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Rotax 912S starter relay diode

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billmileski

Joined: 07 Aug 2006
Posts: 68
Location: Ledyard, CT

Posted: Wed Sep 27, 2006 11:30 am Post subject: Rotax 912S starter relay diode

Hi,



Does anyone know if the Rotax 912 series starter relay has a built-in shunt diode?



I am considering adding a diode across my starter switch, but then I came across an application note from Tyco, regarding shunt diodes across their relays. Apparently a zener in series with a diode is better, to allow the relay to develop some EMF, otherwise the opening dynamics can be somewhat thwarted. If so, the starter relay can actually be more likely to fail to open, and/or life can be shortened.  Interesting read at http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf



Anyway, first thing's first. Anyone know if the 912's starter relay has a built-in shunt protection system of any sort?



Thanks,



Bill Mileski

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nuckollsr(at)cox.net
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Posted: Thu Sep 28, 2006 4:37 am Post subject: Rotax 912S starter relay diode

At 12:30 PM 9/27/2006 -0700, you wrote:



 	  Quote: 	
		  



Hi,



Does anyone know if the Rotax 912 series starter relay has a built-in 

shunt diode?


	


   Don't know. If you've got access to a 'scope, it's

   easy to find out. Even easier to add a diode just for

   grins. Two diodes doesn't hurt a thing, no diodes

   is hard on starter switches.

 	  Quote: 	
		  I am considering adding a diode across my starter switch,


	


    Not across the switch, across the contactor coil. See



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



 >   . . . but then I came across an application note from Tyco, regarding 

shunt diodes across their relays. Apparently a zener in series >with a 

diode is better, to allow the relay to develop some EMF, otherwise the 

opening dynamics can be somewhat thwarted. If so, the starter >relay can 

actually be more likely to fail to open, and/or life can be 

shortened.  Interesting read 

at >http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf



    This document is an excellent example of poor decisions

    made on poor science. The title: "Coil Suppression Can

    Reduce Relay Life" certainly gets your attention. The

    writer goes to some effort to explain how coil suppression

    can have a profound effect on RELAY RELEASE TIME which

    is easy to demonstrate and explain. Then in the last

    5 paragraphs on page one, launches into some assertions

    about the effects of a slowed decay of the magnetic field

    on contact separation speeds but no demonstrations or

    even a mathematical modeling of the event to support

    the assertions.



    This discussion came up on the List some months ago.

    I've excerpted portions of that discussion below:



-------------------------

 	  Quote: 	
		  Coil Suppression:



MOVs are considered to be better than diodes, although they have higher 

impedance. But MOVs typically have a limited lifetime.


	


    The lifetime issue controls when the MOV is stressed

    repeatedly to its maximum rated energy levels . . . in

    coil spike suppression, the energies are tiny by comparison

    and life-limits do not become an issue.

 	  Quote: 	
		  Diodes are not the best method today. Not even the second-, third-, or 

fourth- best method. But in the 1960's they were the way to go.


	


    <snip>



    Measurements on my bench have failed to demonstrate

    the suggestion.



    A number of papers have been cited over the years, some

    written by some folks who work for big name companies

    like Tyco-Amp, Teledyne, etc. wherein authors have suggested

    that the readily observable effects of plain diodes on

    opening delay (no big deal in 99.99% of applications)

    directly translates to slower contact spreading velocity

    (extended arcing during contact break).



    Two separate conditions are in play. (1) slower rate

    of decay in the relay or contactor's coil current and (2)

    rate of decay in the magnetic attraction force as the

    moveable armature begins to separate from its seated

    position within the device. The diode has a profound

    effect on (1) but a small effect on (2).



    Most of the papers I've read accurately observed

    and discussed (1) and even did some measurements but

    then went on to improperly assume that similar effects

    would be noted for (2) as well.



    The trace at:



http://www.aeroelectric.com/Pictures/Curves/704-1DelayNoDiode.gif



    Shows contact OPENING DELAY of an S704-1 plastic high

    current relay when no diode was present across the coil

    and the coil current was being interrupted by the "perfect

    switch". Note the expected coil spike on channel 2 and the

    opening delay of about 2.5 mS. You can see the

    arcing across spreading contacts if you look carefully

    at the falling trace on channel 1.



http://www.aeroelectric.com/Pictures/Curves/704-1DelayWithDiode.gif



    This trace shows what happens when we use the plain-vanilla

    diode across the coil. Yes, OPENING DELAY goes up by a factor

    of 500% to about 12.5 mS.



    Now, let's go take a close look at the arcing phenomenon noted

    in the two traces above . . .



http://www.aeroelectric.com/Pictures/Curves/704-1OpeningTimeNoDiode.gif



    . . . with no diode, a series of about 10 measurements produced

    an opening time (ARC DURATION) that averaged 210 uS.  In the

    next trace . . .



http://www.aeroelectric.com/Pictures/Curves/704-1OpeningTimeWithDiode.gif



    . . . the diode was put back on and we see an average of

    230 uS ARC DURATION for an increase of about 10%



    I did similar experiments with other relays and did not

    formalize the data gathering but got similar results.



    Bottom line is that the use of the lowly diode for coil

    spike suppression does not deserve relegation to the dust

    bins of electronic history as a 5th-rate spike suppression

    technique. Unfortunately, some big names working for big

    companies have stubbed their toes on significant but error

    driven assumptions.



    The repeatable experiment has demonstrated otherwise.

    Continued use of diodes as suggested in the 'Connection

    and on many drawings posted to the website is not a

    recipe for failure. Substitution of a more "modern"

    technique will produce no observable effect on the

    service life of your relays and contactors.

------------------------



    Demonstrable differences for contact spreading velocity

    and potential to arc for diodes versus any other choice

    are tiny. Relay and switch lives in industrial parlance

    is stated in the tens of thousands of cycles. There are

    few switches and relays in our airplanes that will get even

    1,000 cycles in the lifetime of the airplane. Nonetheless,

    we suffer significant switch and contactor failures . . .

    mostly for  environmental stresses over long periods of time . . .

    commonly known as old age.



    There's nothing inherently BAD about coil suppression

    techniques other than plain diodes but nothing overwhelmingly

    good about them either.



    Bottom line is use what ever technique suits your fancy

    but don't discount the lowly diode based on the assertions

    of folks who demonstrate one effect and then extrapolate

    a second effect without supporting data. Further, be

    especially wary of papers like the one cited above where

    prestige of either people or the companies they work for

    is the motivating force to do or not to do any particular

    thing.



    I've sat across the table many times with folks from

    big name companies attempting to deduce root cause of

    failures for their products on our products. When the

    problem is finally solved, it's all too often that we

    end up knowing more about the supplier's product than

    they do.



    Bob . . .

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billmileski

Joined: 07 Aug 2006
Posts: 68
Location: Ledyard, CT

Posted: Thu Sep 28, 2006 12:07 pm Post subject: Re: Rotax 912S starter relay diode

Thanks for the info, and the pdf reminding me that what I really wanted was a diode across the coil, not the switch. And I'll use a scope as you suggested if no protection seems to be already in place.



Regards,



Bill

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