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The E-bus alt feed: a different idea

N546RV

Well Known Member
So I've been thinking about the Nuckolls E-bus setup, specifically the handling of having an alternate feed off an always-hot battery bus in case of a failure affecting the main bus. Bob's setup takes the same basic form everywhere I've seen it: the main bus feeds the E-bus through a diode, and the alternate feed is provided through a switch and/or relay (depending on expected E-bus loads).

The reasoning behind the diode between the main and E-bus is to prevent the E-bus from feeding the main bus and possibly popping a fuse/breaker. Bob's assertion seems to be based on an alternative scenario where two switches would be provided, and it would be incumbent on the pilot to ensure the main->E-bus feed switch was opened before closing the E-bus alt-feed switch, to prevent the aforementioned backfeeding.

However, it seems this issue can be fairly easily solved with careful switch choices, such that it's not possible to have both feed paths closed simultaneously. For example, if a five-pin relay is used for feed switching, the main bus could feed through the normally-closed contact, and the batt bus through the normally-open one. The choice of how to feed the bus is still controlled by a single switch throw, the only difference being that that switch position definitively disconnects one feed while connecting another one.

Example diagram:

JQyTpEpl.png


This seems elegantly simple enough that I'm immediately suspicious that I'm missing some downsides. Some thoughts that come to mind in this vein:

  • There's the possibility of the relay somehow failing such that both contacts close, thus possibly blowing the batt bus fuse and killing the E-bus in an emergency. Not sure what the likelihood of this is.
  • If the relay somehow fails with both contacts open, the E-bus is now dead. This question seems to come down to the relative chance of failure between the diode and the relay.
  • Switching feed paths means momentarily cutting power to the E-bus. This seems inconsequential since, when switching to the E-bus, presumably power has been lost already.

Any other detractions I'm missing? Any thoughts on the concerns above?
 
Hmmm how interesting, I'm still looking for problems that may arise. Looking forward to seeing what more electron wizards say on this one.

Also, queue the E-Bus haters now...:)
 
Switching to an alternate feed for e-bus isn't usually because power to the e-bus has been lost. It's because the alternator has been lost and there is plenty of time to get it done. Switching off the main bus cuts off all the high amp exterior lighting and such to preserve battery power for the stuff you really need. Closing the alt feed switch first and then opening the main would prevent an interruption to radios and such, though most of the newer EFIS wouldn't shut down and reboot as long as you get the e-bus powered up within 30 seconds of shutting down the main. In my old 6A, I just closed both the e-bus alt feed and the main buss switches as a matter of course. I would close the e-bus before start and listen to ATIS, so the alt feed was tested before flight. When I did actually lose my alternator, which did happen, all I had to do was open the main switch. No muss, no fuss.

My new 6 carries 2 batteries and I have 3 position switches for both - OFF-EBUS-ON. I have fusible link protected wires from the batteries to the center E-BUS terminals. I can power the e-bus or the main with either or both batteries. Normal position is both ON with both charging. If I lose the alternator, I'll shut off one battery entirely and hold it in reserve. The other battery goes to E-BUS and when it's used up, I'll turn on the other battery and start looking for someplace to land. I did it this way because I have 2 electronic ignitions.

An alternative would be to have a back-up alternator and you wouldn't even need an e-bus or a second battery.

Ed Holyoke
 
Also, queue the E-Bus haters now...:)

I'll certainly confess that I've wondered if the E-bus is unnecessary complexity in my case. I'm already planning a dual alternator setup, with the #2 alternator set up as a standby to come online automagically should the primary fail. I'll also have a dedicated backup battery for each EFIS screen, along with a backup battery system for the dual SDS CPI2 ignition.

So there's a lot of redundancy there (naively speaking), but then again we're talking about an electrically dependent airplane as well. And that, in a nutshell, defines my wondering on this topic.

As Ed alludes, one might wonder whether the E-bus is still necessary with a second alternator. My general thought is that it adds protection against a wiring or contactor fault that causes the main bus feed to go dark. That wouldn't be a code-brown moment in my airplane - dedicated backup battery systems would ensure that the ignition kept firing and the EFIS displays stayed on and kept an attitude reference - but it'd be a bare-bones affair finishing the flight. Adding the E-bus alt feed idea allows me to retain stuff like pitot heat, IFR navigator, etc. Worth the system complexity? My current opinion is yes, but I'm certainly willing to entertain contrary viewpoints.
 
