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Diodes versus switches 101 ?

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Bill Schlatterer

Joined: 09 Jan 2006
Posts: 195

Posted: Sun Mar 09, 2008 10:13 am Post subject: Diodes versus switches 101 ?

There was some discussion about Z19 using diodes in a critical ECU circuit and the question arose as to what kind of failure modes apply to diodes and their reliability as compared to switches.  I think the trust of that thread (sub list) and post below is that diodes might be "automatic" but switches are safer and less likely to fail?  ( I have no opinion but would like to understand the rationale )  This was posted and I was just wondering what the thought might be on the AE list since most of Bobs diagrams "suggest" a diode in the E-Bus circuit as opposed to a switch.  That might not be AS critical as the ECU/EFI circuit on an all electric engine but the same concerns still apply?  

Posted to the Sub list:   

"After 30 years in the industrial electrical construction and maintenance field I can say without equivocation that a robust switch is much less likely to fail than a robust diode. The science of the switch is much simpler - use a lot of good conducting material that is not likely to corrode and support it with stout mechanical parts that hold tightly.   

Now, by contrast, the science of the diode starts with getting good silicon material and contaminating it in just the right proportion with just the right material at just the right temperature for just the right time. THEN you can start on building the junctions onto some sort of heat dissipating holder. I have replaced dozens of solid state devices that failed (always failed open) and I never found the root cause of failure. I put in an exact replacement and the circuit worked just fine for years. The trade off you get for the "automatic" switching of a diode vs. the manual switch is in the area of RELIABILITY. I'll be using manual switches for all critical loads."  

Not wanting to stir anything up, just want to understand the thinking and factual matter in the post above?  

Thanks Bill S  

7a  Z13/8  Z32(HD E-Bus)  Z35(7ah)       [quote][b]

retasker(at)optonline.net
Guest

Posted: Sun Mar 09, 2008 11:45 am Post subject: Diodes versus switches 101 ?

Here is an excerpt of something I posted on the Sub list which relates 

to this email:



Regarding the reliability of switches and diodes...  According to Mil handbook 217F - the "bible" of reliability prediction - a Schottky power diode (which is the only type anyone should be using in any power circuit in an airplane) has a predicted failure rate of 0.0028 failures per million hours.  A mil rated toggle switch has a predicted failure rate of 0.1 failures per million hours.  Now, realistically, this prediction for a switch is partially related to the toggle mechanics and partially related to the individual contacts.  The mil handbook relates to a single set of contacts while the specified switch has four sets of contacts so the switch in question is probably less reliable that the handbook predicts for a simple toggle switch.  But even if we ignore that detail, for what it is worth, a diode of the type that should be used is 35 times more reliable than the type switch specified.  So much for the superiority reliability of a switch that some have been bandying ab

 out...



A properly designed diode circuit provides automatic, uninterrupted power transfer in the event of the loss of the main supply.  A switched circuit requires the pilot to notice the problem and take some action (toggle the switch to the other position).



A diode circuit has not been specified (in the Subaru application), so one is on their own if that were to be the choice of a user.  While this is not an insurmountable problem, the diode circuit is more susceptible to a poor design.  One must choose a diode with sufficient current and voltage ratings for the desired circuit conditions.  Additionally, since there can be considerable heat generated in a power diode, proper heat sinking must be provided (not particularly hard to do as long as you know how to calculate what is needed).



So...  If you are electrically challenged use the factory recommended circuit design - good or bad as it may be, it has full factory support.



If you really know what you are doing, a diode circuit has certain advantages.  However, if you choose to depart from the factory design, do not expect any assistance or support from the factory for your design and if you have a problem it is your problem.



I have already wired my RV9A (long before this discussion and long before "diode" became a dirty word) and I am using two very beefy Schottky diodes to connect both battery buses to the engine bus.  If either bus goes down for whatever reason, the engine keeps on running and an annunciator tells me which bus is down.  Additionally, a voltmeter on each bus keeps me informed of its health continuously.



It should also be noted that in the instance in question that the ECU was provided power from a, probably, inadequately heat sinked bridge that was, possibly, underrated for the application.



For what Bob recommends the bridge - the Ebus - it is an excellent solution for the typical experimental plane builder.



