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  #31  
Old 02-27-2019, 05:48 PM
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Carl Froehlich Carl Froehlich is offline
 
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Originally Posted by RV7A Flyer View Post
There are always "better" options. But what is unacceptable about the original solution?
The orginal designed failed and the OP lost all avionics. What else need be said?

Blind adherence to Knuckes or any other design without understanding pros and cons is not what builders should strive for.

Carl
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  #32  
Old 02-27-2019, 10:02 PM
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N804RV N804RV is offline
 
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Note 12, page Z-9 of "The Aeroelectric Connection" addresses this problem. I'm guessing the E buss was pulling more current than the isolation diode could handle without a heat sink.

Carl, what are the specs on the Schottky Diodes you picked out?
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Last edited by N804RV : 02-27-2019 at 10:04 PM.
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  #33  
Old 02-27-2019, 10:57 PM
Mjuckes Mjuckes is offline
 
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Default Still have questions

I wonder does the aircraft have a E buss switch or a Avionics switch? If it truly is a E buss switch what difference does the diode make? The drawing posted earlier shows a avionics switch. E buss is to be powerd directly from a battery. The diode is used to stop flow to the main buss, If it failed and there is no power to the E buss then I would guess there is no E buss switch?
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  #34  
Old 02-27-2019, 11:13 PM
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N804RV N804RV is offline
 
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Originally Posted by Mjuckes View Post
I wonder does the aircraft have a E buss switch or a Avionics switch? If it truly is a E buss switch what difference does the diode make? The drawing posted earlier shows a avionics switch. E buss is to be powerd directly from a battery. The diode is used to stop flow to the main buss, If it failed and there is no power to the E buss then I would guess there is no E buss switch?
I misread your post first time. I was thinking about this also. I wonder if the OP had this switch closed during normal operation, thinking it was an "avionics master" when its really intended to be "normally open" and used as an alternate means to power the E buss.
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  #35  
Old 02-27-2019, 11:14 PM
Mjuckes Mjuckes is offline
 
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Default Yep

That?s what I think also. If the OP had an E buss/alternate switch he could have flipped said switch and had power to the panel?
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  #36  
Old 02-28-2019, 08:48 AM
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Carl Froehlich Carl Froehlich is offline
 
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Carl, what are the specs on the Schottky Diodes you picked out?
The only place I use diodes is on the output of the standby alternator. I use a total of four, 20 amp dual packages mounted on a nice heat sink. One set of two dual packages feeds the left avionics buss, the other set the right. The left and right avionics busses are powered via a set of 30 amp relays, two on each battery (primary and backup to cross connect - e.g. left avionics to right battery, etc). Relays are connected directly to the battery. The two master solenoids provide power to only the non-avionic loads and starter.

Four dual 20 amp packages are way overkill for a 20 amp alternator, but they are the same price, size and weight of smaller units - so why not?

The point - the diodes are installed for the sole reason to isolate a fault on one buss from taking down both busses. No avionics buss gets power just via a diode.

Of note - the new project will not have a standby alternator, just the two PC-625 batteries. I may add one later if mission creep sets in.

Most IFR setups now have dual screen glass panels, more than one GPS receiver and two radios. Having everything on one avionics buss (essential buss or whatever you call it) builds in a single fault risk. Split the panel into two busses - power them as you see fit.

As I posted in the past, anyone interested how I do power distribuition should PM me your email address.

Carl
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  #37  
Old 02-28-2019, 10:16 AM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
The orginal designed failed and the OP lost all avionics. What else need be said?

Blind adherence to Knuckes or any other design without understanding pros and cons is not what builders should strive for.

Carl
Seems like the failure was not the design, but the implementation, as noted below (inadequate heat sink), perhaps. There must be hundreds or even thousands of planes flying around with this design, with no problems, and no need to go to two batteries and two alternators to mitigate a fault.
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  #38  
Old 02-28-2019, 10:35 AM
Jonathan Alvord Jonathan Alvord is offline
 
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Originally Posted by Mjuckes View Post
That?s what I think also. If the OP had an E buss/alternate switch he could have flipped said switch and had power to the panel?
Yes the plane has a Master Switch (dual pole), Avionics switch, Nav lights, Strobe, Ebus, and Fuel Switch.

I would be interested in isolating the navigation instruments from the radios. In my mind the Radio is the most important piece to have, so that contact with ATC can be made in the event of emergency, or to cont Flight Following. In this case I was on flight following and then abruptly had no more contact. I did however have Foreflight and a way to navigate safely to home Airport.
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  #39  
Old 02-28-2019, 11:11 AM
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In my mind the Radio is the most important piece to have, so that contact with ATC can be made in the event of emergency, or to cont Flight Following.
I disagree. The *flight instruments* are most important (if you're in IMC), followed by the navigation. ATC will see you and get people out of your way, and you can deal with the lost comm later.

If you're VFR, navigate to the nearest airport and land.

Aviate
Navigate
Communicate
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  #40  
Old 02-28-2019, 12:59 PM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by RV7A Flyer View Post
Seems like the failure was not the design, but the implementation, as noted below (inadequate heat sink), perhaps. There must be hundreds or even thousands of planes flying around with this design, with no problems, and no need to go to two batteries and two alternators to mitigate a fault.
Sorry but disagree. All designs should be examined for the result of any component be it a switch, relay, terminal or equipment failure. If the result is not acceptable for the mission, then the builder needs to address the design. In this case a single failure resulted in the loss of all avionics. The cause of the failure is not the focus. Everything you put in your airplane can fail, design to mitigate such failures if you cannot live with the result.

For a day VFR airplane less rigor is required.
Carl
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