avionics bus and move everything else to the stabilizer. Now I have my display, engine interface, and the whole navigation system able to run uninterrupted during start (still only 2.3A total).

The avionics bus now just contains the radio, transponder, and ELT. So if I want to I can switch on that bus prior to start, get ATIS/clearance, let the ADSB download data or whatever, then turn it back off while I start the engine. In fact I think I will move the ELT to the stabilizer too, since it interfaces directly with the GPS box and not the transponder. So far I think this makes more sense.

[gammaxy]

I don't think you need to worry about voltage spikes from the alternator on startup. Our permanent magnet alternators (or generators) don't generate enough voltage to even charge the battery until quite a bit over idle (1500+rpm?), so it definitely shouldn't be a concern on startup. If you read a lot of Bob Nuckolls's material, you will see that he believes most voltage spike concerns in small airplanes to be founded on myths. Most voltage spikes are caused when switching off a large inductive load and will appear across the switch, so it's a good idea to have diodes or resistors installed across the coils to prolong switch and relay life.

The voltage sag when engaging the starter is real. I wouldn't consider it a hazard as much as an annoyance if it causes your EFIS to reset. In my experience, I can usually start the airplane without my EFIS rebooting, but when it's cold outside with an old battery, it will shut down. My EFIS allows the connection of an auxiliary battery for this purpose, but I took the simple approach and it hasn't bothered me yet. Even when it does reboot, it comes back pretty fast.

[racaldwell]

One way to prevent voltage sag when cranking the engine is separate the engine monitor from that electrical circuit.

That is what I am doing for my motorglider. I have a separate battery (K2 soaring battery) that feeds the avionics bus that is isolated from the main bus with a switch. The main bus has the starter battery (ETX680.) After starting, flip the switch to connect the K2 battery to the main bus so it can get charged when the engine is running.

In your case, you can use the 2nd battery as a back up. The Lifepo batteries means they are not a weight detriment anymore.

I have a schematic if anyone is interested.

Rick Caldwell
Xenos 0057

[bvolcko38]

I would like to see your schematic.

[N190YX]

When you operate the starter, you have crazy amperage spikes resulting in voltage spikes up and down as the starter is actuated, and then lugs down a little on each compression stroke as it slows down with the load on the compression stroke, using more amperage, and eases up while the engine is not on a compression stroke and turns more easily. I damaged the gas discharge numbers on a KX155 by accidentally starting my engine with the avionics turned on after the engine stalled on a landing roll out. Should have turned the avionics off before using the starter. I recommend turning everything off while using the starter! Those avionics are expensive, to purchase, and to repair!

[lakespookie]

I also don't see any of the power draw for the engine sensors does the GEA24 module power account for the senders? My power budget is a little worse than yours not that I need to worry about that for at least a few more years but my average power draw is currently at 18.52 Amps and 28.51 Amps peak(Although I doubt I would ever be transmitting from two radios at the same time so this is actually lower conservatively closer to 25 amps), but I have still not accounted for all the engine sensors. Honestly its a little concerning given how little head room that leaves for charging the battery. Note my power budget is worst case at night when running all the lights and with the exception of the caveat above in general is significantly more conservative during daytime operations. as you can cut 4.5 amps from the average and total power and even at night I would only have this power draw when approaching to land. the rest of the time that's 2 amps removed from the overall loading so I think its still doable. I also currently have a GPS20 A included in the power budget and that will most likely be removable since I want to throw a GNX375 into the system and I belive I would not need the sepreate box for that since the navigator will pump what I need into the system. (My reason for this an not going the GPS 175 route after the fact is mostly price dependent but I budgeted with the swap in mind in case I want to lower overall package costs. but the difference between the GPS175 and the GNX 375 is slightly less than 3k and I like the idea of having an extra entry field for redundancy in case the G3x goes down with the G5 and the GNX I still have transponder control and enough flight data to get around even in the worst conditions).

Other things to note my planned electrical system includes 3 servos, I want to do long leg cross countries so I want to throw in a yaw damper if possible although I suspect that either that wont be possible or I wont find an elegant mounting point for the YD servo so that would improve power budget. My Avionics plan also includes a G5 with a backup battery and the interface unit. YMMV. I probably should worry about building the damn thing before the avionics but I am an engineer by trade and have had about 2 months to think about random things since I ordered my tail kit.


I also would add that most of the Garmin avionics support dual bus power so you could potentially have a completely independent power bus for the avionics. it would take a little more wiring and an extra switch but it would allow you to have an independent power source as a backup, and allow for starting with your avionics switch off while still powering the Garmin panel for engine monitoring. and a fall back bus in case of an electrical system failure. At least that was my plan even though I have the g5 with the battery backup for when I'm in flight I have been bouncing the engine monitoring stuff around in my head as well on startup and that's probably the best solution heck you can probably wire it to cut bus two when the main avionics is on if you wanted true bus isolation but the internal circuits in the Garmin units should draw from the better power source which I suspect in most cases would be the main power bus unless you were really low on your battery.

[Spaceman]

One way to prevent voltage sag when cranking the engine is separate the engine monitor from that electrical circuit.

That is what I am doing for my motorglider. I have a separate battery (K2 soaring battery) that feeds the avionics bus that is isolated from the main bus with a switch. The main bus has the starter battery (ETX680.) After starting, flip the switch to connect the K2 battery to the main bus so it can get charged when the engine is running.