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I'll certainly confess that I've wondered if the E-bus is unnecessary complexity in my case. I'm already planning a dual alternator setup, with the #2 alternator set up as a standby to come online automagically should the primary fail. I'll also have a dedicated backup battery for each EFIS screen, along with a backup battery system for the dual SDS CPI2 ignition.

So there's a lot of redundancy there (naively speaking), but then again we're talking about an electrically dependent airplane as well. And that, in a nutshell, defines my wondering on this topic.

As Ed alludes, one might wonder whether the E-bus is still necessary with a second alternator. My general thought is that it adds protection against a wiring or contactor fault that causes the main bus feed to go dark. That wouldn't be a code-brown moment in my airplane - dedicated backup battery systems would ensure that the ignition kept firing and the EFIS displays stayed on and kept an attitude reference - but it'd be a bare-bones affair finishing the flight. Adding the E-bus alt feed idea allows me to retain stuff like pitot heat, IFR navigator, etc. Worth the system complexity? My current opinion is yes, but I'm certainly willing to entertain contrary viewpoints.

Ya with a 2nd alternator (assuming it is large enough to power all your goodies an E-bus is kind of redundant it would seem). However, you mention a valid point, if the main bus is taken out for some reason you still have the devices on your E-bus. Someone is probably going to chime in and say "even under IFR you can fly without those things..." yadayada but that doesn't mean it's as safe or certainly as comfortable to you. Flying in less than an optimal aircraft in IFR is not joke, especially if you don't do it for a living.

I also like the E-bus for what it was intended for. Bob wanted a quick one switch method to continue a flight should the alternator fail. I really like the idea of flipping a switch and flying to my destination instead of worrying about battery life or tracking down another alternator etc. (once again mitigated by your second alternator)

Finally, the biggest thing that I can never understand is how many people mention how complex it is. Uhhh... The only thing extra is a diode and a relay. granted there are a few extra connections but it's nothing like the internals of a Falcon 9. Even if I didn't have an E-bus I don't think I could have fit all my circuits on one single bus. (unless there are some large ones I didn't know about)

Long story short, I like the E-bus for the reasons mentioned but mainly due to a teeny bit extra redundancy should the main bus take a dump.
 
I think you might be missing how I at least thought this would be used. First I would normally have both the ebus and the main bus switched on. Then in the case where loads need to be shed then the main bus is switched off. There is no power interruption to the ebus since the power is provided by the ebus switch/relay. The diode prevents the main bus from back feeding from the ebus.

Also, I would suspect that it is more likely for relays to fail in a shorted position. The contact usually stick versus the actuator not moving to close the switch.
 
changing poles on that switch/relay will create a momentary open to your e bus feed. That will cause a reset/brown out for any circuitry, requiring a reboot/restart for advanced equipment.

creative wiring of a triple throw switch could avoid that, but it would combine the two busses for a brief moment.

Larry
 
. SNIP
I also like the E-bus for what it was intended for. Bob wanted a quick one switch method to continue a flight should the alternator fail. I really like the idea of flipping a switch and flying to my destination instead of worrying about battery life or tracking down another alternator etc. (once again mitigated by your second alternator) SNIP

Very good design point - not only for shedding loads to conserve battery capacity, but for rapid isolation of big loads in the event of an electrical fault.

This can also be achieved by having ?critical for IFR flight? loads on the battery side of the master solenoid(s). All the big loads (engine starter, pitot heat, landing lights, boost pump, etc.) on the output of the master solenoid.

With two ?avionic master? switches, each getting power from the battery(s) to run half the panel, you add redundancy to mitigate a fault on one of the avionic busses taking down both EFIS screens, both radios, etc.
Carl
 
Any other detractions I'm missing? Any thoughts on the concerns above?
Bob N's diagrams have two current paths supplying power to the E-Bus, one
through a diode and one through a relay (or switch). Eliminating one of those
paths greatly reduces the reliability of the E-Bus. Relays are much more likely
to fail than a diode.
 
Very good design point - not only for shedding loads to conserve battery capacity, but for rapid isolation of big loads in the event of an electrical fault.

This can also be achieved by having ?critical for IFR flight? loads on the battery side of the master solenoid(s). All the big loads (engine starter, pitot heat, landing lights, boost pump, etc.) on the output of the master solenoid.

With two ?avionic master? switches, each getting power from the battery(s) to run half the panel, you add redundancy to mitigate a fault on one of the avionic busses taking down both EFIS screens, both radios, etc.
Carl

All good points as well Carl. I like your double avionics master idea. Another way to skin the cat. I like how you pointed out conserving battery capacity as a nice feature of the quick load shedding method should the backup alternator not work/ not come online for some rare reason.
 