Dick Tasker

Bill Schlatterer wrote:

 	  Quote: 	
		  

 There was some discussion about Z19 using diodes in a critical ECU 

 circuit and the question arose as to what kind of failure modes apply 

 to diodes and their reliability as compared to switches.  I think the 

 trust of that thread (sub list) and post below is that diodes might be 

 "automatic" but switches are safer and less likely to fail?  ( I have 

 no opinion but would like to understand the rationale )  This was 

 posted and I was just wondering what the thought might be on the AE 

 list since most of Bobs diagrams "suggest" a diode in the E-Bus 

 circuit as opposed to a switch.  That might not be AS critical as the 

 ECU/EFI circuit on an all electric engine but the same concerns still 

 apply?



 Posted to the Sub list:



 "After 30 years in the industrial electrical construction and 

 maintenance field I can say without equivocation that a robust switch 

 is much less likely to fail than a robust diode. The science of the 

 switch is much simpler - use a lot of good conducting material that is 

 not likely to corrode and support it with stout mechanical parts that 

 hold tightly.



 Now, by contrast, the science of the diode starts with getting good 

 silicon material and contaminating it in just the right proportion 

 with just the right material at just the right temperature for just 

 the right time. THEN you can start on building the junctions onto some 

 sort of heat dissipating holder. I have replaced dozens of solid state 

 devices that failed (always failed open) and I never found the root 

 cause of failure. I put in an exact replacement and the circuit worked 

 just fine for years. The trade off you get for the "automatic" 

 switching of a diode vs. the manual switch is in the area of 

 RELIABILITY. I'll be using manual switches for all critical loads."



 Not wanting to stir anything up, just want to understand the thinking 

 and factual matter in the post above?

 Thanks Bill S

 7a  Z13/8  Z32(HD E-Bus)  Z35(7ah)




	


-- 

Please Note: 

No trees were destroyed in the sending of this message.  We do concede, however,

that a significant number of electrons may have been temporarily inconvenienced.

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matronics(at)rtist.nl
Guest

Posted: Sun Mar 09, 2008 12:25 pm Post subject: Diodes versus switches 101 ?

I think it's important to take into account what the purpose of the switch 

and/or diode is, and more important, what the failure modes of the circuits 

around it is. A diode may be much more reliable when used within it's 

designed specs, but if you overload it, it will fail much quicker than a 

switch. Compare a diode and a switch, both rated at 25A. A short-circuit 

current of 100A through the diode for a few seconds will make it fail, most 

likely open circuit. The same current through a switch may heat it up and 

damage the contacts, even cause some sparks, but it may still limp along.



Rob



---

retasker(at)optonline.net
Guest

Posted: Sun Mar 09, 2008 5:39 pm Post subject: Diodes versus switches 101 ?

True, but it is easy to get a diode that is rated for far over the 

normal operating current.  In fact, doing so costs very little extra and 

has other advantages.



Additionally, while your example as started is true, hopefully no one on 

this forum would end up with a design where it is possible to draw 100A 

on a 25A circuit without some sort of protective device interrupting the 

current before "a few seconds" are up...

Rob Turk wrote:

[quote] 

 I think it's important to take into account what the purpose of the 

 switch and/or diode is, and more important, what the failure modes of 

 the circuits around it is. A diode may be much more reliable when used 

 within it's designed specs, but if you overload it, it will fail much 

 quicker than a switch. Compare a diode and a switch, both rated at 

 25A. A short-circuit current of 100A through the diode for a few 

 seconds will make it fail, most likely open circuit. The same current 

 through a switch may heat it up and damage the contacts, even cause 

 some sparks, but it may still limp along.



 Rob



 ---

nuckolls.bob(at)cox.net
Guest

Posted: Mon Mar 10, 2008 7:44 am Post subject: Diodes versus switches 101 ?

At 09:21 PM 3/9/2008 +0100, you wrote:



 	  Quote: 	
		  

I think it's important to take into account what the purpose of the switch 

and/or diode is, and more important, what the failure modes of the 

circuits around it is. A diode may be much more reliable when used within 

it's designed specs, but if you overload it, it will fail much quicker 

than a switch. Compare a diode and a switch, both rated at 25A. A 

short-circuit current of 100A through the diode for a few seconds will 

make it fail, most likely open circuit. The same current through a switch 

may heat it up and damage the contacts, even cause some sparks, but it may 

still limp along.


