Today's Message Index:
----------------------
1. 02:41 AM - Test (Bill Settle)
2. 07:29 AM - Re: Re: "Coat Hanger" antenna (Robert L. Nuckolls, III)
3. 08:09 AM - Re: Z-19 Series Architecture (Mark Sletten)
4. 08:53 AM - Re: Z-19 Series Architecture (mikef)
5. 09:50 AM - Re: Re: Z-19 Series Architecture ()
6. 10:33 AM - Re: Re: Z-19 Series Architecture (Robert L. Nuckolls, III)
7. 10:34 AM - Re: Re: Z-19 Series Architecture (Robert L. Nuckolls, III)
8. 02:33 PM - Re: Diodes versus switches 101 ? (Charlie England)
9. 05:31 PM - Re: switch wiring (Ken)
10. 06:57 PM - Re: switch wiring (Robert L. Nuckolls, III)
11. 07:47 PM - Re: Diodes versus switches 101 ? (Robert L. Nuckolls, III)
12. 09:02 PM - Re: Diodes versus switches 101 ? (Bill Schlatterer)
13. 09:56 PM - Re: Diodes versus switches 101 ? (ZuluZephyr)
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Subject: | Re: "Coat Hanger" antenna |
At 05:30 PM 3/11/2008 -0700, you wrote:
> >From: "Robert L. Nuckolls, III"
> <<mailto:nuckolls.bob@cox.net>nuckolls.bob@cox.net>
> >Subject: Re: "Coat Hanger" antenna
>
> >>At 07:59 AM 3/8/2008 -0800, you wrote:
>
> >>No offense to Ernest or Bob, the old coat hanger
> >>antennas that you terminated the coax with crimp on
> >>lugs has not been used in production airplanes since
> >>the 50's or early 60's.
> >>
> >>Coat hanger antenna
>
>
> >><<http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php>http://www.aircraftspruce.com/catalog/avpages/av534.php
> >>
> >>What Eric says about "EM field energy", which I recall
> >>bits from physics and armature radio, the coax,
> >>BNC connector is way more efficient. That inch of
> >>exposed shield/core & important lost insulation does
> >>count. The "coat hanger antenna connections are
> >>subject to corrosion and fatigue way more than a BNC
> >>connector. Just my opinion.
>
> >But without quantification. The advantages of the
> >modern antennas are mainly mechanical. They don't
> >twist in the insulator. In high-dollar models there
> >are p-static immunities due to the DC grounded
> >fabrication . . . and they look sexier and have lower
> >maintenance costs.
>
>Bob its more than sexy it is indeed that last inch of exposed
>dielectric, often close to some conductive material, even
>in a composite plane (don't they need a ground plane for a 1/4
>wave dipole?) that is the issue, at least to me.
Sure. But let's consider the "loaded" antenna. An antenna that's
physically too short for efficient operation at the desired frequency.
The ideal, full sized quarter-wave mobile antennas operating at say 14 Mhz
would LIKE to have about 17.5 feet of antenna on the bumper. See:
http://tinyurl.com/2dyfss
This is not practical on a moving vehicle so some means of electrically
lengthening a "too short" antenna is called for. This is done in a variety
of ways but all involve adding some reactive component (inductive or
capacitive)
to the antenna.
My last HF mobile installation featured a 4' stub mast, an adjustable
center loading coil and an 8' whip antenna above all this. Here's
a modern example of this style of artificial antenna lengthening:
http://www.hiqantennas.com/images/MVC-462.jpg
Let's examine a practical example where there's value
in making an antenna shorter while minimizing effects on
performance. A few years ago I got a call from the production
line on some new ELT transmitters that were shutting down
with a high SWR fault. Here's the antennas in question:
http://tinyurl.com/2u5c8u
The original system integrator installed dual antennas
(125.5 and 406 Mhz) under the fiberglas 'instep cap'
of the vertical fin. There were some composite
air ducts with metallic components also located
in the fin cap. These not only made it necessary to
fold the longer antenna over, proximity of the metal
further de-tuned it. See:
http://tinyurl.com/38symp
Older versions of the ELT transmitter would tolerate
this degradation of antenna performance, the new ones would
not.
I wrote a white paper suggesting a variety of options.
