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  #111  
Old 05-18-2018, 12:43 PM
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Josh, are the cross tie and ESS Aux switches open or closed for normal flight?
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  #112  
Old 05-18-2018, 01:14 PM
unitink72 unitink72 is offline
 
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Josh, are the cross tie and ESS Aux switches open or closed for normal flight?
Cross Tie Open, ESS Aux switch closed
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  #113  
Old 05-18-2018, 03:16 PM
rv7charlie rv7charlie is offline
 
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"I am suspicious of parallel operations. I have tried paralleling power supplies, coils, and transistors and due to imperfect matching things get hot due to oscillations: on and off and the switching causes heat. Getting the parallel devices to conduct equally is not easy. It also does not offer the same safety factor as having tested redundant components that can be switched out and in as needed. Some math analyses would be needed to verify this.

Another concern is that diodes can fail either open or short. I am not sure and will do some more thinking about the impact of a shorted diode. An open diode by itself would not be a problem, other than being able to detect and replace when back on the ground. What is not wanted is one battery draining into the other; one or both batteries might fail. To me a switching arrangement would be better. That is switching in and out either power system to either critical component set. The best would be on the ground checks, before air, of both power systems and both sets of critical components and then the capability of switching."


The caution about paralleling power supplies is worthy, but not a total prohibition. You just need to know how the system will act with the chosen components. (But don't confuse a battery with a power supply in this context.)

As Carl mentioned, paralleling batteries, while not perfection, can work fine if the design calls for it. The only danger of one battery 'feeding into' another is if there's a catastrophic short in one of the batteries, or one is almost totally discharged. If both are healthy and at a reasonable state of charge, neither battery has enough voltage to 'feed into' (charge) the other. As evidence, consider the open circuit voltage of a fully charged lead-acid battery (~12.5V), and that it takes almost 14V from the charge source (alternator or charger) to fully charge that battery. The issues with paralleling batteries, while real, are of more academic than practical significance in our a/c applications, where batteries are seldom discharged by more than 5-10% (during the start sequence).

Paralleling devices: Look carefully at any power supply bus and its attached devices. Having said that, paralleling *components* feeding a single device *might* have implications. For an example, see the AEC book for a description of how to safely parallel multiple subD pins in a connector to share a higher current than one or two pins can handle safely.

Diode failures: Yes, they can fail. Just like switches, which can also fail either open or shorted. The proper way to consider them (and any other component) is to ask whether something bad happens with a single failure, and what (if anything) needs to be done with a simple operation to work around that failure until safely on the ground. Just assume that any one random component *will* fail on a given flight, and try to have a plan B (if needed) for that failure. FWIW, a properly selected and installed diode will have many orders of magnitude more reliability than a switch/relay/etc that's mechanical. But on any given flight, it either will or won't fail, just like any other component. So you plan your *system* design expecting failure of any one component.

My take would be that you pick diode or switch, as appropriate, for desired circuit operation; not on reliability.

FWIW...

Charlie

Last edited by rv7charlie : 05-18-2018 at 03:21 PM.
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  #114  
Old 05-18-2018, 05:00 PM
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Quote:
Originally Posted by unitink72 View Post
Cross Tie Open, ESS Aux switch closed
(1) You're on the run up pad. How will you determine if the aux feed is actually working?

(2) 8500 ft, smoke in the cockpit. I'm not going to tell you the source. just like you won't know in the air. What will you do?
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  #115  
Old 05-18-2018, 08:18 PM
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Default Diode shorted?

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Originally Posted by DanH View Post
Sure.

Note this popular maxim, from Saint-Exupery:

..perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away...

Now consider these two sketches. What is the reliability difference between the one on the left (your original) and the one on the right?



BTW, I am not suggesting that the one on the right is optimum. It's just an example, an illustration of a point.
On the right... we can preflight for an open diode but how do we know if a diode is shorted?
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Last edited by johnbright : 05-18-2018 at 08:34 PM.
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  #116  
Old 05-18-2018, 08:29 PM
svyolo svyolo is offline
 
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Quote:
Originally Posted by DanH View Post
(1) You're on the run up pad. How will you determine if the aux feed is actually working?

(2) 8500 ft, smoke in the cockpit. I'm not going to tell you the source. just like you won't know in the air. What will you do?
I would rather be riding in a glider, than a Roman Candle.
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  #117  
Old 05-18-2018, 09:21 PM
rv7charlie rv7charlie is offline
 
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With controllable alternators, you can check the diodes for shorts by shutting down the alts individually [edit: Should have completed the thought:] and individually monitoring bus voltages on either side of each diode.


On a more general note. After 12 pages of discussion, I went back to the 1st post. The OP mentions using a modified AEC Z-14, but with electronicly controlled engine. Is there any reason for not going directly to Z-19? I confess it seems unlikely that those of us with limited to no a/c electrical design experience can beat what 40+ years of experience, seeing both the good and bad of a/c design in the certified world, brought to designing Z-19.

Charlie
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  #118  
Old 05-19-2018, 05:12 AM
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Quote:
Originally Posted by rv7charlie View Post
On a more general note. After 12 pages of discussion, I went back to the 1st post. The OP mentions using a modified AEC Z-14, but with electronicly controlled engine. Is there any reason for not going directly to Z-19? I confess it seems unlikely that those of us with limited to no a/c electrical design experience can beat what 40+ years of experience, seeing both the good and bad of a/c design in the certified world, brought to designing Z-19.
I'm the OP

Z-19 is one alternator, two batteries. If the alternator goes down, then you're down to the capacity of those two batteries to handle the engine and run everything else. I didn't feel comfortable with that limitation, given the power demands of SDS and an all-glass, likely IFR panel.

Z-19 has diode modules for the ECU and primary fuel pump; taking that strategy with the SDS system would have lots of diode modules (one for each component). It seems simpler to have one engine bus fed by two sources through diodes.

Z-19 also uses a voltage-sensing relay to switch engine power source from main to engine battery. I'd rather keep the two through simple switches and diodes, and with a modern EFIS I can monitor voltages on both batteries easy enough.
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Last edited by rmartingt : 05-19-2018 at 08:53 AM. Reason: clarifying reason 1
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  #119  
Old 05-19-2018, 08:51 AM
rv7charlie rv7charlie is offline
 
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Well, I guess that this is my 'Duhhh!' moment for the day...

My apologies. I breezed right past the 'single alternator' item while looking for 'electronically controlled engine' spec.

I may raise the issue on the AEC list, to see of Bob's interesting in revising or adding a drawing to allow dual alt/dual battery/electronic engine.

Charlie
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  #120  
Old 05-19-2018, 10:29 AM
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Default Prospective Z diagram for EFI

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Originally Posted by rv7charlie View Post
... I may raise the issue on the AEC list, to see if Bob's interested in revising or adding a drawing to allow dual alt/dual battery/electronic engine.

Charlie
Be sure to point out there are dual ECUs, dual fuel pumps, and dual coilpacks, but only one set of injectors. The injectors need B+; they are not powered by the ECU but switched to ground by it, this function relay selectable between primary and backup ECU. (The coilpacks are powered by B+ also and triggered by the ECU but there are two coilpacks; each dedicated to its own ECU.

AEC states Z-14 is suitable for an electrically dependent engine; it shows electronic ignition and electric fuel pumps but does not speak to electronic fuel injection.

Z-19:
* States it's for electronic fuel injection and shows diodes arranged from two battery buses like post 86.
* Does not show coilpacks or injectors.
* Shows only one ECU so hence the diodes to automatically power it from one or both battery buses; like we could do with the injectors.
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