	


   I've never replaced a diode that failed open. I don't

   mean to imply that they NEVER fail open but in what must

   be dozens of personal observations, I've never seen one

   open.  Oh yeah, take that back. I did have a plastic 1A

   device that blew up such that the stuff between the wires

   just disappeared. I guess that could be classified as

   an "open".



   Diodes are generally pretty robust. Exemplar devices

   might include the diminutive 1A, plastic 1N4001 described

   here . . .



http://www.aeroelectric.com/Mfgr_Data/Semiconductors/1N4001.pdf



   and his big brother 25A bridge rectifier . . .



http://www.aeroelectric.com/Mfgr_Data/Semiconductors/gbpc12.pdf



   Note that the single cycle surge (approx 8 milliseconds) for

   the 1A device is 30A. The 25A device will stand off 300A

   for 8 mS.



   Power diodes for steering energy amongst the busses in

   the heavy iron birds have been used for 50+ years. Their

   failure rates have been exceedingly low.



   Diodes also have an exceedingly long service life. I've

   never seen one 'wear out'. On the other hand, switches do

   have a service life . . . one that is profoundly affected

   by numbers of operations per year and under what electrical

   load and environmental conditions. I've seen switches open

   up with very few cycles on them after ten+ years of sitting

   in an airplane.



   99.9% of observed diode failures in light aircraft were

   in alternators where the designers of the installation

   did not exercise due diligence with respect to cooling.

   Concerns for a 25A diode used as the e-bus normal feedpath

   management are not well founded.



   Bob . . .

ceengland(at)bellsouth.ne
Guest

Posted: Wed Mar 12, 2008 1:33 pm Post subject: Diodes versus switches 101 ?

 	  Quote: 	
		  

 Bill Schlatterer wrote:

>

> There was some discussion about Z19 using diodes in a critical ECU 

> circuit and the question arose as to what kind of failure modes apply 

> to diodes and their reliability as compared to switches.  I think the 

> trust of that thread (sub list) and post below is that diodes might 

> be "automatic" but switches are safer and less likely to fail?  ( I 

> have no opinion but would like to understand the rationale )  This 

> was posted and I was just wondering what the thought might be on the 

> AE list since most of Bobs diagrams "suggest" a diode in the E-Bus 

> circuit as opposed to a switch.  That might not be AS critical as the 

> ECU/EFI circuit on an all electric engine but the same concerns still 

> apply?

>

> Posted to the Sub list:

>

> "After 30 years in the industrial electrical construction and 

> maintenance field I can say without equivocation that a robust switch 

> is much less likely to fail than a robust diode. The science of the 

> switch is much simpler - use a lot of good conducting material that 

> is not likely to corrode and support it with stout mechanical parts 

> that hold tightly.

>

> Now, by contrast, the science of the diode starts with getting good 

> silicon material and contaminating it in just the right proportion 

> with just the right material at just the right temperature for just 

> the right time. THEN you can start on building the junctions onto 

> some sort of heat dissipating holder. I have replaced dozens of solid 

> state devices that failed (always failed open) and I never found the 

> root cause of failure. I put in an exact replacement and the circuit 

> worked just fine for years. The trade off you get for the "automatic" 

> switching of a diode vs. the manual switch is in the area of 

> RELIABILITY. I'll be using manual switches for all critical loads."

>

> Not wanting to stir anything up, just want to understand the thinking 

> and factual matter in the post above?

> Thanks Bill S

> 7a  Z13/8  Z32(HD E-Bus)  Z35(7ah)

>



Not a direct answer to the question of whether a diode is more likely to 
	


fail than a switch, but a response to the method used to reach his answer.



Old pre-1970's era point-type ignitions are based around switches (the 

points). Just about every production car since then has used variations 

on diodes (doped semiconductor junctions) in all the various components 

of modern solid state ignitions (transistors, FETs, photocells, etc). 