The elegant solution was a top-loaded antenna physically
short enough to mount under the fin cap and avoid
close proximity to the duct-metal.
http://tinyurl.com/2vv5e4
I crafted an exemplar antenna that was only 9" tall
and took it to the lab for comparison with a full
sized 24" antenna. It's radiation angle was higher
and maximum radiation levels were down by less than
2 dB from the full sized quarter wave. Entirely
satisfactory for this installation.
We could have considered some other scenarios. See:
http://tinyurl.com/3aaocs
There are an infinite number of combinations of
top, center and base loading that would minimize
SWR at the frequency of interest. However, all versions
other than top-loading increased fabrication labor
and produced a lower performance.
Let's take the ideas illustrated above and consider
when one chooses to use the stone-simple, rod and
feed-thru antenna.
Open conductors have inductance. As a rule of thumb,
we ball-park an open wire (and in this case, an extension
of the antenna base below the ground plane) at 20 nH/inch.
Assuming 1.5" of bare conductors total, we can expect
something on the order of 30nH of series inductance at
the base of the antenna. This works out to about 25 ohms
of reactance at 130 Mhz. A full length, 24" antenna's
center frequency would shift downward due to this loading
inductance.
A purest would get out the antenna analyzer and trim
the antenna's physical length (shift toward capacitive
reactance) to balance reactive effects of the base loading
inductance.
The next time I have occasion to get the test ground
plane out, I'll take a look at the effects of exposed
conductors at the base of the stone-simple antenna
and report the amount of shortening required to shift
the antenna's center frequency back to the design
optimum.
These exposed conductors ARE at the base of the
antenna where the highest currents are encountered
which go hand-in-hand with the highest radiation.
So indeed, we are squirting some RF into the aircraft's
interior spaces that would not be present with other
antennas . . . but in practice this is seldom a problem
either for EMC issues or overall performance.
>
> >>No one is seriously using the coat hanger wire antenna
> >>on new OEM aircraft any more.
>
> >for the reasons stated plus some others but
> >none related to observable performance.
>
>Bob I have to take your word for it. However in my experience
>I have seen these antennas SRW go up and up and up with time
>because of dissimilar metal corrosion and bad crimps.
Absolutely! But connectors corrode too . . . and
suffer from bad craftsmanship at assembly. I tossed
out the large-area, low-pressure ground terminal supplied
with the stone-simple antenna and substituted a PIDG
terminal properly bonded to the skin with a 10-32 screw
torqued up tight. Similarly, I made sure that the
PIDG terminal at the base of the antenna had the
super-mash on it between two nuts. If you don't
have gas-tight joints, they will degrade with time.
> Like I
>said most pilots only demand short range line-O-sight
>communication, not DX comm. The term observable
>performance should include maintenance and durability in
>my humble opinion.
To be sure. The DIY antennas do have cost of ownership issues
that are strongly affected by craftsmanship of the original
installation. But if one understands the potential 'gotchas'
with ANY antenna and exercises due diligence for craftsmanship,
there's good value to be realized from either of these
antennas.
't know what this "ducting" stuff is.
>
>Well Wikipedia to the rescue:
><http://en.wikipedia.org/wiki/Coaxial_cable>http://en.wikipedia.org/wiki/Coaxial_cable
>
>I use the word "ducting" in an informal way to describe how
>a coaxial cable propagates elect mag wave energy.
>
> "Coaxial lines solve this problem by confining the
> electromagnetic wave to the area inside the cable,
> between the center conductor and the shield. The
> transmission of energy in the line occurs totally through
> the dielectric inside the cable between the conductors."
>
>"ducting" - was a casual my way of describing how coaxial
>propagates EMW. Where open wire transmission lines, like
>you have that inch before the "coat hanger" antenna, two
>parallel wires, have the property where the electromagnetic
>wave propagating down the line, extends into the space
>surrounding it, an undesirable characteristic. You say the
>loss is small? OK I guess, sure but every little bit helps
>or hurts. The fact is radios are so good with filters and
>excellent sensitivity and stability, a less than optimal
>antenna can be some what tollerated.
Understand.
>
>
> >"Weak" is un-quantified. However, just like concerns about
> >"iron poor blood" it may help sell alternative antennas.