In your case, you can use the 2nd battery as a back up. The Lifepo batteries means they are not a weight detriment anymore.

I have a schematic if anyone is interested.

Rick Caldwell
Xenos 0057

I keep going back and forth about doing that. The power stabilizer seems very simple to plan and install and it costs $250. Perhaps I could do a backup battery system for less than that, I just don't know yet how to wire that off the top of my head. I wouldn't mind seeing your schematic if you have a picture of it!

I would like to see your schematic.

I think I'll post it in another thread eventually. I worked on it a lot today so it's looking sort of decent although it still needs work. Figuring out the big picture bus setup has allowed me to make some progress on it finally.

When you operate the starter, you have crazy amperage spikes resulting in voltage spikes up and down as the starter is actuated, and then lugs down a little on each compression stroke as it slows down with the load on the compression stroke, using more amperage, and eases up while the engine is not on a compression stroke and turns more easily. I damaged the gas discharge numbers on a KX155 by accidentally starting my engine with the avionics turned on after the engine stalled on a landing roll out. Should have turned the avionics off before using the starter. I recommend turning everything off while using the starter! Those avionics are expensive, to purchase, and to repair!

Makes sense. Definitely seems like you need to address it one way or another. I don't think I want to accept starting the engine with no engine data as a solution, so I have to have at least some expensive stuff powered in some way!

I also don't see any of the power draw for the engine sensors does the GEA24 module power account for the senders?

The number I used for the GEA 24 is from page 2-16 of the G3X Touch manual; it says "typical external sensor loading." That's probably close enough for now. Seems like most of the G3X users are doing way more complicated airplanes and engines than me so I really doubt I'll end up with more than a "typical" amount of stuff hooked up to mine!

My power budget is a little worse than yours not that I need to worry about that for at least a few more years but my average power draw is currently at 18.52 Amps and 28.51 Amps peak(Although I doubt I would ever be transmitting from two radios at the same time so this is actually lower conservatively closer to 25 amps), but I have still not accounted for all the engine sensors. Honestly its a little concerning given how little head room that leaves for charging the battery. Note my power budget is worst case at night when running all the lights and with the exception of the caveat above in general is significantly more conservative during daytime operations. as you can cut 4.5 amps from the average and total power and even at night I would only have this power draw when approaching to land. the rest of the time that's 2 amps removed from the overall loading so I think its still doable

I think my typical inflight load will be around 15-17 amps, but could definitely exceed 20 amps with seat heat, landing lights, radio transmitting, and autopilot on. Landing lights will only be for in the pattern and I don't think I've ever made a radio call longer than like 3 seconds while VFR though so I think it'll be doable. I agree though 20 amps from the alternator barely seems like enough!

I also currently have a GPS20 A included in the power budget and that will most likely be removable since I want to throw a GNX375 into the system and I belive I would not need the sepreate box for that since the navigator will pump what I need into the system. (My reason for this an not going the GPS 175 route after the fact is mostly price dependent but I budgeted with the swap in mind in case I want to lower overall package costs. but the difference between the GPS175 and the GNX 375 is slightly less than 3k and I like the idea of having an extra entry field for redundancy in case the G3x goes down with the G5 and the GNX I still have transponder control and enough flight data to get around even in the worst conditions).

I'm with you there, it kind of stings spending the money on the GPS 20A when I could get the WAAS position source plus an IFR nav system with one of their navigators... But that would add a couple grand to my avionics budget which I already think is too high as it is. Maybe that'll be a future upgrade.

I'm going to do a single GDU 450 display plus a powered iPad Mini dock. So my backup navigation will be the iPad with foreflight. I figure even with total electric failure I'd be left with a fully charged iPad which would last me longer than a full tank of gas, so that seems like more than enough. I might throw a Stratus behind the seat and call that my backup nav system.

Other things to note my planned electrical system includes 3 servos, I want to do long leg cross countries so I want to throw in a yaw damper if possible although I suspect that either that wont be possible or I wont find an elegant mounting point for the YD servo so that would improve power budget. My Avionics plan also includes a G5 with a backup battery and the interface unit. YMMV. I probably should worry about building the damn thing before the avionics but I am an engineer by trade and have had about 2 months to think about random things since I ordered my tail kit.

You could probably put a yaw servo on the floor way in the back of the tail and attach it with a pushrod to the control mixer. Or maybe use longer rudder cables, and bring them together in the center to be driven by a capstan if you really wanted to. I've never seen a need for a yaw damper on a straight wing single engine plane though so I ditched that idea pretty early!

I also would add that most of the Garmin avionics support dual bus power so you could potentially have a completely independent power bus for the avionics. it would take a little more wiring and an extra switch but it would allow you to have an independent power source as a backup, and allow for starting with your avionics switch off while still powering the Garmin panel for engine monitoring. and a fall back bus in case of an electrical system failure. At least that was my plan even though I have the g5 with the battery backup for when I'm in flight I have been bouncing the engine monitoring stuff around in my head as well on startup and that's probably the best solution heck you can probably wire it to cut bus two when the main avionics is on if you wanted true bus isolation but the internal circuits in the Garmin units should draw from the better power source which I suspect in most cases would be the main power bus unless you were really low on your battery.

I'm sure you're right I'm just not electrically fluent enough yet to sketch that out! So far I've been looking for the simplest possible way!