Using his analysis technique, it's obvious that point-type ignitions are 

the way to go, since they are in fact cheaper & *much* simpler. Yet we 

don't see them in new cars.



The real question is, which is more reliable in real world applications?



Charlie

nuckolls.bob(at)cox.net
Guest

Posted: Wed Mar 12, 2008 6:47 pm Post subject: Diodes versus switches 101 ?

There was some discussion about Z19 using diodes in a critical ECU circuit 

and the question arose as to what kind of failure modes apply to diodes and 

their reliability as compared to switches.  I think the trust of that 

thread (sub list) and post below is that diodes might be "automatic" but 

switches are safer and less likely to fail?  ( I have no opinion but would 

like to understand the rationale )  This was posted and I was just 

wondering what the thought might be on the AE list since most of Bobs 

diagrams "suggest" a diode in the E-Bus circuit as opposed to a 

switch.  That might not be AS critical as the ECU/EFI circuit on an all 

electric engine but the same concerns still apply?



Posted to the Sub list:



"After 30 years in the industrial electrical construction and maintenance 

field I can say without equivocation that a robust switch is much less 

likely to fail than a robust diode. The science of the switch is much 

simpler - use a lot of good conducting material that is not likely to 

corrode and support it with stout mechanical parts that hold tightly.



Now, by contrast, the science of the diode starts with getting good silicon 

material and contaminating it in just the right proportion with just the 

right material at just the right temperature for just the right time. THEN 

you can start on building the junctions onto some sort of heat dissipating 

holder. I have replaced dozens of solid state devices that failed (always 

failed open)



    Interesting! Most solid state failures I've observed were

    shorted devices (overheated and or voltage spiked). The

    devices that did go OPEN were transistors that shorted first

    and then opened their emitter bond-wires due to over current.



  . . . and I never found the root cause of failure. I put in an exact 

replacement and the circuit worked just fine for years. The trade off you 

get for the "automatic" switching of a diode vs. the manual switch is in 

the area of RELIABILITY. I'll be using manual switches for all critical loads."



Not wanting to stir anything up, just want to understand the thinking and 

factual matter in the post above?



    About 1980 I was designing a new solid state speed

    controller and runaway monitor system for the Lear 55

    and ultimately the entire fleet of 30 series aircraft.

    It was the first time I was tasked with doing a formal

    failure mode effects analysis and mean-time-between-failures

    (MTBF) study on a new product.



    I dug out Mil-HDK-217 and began slogging through the

    part-by-part service life prediction algorithms.

    By the time some 200+ items of solder joints, transistors,

    resistors, capacitors, etc were all accounted for, I was

    pleased to turn the crank and a really nice MTBF number

    on the order of 10,000 hours fell out. But wait, there

    was this really expensive, mil-spec, hermetically sealed

    25A power relay used as a last-ditch backup disconnect.

    After factoring this device into the grand scheme

    of things, my shinning MTBF number fell to something

    on the order of 900 hours!



    Several times over the last 25 years I've asked the

    guys at Electromech's field service shop how the Lear

    trim controllers are holding up. The failure items

    they have to report were surprising. MOST of the field

    failures were in mass-terminated ribbon cable connectors

    followed by loss of ground through the mounting hardware

    that held the two boards together. Seems this was before

    I learned about having solid, on-purpose wiring grounds

    and NOT to depend on mountings that loosen and/or corrode.

    Electronically, most failures were the usual gang of

    jelly bean components, solder joints, etc. Most failures

    were in the monitor boards that had 3x the parts count

    of a controller board. There were NO failures of the

    controller that caused a runaway and a tiny fraction

    of the failures incapacitated the system. There had

    been no failures for that piece-o-#(at)(at)$, high-dollar

    relay that crashed my MTBF study.



    Bottom line: Was this relay badly misjudged? No,

    I had a pretty good understanding of contact physics

    and designed the electronics such that relay contacts

    were closed before electronics actually caused the motor

    to run. Similarly, electronics shut the motor down and

    waited some tens of milliseconds before opening the

    relay. Hence, the relay was never required to actually

    switch any power. No current was flowing when the relay

    contacts opened or closed and current was not allowed to

    flow until contacts stopped bouncing on closure.