>
>True it is unqualified, but we could get into dB or signal strength,
>but all I am saying is in Aviation we are talking about strong
>signals at short distances, most of the time, period. However if
>you want real long range communications at distance than it
>does matter. Yes "matter" is an unqualified term as well.
Wouldn't argue that the stone-simple antenna does not
represent the best-we-know-how-to-do. But neither is
a Toyota starter bolted to a Lycoming on an adapter
plate carved out on a bandsaw.
I would recommend that any builder take advantage of
the convenience of pre-fabricated, optimized antennas.
But I think we would be remiss as teachers if we did not
define the simple-ideas that control performance
and service life of the DIY approaches. I would not
discourage anyone from doing the best they can with
what they have to work with . . . whether by choice or
circumstance.
<snip>
>
> >Bottom line is that we're fabricating OBAM aircraft.
> >Yes, there are sexier, more convenient and probably
> >more 'efficient' antennas to be considered over the
> >stone-simple, rod and feed-thru insulator antennas
> >that were quite popular 40 years ago. It all comes
> >down to the builder's preferences for trading his/her
> >personal expenditure of $time$ versus purchasing
> >the product of someone else's $time$ for a more
> >sophisticated design.
>
>Well sexier I guess, but cost is only times 2 and when you
>have a $1000 or $3000 radio, $60 more is a drop in da bucket.
Agreed. But one can do the stone-simple antenna with
self procured components for much less than the $60
sale price . . . and there are very few builders who
have $3000 tied up in a radio.
>
> >Performance risks for going the low-dollar, DIY
> >route are low and easily managed.
> >
> >Bob
>
>Well I guess we disagree with the last part, but I agree
>making antennas and testing them is FUN! VHF on a plane
>is pretty easy, and I have no problem with a DIY, however
>as I said in my opinion, if you are going to buy an antenna,
>like most builders, get a $120 antenna with a BNC connector.
>My focus is all towards metal planes, composites have more
>flexibility to experiment with internal antennas.
>
>A BNC will be more secure & robust. You can have BNC
>problems and other issues, but keeping the coaxial cable
>intact to the last inch, is not totally trivial.
True. We could build a bracket to mount the connector
but it needs to be attached to the airframe with rivets,
not by assembly forces of the ceramic feed-thru insulator.
The problems of establishing gas-tight joints at both
the ground plane and at the antenna feed-point are the
same for both metal and plastic airplanes. See:
http://tinyurl.com/yu38rz
I would hope that the efforts I made toward that goal
over 35 years ago paid off in trouble-free performance
over the lifetime of the installation. The last time
I had occasion to install a rod-n-cone antenna was
on a wrecked straight-tail, C182 during the rebuild.
It was a belly mounted antenna for a #2 nav/com.
The antenna survived the accident but the belly skin
was wrinkled and got replaced.
>
>With a composite plane the connection will be on the "back
>side of the ground plane? Right? There will be signal loss and
>reflected RF into the airplane, IMHO.
True. The effects are non-zero but for most users
are not operationally significant.
>
>Good discussion. The proof is in the eating of the pudding.
>Fly a plane with both antennas and test them against a
>ground station for both TX and RX. I think you will see a
>difference in one antenna getting a weak incoming signal
>and getting out a stronger signal. Of course bent whips
>are a compromise. The radiating element should be as
>perpendicular to the ground plane as possible but most
>pilots like the looks and lower drag of the bent whip for a
>little more SRW. Also the "Blade" high speed antennas
>have "wider" band width. There are good enough antennas
>and better, best antennas. It is definitely esoteric but for
>just secure connection the BNC has it over the crimp
>and screw exposed twin lead approach, at least in the
>VHF band of freq's, in my opinion. Old ways are still
>good but there is a reason new methods & connections
>where developed. It may take more sensitive lab equip
>to notice but I can't believe it makes NO difference,
. . . no perceivable difference from the pilot's
seat. We often get wrapped around a performance axle
trying to tweak things based on test measurements
that return no operational benefits. I had one
instructor tell me there was benefit after a
balked landing to turn off the alternator so that we
could get more horsepower to the prop! Intuitively
we know there's a measurable difference but
the practical benefits are someplace between zero and not
much.
> . . . but than you say you tested it, and I have not.
All of the testing has been in the lab were we
do indeed see quantifiable differences. But in
practice, tens of thousands of these antennas have
flown and offered the user good return on investment
too.