All good points as well Carl. I like your double avionics master idea. Another way to skin the cat. I like how you pointed out conserving battery capacity as a nice feature of the quick load shedding method should the backup alternator not work/ not come online for some rare reason.

I like cats...dogs too.
 
As I plan out my ebus items most of my avionics is able to be on that bus. So you effectively have two paths to power most of the avionics. My GNS430W is a power hog so for now it?s on the main bus. I?m not sure I really like that idea but it seems like it consumes a lot compared to everything else.
 
Bob N's diagrams have two current paths supplying power to the E-Bus, one
through a diode and one through a relay (or switch). Eliminating one of those
paths greatly reduces the reliability of the E-Bus. Relays are much more likely
to fail than a diode.

I don't think this idea qualifies as "eliminating one of those paths." It still provides two possible paths to power the E-bus, just like Bob's diagrams. The primary difference is that the alt feed switch in Bob's diagram is additive - closing that switch connects the second feed to the E-bus but does not disconnect the feed from the main bus.

The comment on failure probability for a relay vs diode is good info, thank you - and it fits with my gut feeling, if for no other reason than just comparing solid state vs mechanical parts.

Lots of good discussion here.
 
changing poles on that switch/relay will create a momentary open to your e bus feed. That will cause a reset/brown out for any circuitry, requiring a reboot/restart for advanced equipment.

creative wiring of a triple throw switch could avoid that, but it would combine the two busses for a brief moment.

Larry


Add a large(ish) capacitor to the E-bus to supply power during the brief switch-over.
 
Add a large(ish) capacitor to the E-bus to supply power during the brief switch-over.

That would solve the brown out problem but I will say that from my HVAC days capacitors were without question the part with the highest failure rate. Perhaps they sell better grade ones with good records but my experience doesn't show it.
 
Dennis,

You should post this question on Matronics list under Aeroelectric. Then you?ll get Bob himself to give you the pros and cons. With your permission I can post it for you.

Jeff
 
the alt feed switch in Bob's diagram is additive - closing that switch connects the
second feed to the E-bus but does not disconnect the feed from the main bus.
I consider that an advantage. Why would you want the E-bus disconnected from the main bus?
 
Add a large(ish) capacitor to the E-bus to supply power during the brief switch-over.

I simply check the battery buss works correctly by engaging the ebuss before engaging the master/alt prior to start, then leave the ebuss enabled all the time. I don't see any need to turn the ebuss back off.

If the alt fails then all I need to do is open the alt/master and not two actions.
 
The spilt bus idea is designed to cope with an airborne alternator failure in a single alternator, single battery aircraft. Soon after an alternator failure is identified 2 switch selections will vastly reduce the electrical load and give the pilot plenty of time to figure out what to do next. Eliminating the alternator field and master relay current can save between 3 and 5 amps, on top of the high power services, lights, heated pitot, etc.

Flying with the direct feed from the battery to the endurance bus on will mean it will be left on at some point, draining the battery. Normal use is master only on. The e-bus feed can be used as a "Clearance Delivery" switch, depends how it is wired up.

There are plenty of other methods to implement this scheme, including switches instead of the diode. It will achieve the same end, but will need some careful labeling to make sure operation is clear when the alternator fails 5 years after the aircraft was built.

There are draw backs to using a diode, mainly the voltage available to the e-bus side of the bus is around a volt lower than the main bus. Usually that is no factor, but it can be significant. Reliability usually comes with low parts count which is why the diode scheme was invented. With a second alternator the need to quickly shed the high power loads is less important so the e-bus layout is less useful. For brown-out protection something like the TCW Intelligent Power Stabilizer can be useful.
Pete
 
I personally I think the addition of a capacitor is just adding unneeded components. The most common failure of capacitors is shorting so it will effectively take out the bus it?s connected to if it fails.

My planned sequence is slightly different from Joes but it?s similar in that there is only one switch operation required in case of an issue.

* turn on ebus to get ATIS
* turn off ebus to protect avionics during start
* turn on main alt and do engine start
* turn on ebus and I?m in my normal run state

During this last step I don?t expect any voltage spiking. The load is being supplied by the main bus. It will be a diode voltage drop lower than the battery voltage. Let say between 0.7 an 1.0 volts. Then when the ebus is switched on the ebus voltage will rise to the battery voltage.

In the case of an alternator failure I switch off the main bus. The voltage will stay constant since the diode in normal operation is reversed biased (turned off). This is due to the ebus voltage being higher than the main bus voltage by the diode drop.
 