    This is why Lear bought my design with the somewhat

    distorted MTBF numbers because in thousands of hours

    of flight, the relay never really saw any switching

    service. Nowadays, there are more accurate considerations

    of how a part is used that will provide a more realistic

    prediction of failure rates.



    Does this mean that the very robust, mil-spec relay

    is the golden child of the design and the electronics

    were left holding the bag for all the failures?

    No, most failures were due to bonding/connection

    issues. This controller resides in the vertical fin,

    just under the leading edge of the stabilizer. This

    has to be the worst environment in the airplane for

    environmental extremes. For the most part, the electronics

    seem to be living up to predictions for a long and

    quite satisfactory service life in thousands of hours.



    My personal experience since supports an assertion

    that the silicon rectifier diode has an expected

    service life that will far outpace any electro-mechanical

    device (relay or switch) that is tasked with controlling

    current flow.



    Some of my most vexing field failures problems to solve

    involve relays or switches. I've never had to chase down

    root cause for failures of energy steering diodes

    in an airplane.



    Now, let us consider the gentleman's contrary assertion.

    No doubt he has replaced blown diodes and perceives them

    to be less robust than a switch. I cannot help but wonder

    if his problem children were not subject to overheating

    and or over-voltage due to industrial line transients

    or local lightning.  Whatever the root cause of his

    observed failures, there is nothing in my experience

    or that of my colleagues in the aircraft industry that

    parallels his experience. I'm not suggesting that he is

    being untruthful; only that we're probably observing

    an apples/oranges situation. The environments

    in which our diodes live and the manner in which they

    are applied in airplanes are sufficiently different

    from his industrial environment to prevent a useful

    comparison.



    So please folks, don't rip out your diodes and put

    in switches. There are two reasons to support this

    suggestion:



    First, we design for failure tolerance.

    What are the risks if a diode DOES fail? How will

    the pilot know it? Is it pre-flight detectable? Will

    the failure cripple the system such that it becomes

    a hazard? If the answers to any of these questions

    give you cause for concern about comfortable termination

    of flight, then redesign the system to eliminate

    the potential for stress. Consider his statement:

    "I'll be using manual switches for all critical loads."



    Our common usage of steering diodes for power is

    one of TWO power paths to an e-bus. So even if it

    does fail open, it has backup. If it fails shorted,

    we can detect this during pre-flight.



    Second, there is nothing in the service history of

    diodes (or any other solid state device) to suggest

    they are recognized problem children waiting for a

    chance to ruin your day or drive up your cost of

    ownership.



    Bob . . .

Bill Schlatterer

Joined: 09 Jan 2006
Posts: 195

Posted: Wed Mar 12, 2008 8:02 pm Post subject: Diodes versus switches 101 ?

Thanks Bob, that tells me what I need to know.  Wasn't planning on taking

the diode out anyway but it's good to understand the why and why not?  My

only regret about my building process is that once I'm done, I won't have

much use for the things I have come to understand from this list. Education

is a good thing but practical use makes it excellent.  In my case, I have

learned just enough from this list to recognize what I don't know yet?  That

may be the reason I become a repeat builder 



Thanks Bill S



--

ZuluZephyr

Joined: 12 Mar 2008
Posts: 6
Location: Upland CA

Posted: Wed Mar 12, 2008 8:50 pm Post subject: Re: Diodes versus switches 101 ?

I have followed the switch vs diode controversy on a number of forums with interest since I am building a Sportsman with a Subaru H6 Engine. I am aware of crash involving a failed diode.  I believe the diode was not rated for the actual amp load which was the most likely reason for the diode failure and the ensuing controversy.



There is another design recommending the elimination of the diodes and using one switch to supply the fuel pumps, ECU and EFI.  This provides a single point of failure which I think violates the design goals of the Z19 drawing of providing redundant paths, components and circuits.



Being a committed party ( I have the engine) and wanting the simplest most reliable electrical system possible, I have designed a variation of the Z19 drawing using the same parts count with diodes (rated for the load) than includes a bypass circuit in case of diode failure (unlikely).



Please see attached drawing.  I welcome all comments and suggestions.



