Ceengland
Joined: 11 Oct 2020
Posts: 391
Location: MS
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 Posted: Fri Jan 13, 2023 11:08 am Post subject: is this a MOV and should I remove or replace it? |
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On 1/13/2023 12:41 PM, Robert L. Nuckolls, III wrote:
| Quote: | At 04:12 PM 1/9/2023, you wrote:
| Quote: | Hello Bob,
I tried researching this on your forums and did not find an answer. I
also tried vans airforce and stumped them there for the most part.
Also stumped the eaa techs in my hangar.
My RV4 has an electrical diagram that apparently you created, in 1993,
for Ed Beach. It shows a MOV in parallel with the master switch and
the starter switch.
See the attached images. I find this potted block with 4 wires going
into it, 2 wires from the master and 2 from the starter switch. The
block looks a little burned on top, though I am not sure what that
actually is. I only just found the block as I am doing some minor
panel upgrades.
Are you familiar with a MOV in this form factor? Is it an actual MOV
or diode, etc? Any advice on what to do with it (everything works
fine as is).
Thanks,
Dave K
PS here is a link to my question on VAF
https://vansairforce.net/community/showthread.php?t=213172 |
An excellent question my friend . . . thank you
for bringing it up. Indulge me for a few moments
of study in the history of MOVs (Metal Oxide
Resistor) in DC power systems . . . but your
welcome to skip to the short answer further
down.
In the earliest days of my participation in
OBAM (Owner Built and Maintained) aircraft,
I was some 20 years into a professional history
with certified aviation. My jobs to that point
were mostly focused on black box ('appliance' in
FAA-speak) design, development and certification.
This included a good working relationship with
the systems folks but I had little need or
interest in studying the physics of their
activities.
"Spike" was a popular but ill-defined concept
that circulated amongst individuals with
experience that ranged from the shade-tree
mechanics to IR&D scientists. While design
and qualification protocols already cited
technical publications like DO160 and Mil-STD-704
(quantified definitions of potential
threats), there was a pervasive, popular concern
for "spikes" being generated somewhere within
the system that pose functional threats to
"sensitive/vulnerable" components in the
system.
In my first tour of duty at Cessna (Circa
1965), I recall the first studies of stored
energy on inductive devices (like battery
contactors and large relays).
One of our electro-wizzards was
a PHd physicist named Gordon Wood who bought
a Hewlett-Packard, peak-reading voltmeter.
This instrument captured and quantified
battery contactor "spikes" on the order
of hundreds of volts.
WOW! . . . we sure wouldn't want
that kind of threat wandering aimlessly
about the electron highways in our
airplanes!
Contemporary avionics were just starting
to receive their first transistors . . . too
many of which were failing for reasons
we didn't exactly understand . . . but
for sure . . . the easy reaction was to
hunt down and throttle those 'killer' spikes.
That was the era when silicon junction
diodes were becoming commonplace. There
was some prior art in this arena wherein
we find selenium rectifiers tied across
contactor coils for the purpose of suppressing
their stored energy during coil release.
Cessna created a number of diode assemblies
designed to make them assembly-line-friendly.
The rest of the small aircraft community
was similarly invested in 'taming
spikes'.
I was not involved in any design decisions
or problem investigations. However, as a
tech writer, I needed to be plugged into
what was going on as it was my job to
explain it to field technicians. The
down-side of this relationship was that
I would gain no more understanding of the
fundamentals than was discovered and shared
by my compadres across the isle.
So . . . from about 1965 to 1985 or so,
I had little cause to ponder the necessity
for putting diodes across the contactor
coils in airplanes.
Then about 1988 the seeds were planted
for publication of The AeroElectric
Connection when I found myself gaining
a toehold in Owner Built and Maintained
aircraft. By then, the value of adding
contactor coil energy suppression was
'old news' but as a tech writer by 'birth'
I pondered ways to introduce the practice
and physics to my newest readership.
Diodes were a natural solution to spike
suppression but I'd become aware of a
new 'spike warrior' on the block. Unlike
the polarity sensitive diode, the Metal
Oxide Varistor (MOV) didn't care about
polarity of the offending energy. Cool!
one could purchase low voltage versions
of the MOV (18V or so) and those should
tame the wily spikes and you couldn't
wire them up wrong . . . they didn't care.
So when I included them in first generation
Z-figures, I thought I was doing a good
thing . . . yeah . . . well . . .
In many years since, we've had numerous
spike suppression discussions here on the
List and I've published some articles on
the website. My own understanding of
MOV functionality expanded in during
my tours of duty in general aviation and
bench studies in support of the 'Connection.
To my dismay, I discovered that the MOV
does not have a sharp conduction threshold
such that an 18V MOV could allow a contactor
energy spike to rise to as much as 50
volts . . . while useful as a prophylactic
against damaging spikes propagating through
a SYSTEM, they would not serve to quench
formation of the arc that strikes between
the opening contacts of a SWITCH that controls
the contactor! A diode, on the other hand,
suppressed the spike to ZERO.
MOVs were subsequently replaced with diodes
in the Z-figures and in our discussions
here on the List. Hence, THE SHORT ANSWER:
I would replace any MOV's found on your ship's
contactors with silicon rectifiers . . . current
and voltage ratings not critical. You're more
concerned with mechanical robustness and
user-friendly installation. I recommend 3A
devices of any voltage rating. 1N5400 through
1N5406 are suitable. These are cheap and
rugged.
Here's a good way to buy them:
https://tinyurl.com/2p82ml35
. . . a fist full of devices for $10 delivered
to your door.
Some of you old timers may recall discussions
on the List where I explored spike management
philosophies and physics with a couple of readers
about 20 years ago.
One such thread included some bench test
demonstrations captured and illustrated
in this document.
http://aeroelectric.com/Reference_Docs/Contactors-Relays/60507_1_Spike_Suppression_Studies.pdf
I'm still trying to find the narrative that
goes with these scope traces. Suffice it to
say that I kinda stubbed my toe with the MOV
experience. The lowly silicon rectifier has
proven to be the elegant solution for contactor
spike suppression . . . in 1965, 2002 and
today.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
|
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_________________
Charlie |
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