So I've been thinking about the Nuckolls E-bus setup, specifically the handling of having an alternate feed off an always-hot battery bus in case of a failure affecting the main bus. Bob's setup takes the same basic form everywhere I've seen it: the main bus feeds the E-bus through a diode, and the alternate feed is provided through a switch and/or relay (depending on expected E-bus loads).

The reasoning behind the diode between the main and E-bus is to prevent the E-bus from feeding the main bus and possibly popping a fuse/breaker. Bob's assertion seems to be based on an alternative scenario where two switches would be provided, and it would be incumbent on the pilot to ensure the main->E-bus feed switch was opened before closing the E-bus alt-feed switch, to prevent the aforementioned backfeeding.

However, it seems this issue can be fairly easily solved with careful switch choices, such that it's not possible to have both feed paths closed simultaneously. For example, if a five-pin relay is used for feed switching, the main bus could feed through the normally-closed contact, and the batt bus through the normally-open one. The choice of how to feed the bus is still controlled by a single switch throw, the only difference being that that switch position definitively disconnects one feed while connecting another one.

Example diagram:

JQyTpEpl.png


This seems elegantly simple enough that I'm immediately suspicious that I'm missing some downsides. Some thoughts that come to mind in this vein:

  • There's the possibility of the relay somehow failing such that both contacts close, thus possibly blowing the batt bus fuse and killing the E-bus in an emergency. Not sure what the likelihood of this is.
  • If the relay somehow fails with both contacts open, the E-bus is now dead. This question seems to come down to the relative chance of failure between the diode and the relay.
  • Switching feed paths means momentarily cutting power to the E-bus. This seems inconsequential since, when switching to the E-bus, presumably power has been lost already.

Any other detractions I'm missing? Any thoughts on the concerns above?

I have a similar setup with a slight variation...I have a guarded DPDT On-On switch that controls the feed for the E-BUS it's either from the main bus or straight off the battery via relay and fuse link. One pole of the switch controls the source of power and the other controls the relay. A single throw of the switch moves power source from the main bus to the hot battery.

The fuse link off the battery is 15A, normal power to off the main bus is protected by 15A fuse, and I limited the EBUS load to 10A.

This design was not for endurance as much as it was for emergencies when attempting to isolate an electrical fault through bus isolation...I have a all electrical IFR cockpit with a single alternator and a single battery and the fuses are not accessible in flight.
 
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Simple is better

I planned for 1 failure back up, Alt goes bad , ebus on with 1 switch (no relays no big thinking, see the blinking alt light and do something)
Same with having one LSE ignition and 1 mag one bad use the other
Mech fuel pump bad turn electric on
No regarding the engine .......Glide

So far so good, but always worth looking at new ways to do things

Peter
 
Flying with the direct feed from the battery to the endurance bus on will mean it will be left on at some point, draining the battery. Normal use is master only on. The e-bus feed can be used as a "Clearance Delivery" switch, depends how it is wired up.

Hasn't been a problem in almost 8 years.

Perhaps I'm more careful.

Its part of my shutdown procedure, and I always eyeball the switches after exiting the aircraft.

Its easy to tell anyway, as my GTN650 would be on. That's a pretty big annunciator.
 
I personally I think the addition of a capacitor is just adding unneeded components. The most common failure of capacitors is shorting so it will effectively take out the bus it?s connected to if it fails.

My planned sequence is slightly different from Joes but it?s similar in that there is only one switch operation required in case of an issue.

* turn on ebus to get ATIS
* turn off ebus to protect avionics during start
* turn on main alt and do engine start
* turn on ebus and I?m in my normal run state

During this last step I don?t expect any voltage spiking. The load is being supplied by the main bus. It will be a diode voltage drop lower than the battery voltage. Let say between 0.7 an 1.0 volts. Then when the ebus is switched on the ebus voltage will rise to the battery voltage.

In the case of an alternator failure I switch off the main bus. The voltage will stay constant since the diode in normal operation is reversed biased (turned off). This is due to the ebus voltage being higher than the main bus voltage by the diode drop.

I turn on ALT/MASTER, EBUSS, E-IGN, MAG, Fuel Pump (as necessary). The switches are in that order furthest to nearest.

My MAG is OFF-ON-MOM so to start I simply push the MAG switch to the right into its MOM position. That forces me to have the MAG on to start and pretty difficult to turn it off again by accident although I'd catch it on runup.

Immediately check OP and RPM on the EIS.

The next two switches are EFIS #1 and then #2 so they go on once the engine is running, then I pick up ATIS and do other checks while the EFIS's are coming online.