Rocky Morrison

Proposed Fuel Pump-ECI-EFI Circuit for Eggenfellner Subaru H6.pdf

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nuckolls.bob(at)cox.net
Guest

Posted: Thu Mar 13, 2008 2:09 am Post subject: Diodes versus switches 101 ?

At 10:57 PM 3/12/2008 -0500, you wrote:



 	  Quote: 	
		  

<billschlatterer(at)sbcglobal.net>



Thanks Bob, that tells me what I need to know.  Wasn't planning on taking

the diode out anyway but it's good to understand the why and why not?  My

only regret about my building process is that once I'm done, I won't have

much use for the things I have come to understand from this list. Education

is a good thing but practical use makes it excellent.  In my case, I have

learned just enough from this list to recognize what I don't know yet?  That

may be the reason I become a repeat builder 


	


    Correct . . . presuming that you do not take what

    you've learned and share it with others. Knowledge and

    understanding are commodities that grow in value the more they

    are given away. We all benefit every day from associations

    with individuals who are willing to share what they've learned.

    After your airplane is finished, you can satisfy your debt

    to all the teachers who participated in your success by

    "paying it forward". See:

http://en.wikipedia.org/wiki/Pay_it_forward



http://tinyurl.com/32cfym

    Bob . . .

nuckolls.bob(at)cox.net
Guest

Posted: Thu Mar 13, 2008 2:19 am Post subject: Diodes versus switches 101 ?

At 09:50 PM 3/12/2008 -0700, you wrote:



 	  Quote: 	
		  



I have followed the switch vs diode controversy on a number of forums with 

interest since I am building a Sportsman with a Subaru H6 Engine. I am 

aware of crash involving a failed diode.  I believe the diode was not 

rated for the actual amp load which was the most likely reason for the 

diode failure and the ensuing controversy.


	


   Due diligence in design cannot be discounted. It

   could just as easily have been a failure due to an

   improperly tightened nut. Had it been a fastener failure,

   is there value in responding to that knowledge by

   replacing all the fasteners in our machines with

   grade 8 hardware and metal locknuts?



   The real issue is to understand what suite of

   components are available to arrive at at an elegant

   solution and then properly apply those components

   to the task.

 	  Quote: 	
		  There is another design recommending the elimination of the diodes and 

using one switch to supply the fuel pumps, ECU and EFI.  This provides a 

single point of failure which I think violates the design goals of the Z19 

drawing of providing redundant paths, components and circuits.


	


   Consideration of design goals and failure mode

   effects are important components of the process . . .

 	  Quote: 	
		  Being a committed party ( I have the engine) and wanting the simplest most 

reliable electrical system possible, I have designed a variation of the 

Z19 drawing using the same parts count with diodes (rated for the load) 

than includes a bypass circuit in case of diode failure (unlikely).


	


   It's unfortunate that your lack of confidence leads

   you to conclude that there is value in "backing

   up" a diode. On the other hand, it's far better that

   you fly this airplane free of worry about it. Worries

   do not enhance your piloting skills and should be

   assuaged irrespective of the physics and/or probabilities.



   Bob . . .

ZuluZephyr

Joined: 12 Mar 2008
Posts: 6
Location: Upland CA

Posted: Thu Mar 13, 2008 8:41 am Post subject: Re: Diodes versus switches 101 ?

Bob,



I appreciate your comments, thoughts and efforts in helping all the builders develop more reliable and better electrical systems.  A worrier I am not, but a simple change to the design that provides backup to a potential component failure makes logical sense to me.  I plan on flying in remote places and reliability is high on my list of design goals.  That is why I am here to learn and listen, ask questions and share if I think something is worthy of the groups time.



Keep up the great work.  Your work has been invaluable to me in the building process.



Bob, Thanks!!



Rocky Morrison

longg(at)pjm.com
Guest

Posted: Thu Mar 13, 2008 10:06 am Post subject: Diodes versus switches 101 ?

Bob,

Thanks for the background and scope of the diode. That said and given

the intended reliability of the diode, I would hope my e-bus capacity

remains below 20A in my Z19 setup. Assuming the use of the B & C cooled

diode setup, should I fear any continuous load up to the 25A suggested

rating?