> All I can go
>by is when trouble shooting poor radio performance on
>old planes, the first place I look is at those old corroded
>connections on the "coat hanger".
All very much on point . . . and I would suggest that
any readers of this List make note of the fact that
some installations are more deserving of preventative
maintenance than others. Use of good assembly practice
when making up the joints will forestall if not eliminate
future maintenance events.
Bob . . .
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Subject: | Re: Z-19 Series Architecture |
Listers,
I've posted this question a couple of times via the Matronics Forum interface,
but I've not gotten a response.
My posts are coming back to me via the daily list digest, but I'm not sure Bob
is getting them. Should I email directly to the Matronics list instead of posting
via the Forum?
Regards,
Mark
Mark Sletten wrote:
> Bob,
>
> On the Z-19 rev M and Z-19/RB rev A drawings the E-BUS normal feed is from
> the Main PWR Dist block thru a 7A breaker and diode.
>
> On all other drawings it appears the E-BUS normal feed is directly off the
> Main PWR Dist Block binding post (not a breaker) thru a diode.
>
> Is there a reason for the different setup on the Z-19 series?
>
>
> Regards,
>
> Mark Sletten
--------
Mark Sletten
Legacy FG N828LM
http://www.legacyfgbuilder.com
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=169244#169244
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Subject: | Re: Z-19 Series Architecture |
Mark,
Your posts are getting through, I've not seen any replies to your question either.
There was a related Z19 question a few weeks before xmas about updating the Z19
to reflect separate power feeds for coils (and possibly injectors). No further
replies seen on that either.
Mike
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=169263#169263
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Subject: | Re: Z-19 Series Architecture |
Mark,
I questioned that too. Not sure I ever got a strait answer. As far as I
am concerned that constitutes the weakest link for what is supposed to
be an essential bus system.
I did see something Bob mentioned about the feed coming from both ends.
One side is fed from the 7A fuse, and the other from the switch which is
powered via the backup battery via its contactor switch. I believe Bob's
logic is that one will backup the other. If you turn the switch off, the
e-bus still has 7A and if you lose the 7A job, the switch will power the
e-bus through via the backup battery contactor. The diode keeps the
juice from going back other way (through the fuse) when powered by the
switch.
Earlier I questioned the 7A limit, but if you start breaking down the
load, that can support a lot of today's panel. A Dynon and an sl30 are
still within values. That's all I need to get down. The fuel pumps,
ignition and engine stuff would all feed off of the secondary power
switch not the e-bus.
To hell with idealism - for my Sub, the e-bus will work both ways based
on the intended design or an improvement in it. 7A is not going to make
anything glow. In an emergency we'll be glad for the 7A.
What I think is missing or confusing is the process (a standard issue
for technos). Be sure you have a sound practice for throwing switches
and knowing what is working etc. I may even include a warning light
which closes if that ole 7A takes a pill.
How'z that project going?
Glenn
http://n661gl.blogspot.com
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Mark
Sletten
Sent: Wednesday, March 12, 2008 11:06 AM
Subject: AeroElectric-List: Re: Z-19 Series Architecture
--> <marknlisa@hometel.com>
Listers,
I've posted this question a couple of times via the Matronics Forum
interface, but I've not gotten a response.
My posts are coming back to me via the daily list digest, but I'm not
sure Bob is getting them. Should I email directly to the Matronics list
instead of posting via the Forum?
Regards,
Mark
Mark Sletten wrote:
> Bob,
>
> On the Z-19 rev M and Z-19/RB rev A drawings the E-BUS normal feed is
> from the Main PWR Dist block thru a 7A breaker and diode.
>
> On all other drawings it appears the E-BUS normal feed is directly off
> the Main PWR Dist Block binding post (not a breaker) thru a diode.
>
> Is there a reason for the different setup on the Z-19 series?
>
>
> Regards,
>
> Mark Sletten
--------
Mark Sletten
Legacy FG N828LM
http://www.legacyfgbuilder.com
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=169244#169244
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Subject: | Re: Z-19 Series Architecture |
>Mark Sletten wrote:
> > Bob,
> >
> > On the Z-19 rev M and Z-19/RB rev A drawings the E-BUS normal feed is from
> > the Main PWR Dist block thru a 7A breaker and diode.