My switches are arranged in groups and ordered such that in most operation I'll turn them on/off in the order they are mounted. Works well for me, simple and pretty failsafe.
 
So I've been thinking about the Nuckolls E-bus setup, specifically the handling of having an alternate feed off an always-hot battery bus in case of a failure affecting the main bus. Bob's setup takes the same basic form everywhere I've seen it: the main bus feeds the E-bus through a diode, and the alternate feed is provided through a switch and/or relay (depending on expected E-bus loads).

The reasoning behind the diode between the main and E-bus is to prevent the E-bus from feeding the main bus and possibly popping a fuse/breaker. Bob's assertion seems to be based on an alternative scenario where two switches would be provided, and it would be incumbent on the pilot to ensure the main->E-bus feed switch was opened before closing the E-bus alt-feed switch, to prevent the aforementioned backfeeding.

However, it seems this issue can be fairly easily solved with careful switch choices, such that it's not possible to have both feed paths closed simultaneously. For example, if a five-pin relay is used for feed switching, the main bus could feed through the normally-closed contact, and the batt bus through the normally-open one. The choice of how to feed the bus is still controlled by a single switch throw, the only difference being that that switch position definitively disconnects one feed while connecting another one.

Example diagram:

JQyTpEpl.png


This seems elegantly simple enough that I'm immediately suspicious that I'm missing some downsides. Some thoughts that come to mind in this vein:

  • There's the possibility of the relay somehow failing such that both contacts close, thus possibly blowing the batt bus fuse and killing the E-bus in an emergency. Not sure what the likelihood of this is.
  • If the relay somehow fails with both contacts open, the E-bus is now dead. This question seems to come down to the relative chance of failure between the diode and the relay.
  • Switching feed paths means momentarily cutting power to the E-bus. This seems inconsequential since, when switching to the E-bus, presumably power has been lost already.

Any other detractions I'm missing? Any thoughts on the concerns above?

Downside:
Contacts will pit and fail. Failure mode could be that it does not switch.

For 20-amps or less, I would rather use the diode and a switch instead of the relay.

In my career before retirement, I have seen more relays fail than I have switches.
 
That's a clever flow!

I turn on ALT/MASTER, EBUSS, E-IGN, MAG, Fuel Pump (as necessary). The switches are in that order furthest to nearest.

My MAG is OFF-ON-MOM so to start I simply push the MAG switch to the right into its MOM position. That forces me to have the MAG on to start and pretty difficult to turn it off again by accident although I'd catch it on runup.

Immediately check OP and RPM on the EIS.

The next two switches are EFIS #1 and then #2 so they go on once the engine is running, then I pick up ATIS and do other checks while the EFIS's are coming online.

My switches are arranged in groups and ordered such that in most operation I'll turn them on/off in the order they are mounted. Works well for me, simple and pretty failsafe.
 
Thanks for all the commentary, folks. After reading all this, I've decided to stick with the diode; overall, I think the talk about failure rates is what really put me over the line.

I'm also liking the idea of having the alt feed switch normally on; one of the things that made me hesitant to use the diode was the voltage drop. I think Bob Nuckolls's argument that the drop is immaterial holds water in a traditional single-alternator system, but I'm planning on a primary and standby alternator, with the standby regulator set lower so it comes online automatically in the event of a failure of the primary.

So in a case where I'm running on the standby, now my E-bus ends up down around 12.5 volts - everything will continue to run, but one critical point is that I'm close to the threshold where the Skyview system won't charge/maintain its backup battery. Probably not a huge deal real-world, but enough to make me consider options.

Anyway, the tactic of having the alt feed normally on makes the diode voltage drop pretty immaterial, outside of a scenario where the master contactor is open. I also like that it makes yet another in-flight incident response require no direct action from me.

I'm getting pretty close to being satisfied with my power distribution architecture.

(Note: I'm building an electrically-dependent airplane, hence my somewhat elevated interest in designing a fault-tolerant system)
 
Philip,

Glad we were able to convince you. In general simpler is better.

Having said that... There is a circuit referred to as a virtual diode. It uses one or more MOSFETs and a control circuit nstead of an actual diode. This results in very little voltage drop. The voltage drop is dependent on the current and how good of a MOSFET you use. These days you can buy MOSFETs which can handle a couple hundred or more amps and have very low in resistance.

I built a circuit card with one of these Virtual Diode circuits for my EBus feed. One of the other advantages of this circuit is the MOSFET can be switched off and on so that it can also perform the function of a solid state relay. Anyway when I can get some time or when I actually need it I?ll finish validating my circuit.
 
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