Thanks too for the tips on switching. I can see a few 2-10's in my

future for saving valuable panel space. Some great examples through 8-9x

in your book.



Glenn 



--

nuckolls.bob(at)cox.net
Guest

Posted: Thu Mar 13, 2008 12:29 pm Post subject: Diodes versus switches 101 ?

At 02:02 PM 3/13/2008 -0400, you wrote:



 	  Quote: 	
		  



Bob,

Thanks for the background and scope of the diode. That said and given

the intended reliability of the diode, I would hope my e-bus capacity

remains below 20A in my Z19 setup. Assuming the use of the B & C cooled

diode setup, should I fear any continuous load up to the 25A suggested

rating?



Thanks too for the tips on switching. I can see a few 2-10's in my

future for saving valuable panel space. Some great examples through 8-9x

in your book.


	


   You're welcome. I would hope your endurance bus is below

   20A too. I get about 2 or 3 drawings a month from builders

   who are asking for a review of their particular implementation

   of a Z-figure. The majority of these drawings have too much

   stuff on the e-bus.



   A review of the endurance bus philosophy might be useful here.

   If one is flying one alternator and one battery, the e-bus is

   intended to offer a minimum power load on a battery of known

   capacity for the purpose of NOT turning an alternator failure

   into an emergency. How much 'stuff' is necessary to stay on-course

   for say 1 to 2 hours with the airplane configured for en route

   phase of flight?



   This list boils down to one nav receiver, transponder, minimal

   panel lighting, etc. The nicely crafted e-bus for a one-alternator

   airplane should be reduced to 4A or less total load. Now, other

   things can drive from the e-bus but should be capable of being

   switched off during alternator out operations.



   If you have an auxiliary 8A alternator then the e-bus loads can

   be elevated to 8A total. Likewise, a 20A aux alternator would

   support a 20A e-bus . . . and perhaps there's little if any value

   in even having an e-bus.



   The 25A diode bridge recommended since day-one for the e-bus

   normal feedpath diode was selected for its MECHANICAL convenience

   of mounting and making connections via fast-on terminals. In

   no way was it intended to imply that e-bus loads could total up

   to that value. Anything over 10A should prompt a review of the

   diode installation for the purpose of deducing thermal adequacy

   of the mounting in carrying away heat.



   Most of my fused feeders to the e-bus in the Z-figures are protected

   at 7A. If you have an SD-8 second alternator, perhaps that fuse

   should go up to 10A. But if your suite of proposed equipment

   combined with an evaluation of how you plan to fly causes you

   to pile everything but the kitchen sink on the e-bus, then

   we need to talk.



   If your e-bus loads are more than 10x the battery contactor

   load then the advantages of shedding the contactor as a

   battery only load are diminished considerably.



   If most of the stuff you really need to run exceeds 8A then

   you need an SD20 aux alternator and Figure Z-12 is probably

   your minimum parts count approach to managing alternator

   out conditions. Z-12 cannot fully exploit the design goals

   for an e-bus so it would not be unreasonable to eliminate

   it entirely and run the whole airplane from the main bus

   with the SD-20 backing up the main alternator.



   That takes care of the worries about diodes and e-bus

   switches and makes the airplane simpler to operate.



   Bob . . .

mprather(at)spro.net
Guest

Posted: Thu Mar 13, 2008 1:17 pm Post subject: Diodes versus switches 101 ?

 	  Quote: 	
		   

snip
	


 	  Quote: 	
		      A review of the endurance bus philosophy might be useful here.

    If one is flying one alternator and one battery, the e-bus is

    intended to offer a minimum power load on a battery of known

    capacity for the purpose of NOT turning an alternator failure

    into an emergency. How much 'stuff' is necessary to stay on-course

    for say 1 to 2 hours with the airplane configured for en route

    phase of flight?



    This list boils down to one nav receiver, transponder, minimal

    panel lighting, etc. The nicely crafted e-bus for a one-alternator


	


Plus any devices required to remain upright (in an all-electric airplane)

- aviate, then navigate?  An EFIS (or an electric attitude indicator).



 	  Quote: 	
		  

    Bob . . .




	




Regards,



Matt-

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