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Diodes vs. Bi-Directional Zeners for Contactors

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Oct 12, 2009 4:00 pm Post subject: Diodes vs. Bi-Directional Zeners for Contactors

At 01:00 PM 10/12/2009, you wrote:

 	  Quote: 	
		  

<colyncase(at)earthlink.net>



Bob,



I agree with you about being prepared for any contactor to fail and 

not to be overly concerned about that.  A failure that results in 

not being able to operate the contactor should not in itself be a 

hazard to flight.



However, a failure that results in smoke, although in other ways 

benign, is undesirable.


	


   The fact that you've identified a potential for smoke

   suggests a point of concern in your failure mode

   effects analysis  . . .



 	  Quote: 	
		      I would be interested in what happens to the coil circuit as a 

 function of the failure mode of the suppression device.  If it 

 fails open, so what, you lost some suppression.  If it fails closed 

 I think you get a dead short on the coil circuit, in other words 

 the wire that goes to your starter key and to ground.


	


    Okay, now what?



 	  Quote: 	
		  On my airplane, I don't have that circuit fused, reasoning that the 

coil itself is going to melt down before the wire does.   but if you 

put a device in parallel with the coil and it shorts, then depending 

on the characteristics of the device you've got smoke in the cockpit.


	


    We've never fused that circuit in a whole lot

    of airplanes . . . generally, such prophylactics

    are set aside when risks are perceived to be

    insignificant . . . like 10 to the minus 6

    failures per flight hour or better. Given a demonstrated

    history that confirms that analysis then perhaps

    we can benefit from lessons-learned shared our brethren

    on the TC side of the house.

 	  Quote: 	
		  So  either you have to choose a device which you are pretty sure is 

going to fail open, or you need to fuse that circuit, right? (I 

don't see a fuse shown in the diagrams in the appendix).


	


    Yup, you've read the schematics correctly. But if you

    find that philosophy, suite of design goals and

    demonstrated service histories uncomfortable,

    please don't fly an airplane that offers you any

   discomfort. Fuses are cheap.

        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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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Oct 13, 2009 12:24 pm Post subject: Diodes vs. Bi-Directional Zeners for Contactors

At 11:58 AM 10/13/2009, you wrote:

 	  Quote: 	
		  



TYCO's document at 

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

says, "It is the velocity of the armature that is most affected by 

coil suppression. If the suppressor provides a conducting path, thus 

allowing the stored energy in the relay's magnetic circuit to decay 

slowly, the armature motion will be retarded and the armature may 

even temporarily reverse direction."



TYCO's document seemed credible until the above paragraph.


	


   <snip>



 	  Quote: 	
		  I agree with Bob.  The TYCO article is BS.


	


    Very astute observations sir. And they

    agree with my own  . . . along with conditions

    I measured on the bench. Drop-out

    delay is strongly influenced by current

    decay in the coil when the armature is

    seated. Contact spreading velocity is a

    function of BOTH coil current AND

    the AIR-GAP in the magnetic path once the

    armature becomes unseated. An air gap in

    any magnetic pathway has a huge effect. Reverse

    direction? Yeah, right.



    So once you allow coil current to fall

    just enough to unseat the armature, the rate

    at which the armature accelerates is more

    strongly influenced by the air-gap than

    by decay in magnetic field. I WAS able to

    detect perhaps 2 or 3% increase in spreading

    velocity for "fancy" versus "hammer-n-tongs"

    coil suppression . . . too small to be

    significant terms of service life.



    I also used a fast 'scope to observe differences

    in arcing during the contact break without

    and then with various coil suppression techniques.

    Yes, there were differences. So small and so

    variable that I couldn't hang my hat on coil

    suppression as having any observable effect

    on service life of the CONTACTORS and LARGE

    RELAYS under investigation.



    These simple-ideas can be observed and

    combinational effects duplicated . . . as

    well they should be. It's the repeatable

    experiment that determines whether you

    have a souffle, scrambled-eggs and cheese,

    or garbage.



    Your attention to detail sir is commendable.



    Bob . . .

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