> >
> > On all other drawings it appears the E-BUS normal feed is directly off the
> > Main PWR Dist Block binding post (not a breaker) thru a diode.
> >
> > Is there a reason for the different setup on the Z-19 series?
No reason other than observance practice for protecting
any small wire feeder of 6" or more in length. If the e-bus
is located right next to the main bus fuse block, then you
can make direct connections through the diode without
fuses or breakers to protect the wires.
Bob . . .
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Subject: | Re: Z-19 Series Architecture |
At 08:50 AM 3/12/2008 -0700, you wrote:
>
>Mark,
>
>Your posts are getting through, I've not seen any replies to your question
>either.
>
>There was a related Z19 question a few weeks before xmas about updating
>the Z19 to reflect separate power feeds for coils (and possibly
>injectors). No further replies seen on that either.
I'm not in a position at present to modify the
Z-figures for the purpose of customizing. Remember,
these are intended to illustrate architectures that
offer attractive failure modes effects analysis,
I.e, failure tolerance.
Exactly what loads are tied to what busses by what
sizes of wire/fuse is up to the builder to work out
as appropriate to their particular engine and
suite of appliances.
Bob . . .
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Subject: | Re: Diodes versus switches 101 ? |
>
> 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
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Subject: | Re: switch wiring |
Kevin
You may need a starter contactor or relay but an ON-OFF-momentary ON
switch can probably be wired to do the mags like that. I don't know if
you will find such a ready made diagram though (or anyone to recommend
it), as I think such a scheme would greatly increase the risk of injury
from a live mag. I can understand an OFF-Batt-Alt switch but generally I
try to avoid those 3 position switches as I find them awkward,
especially at night.
Also it may be more common to use a separate switch for each alternator.
That way one failed switch can't disable both alternators.
Ken
Kevin Klinefelter wrote:
> <kevann@gotsky.com>
>
> Bob and all,
>
> I have a couple questions.
>
> I am wiring a Rotax 914 in my Europa per Z-16(sort of). I want to use a
> switch for each "mag" that will be down-off, middle-on, momentary-up of
> both to start. Is there a drawing somewhere on the site that shows how
> to wire this?
>
> I also want to wire a switch to control two alternators, the rotax built
> indynamo and a Denso IR alternator mounted on the vac pad. I would like
> to use one switch; down-dynamo middle-off and up-main
> alternator(denso).Is there a drawing showing how to wire that?
>
> Thanks, Kevin
>
Message 10
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Subject: | Re: switch wiring |
Kevin Klinefelter wrote:
Bob and all,
I have a couple questions.
I am wiring a Rotax 914 in my Europa per Z-16(sort of). I want to use a
switch for each "mag" that will be down-off, middle-on, momentary-up of
both to start. Is there a drawing somewhere on the site that shows how to
wire this?
I also want to wire a switch to control two alternators, the rotax built
indynamo and a Denso IR alternator mounted on the vac pad. I would like to
use one switch; down-dynamo middle-off and up-main alternator(denso).Is
there a drawing showing how to wire that?
Thanks, Kevin
For the ignition switches you need two, two-pole,
three position, center off, one side momentary
devices.
Another way to describe this switch is 2P3T on-off-(on).
This would be a B&C S700-2-5 switch. See:
http://www.bandc.biz
Wire one side of each switch to their respective ignition
modules such that the switch is closed in the down position
and shorts the ignition module to an OFF condition.
Wire the other sides of the two switches in series such
that both have to be lifted to the START position to
engage the starter contactor.
Recommend you keep the two alternator switches separate.
You don't want failure of one switch to cause both alternators
to become unavailable to you.
Bob . . .
----------------------------------------)
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
----------------------------------------
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Subject: | Re: 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-#@@$, 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 . . .
Message 12
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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
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com
[mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Robert L.
Nuckolls, III
Sent: Wednesday, March 12, 2008 9:41 PM
Subject: Re: AeroElectric-List: Diodes versus switches 101 ?
--> <nuckolls.bob@cox.net>
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-#@@$, 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 . . .
Message 13
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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
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=169493#169493
Attachments:
http://forums.matronics.com//files/proposed_fuel_pump_eci_efi_circuit_for_eggenfellner_subaru_h6_233.pdf
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