Matronics Email Lists :: View topic - is this a MOV and should I remove or replace it?

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

is this a MOV and should I remove or replace it?

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 13, 2023 10:41 am Post subject: is this a MOV and should I remove or replace it?

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?"

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

johnbright

Joined: 14 Dec 2011
Posts: 165
Location: Newport News, VA

Posted: Fri Jan 13, 2023 11:44 am Post subject: Re: is this a MOV and should I remove or replace it?

And note that Bob Nuckolls teaches us to put the flyback suppression diode across the contactor or relay's coil, not the switch to the coil. I mention this because the OP's schematic from 1993 shows the MOV across the switch.



http://aeroelectric.com/articles/spikecatcher.pdf

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

_________________
John Bright, RV-6A, at FWF, O-360
Z-101 single batt dual alt SDS EM-5-F.
john_s_bright@yahoo.com, Newport News, Va
N1921R links

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum