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mikeeasley(at)aol.com
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 Posted: Mon Oct 12, 2009 3:24 am Post subject: Diodes vs. Bi-Directional Zeners for Contactors |
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I'm building a new power grid for a customer and the research on contactors ended up with some digging on the diodes that we use across the coils of the contactors, and whether they're necessary. We're using a couple Tyco Kilovac EV200s and since they have pigtail wires instead of terminals, it's tougher to install the diodes. Tyco says we don't need "back EMF protection" with the EV200s. But we are also using the LEV100 contactors, which do need the back EMF protection.
Tyco has a couple documents that explain the various methods to handle the back EMF.
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf
Eric at Perihelion Design sells SnapJacks, and claims that they are superior to using diodes. Eric's claims match up with the information in the Tyco documents. Both Eric and Tyco claim that bi-directional zeners have all the advantages of protecting the switch without slowing down the contact separation speed, which decreases contactor life.
http://www.mouser.com/ProductDetail/STMicroelectronics/P6KE36CA/?qs=E0mHVHmM7ubVknTkdnW39Q%3d%3d[/url]
[url=http://www.periheliondesign.com/suppressors/SnapJack.pdf]http://www.periheliondesign.com/suppressors/SnapJack.pdf
One concern I have is the failure modes of diodes and bi-directional zeners. If they fail open, no big deal, but if either has a failure mode that would create a short, that could cause problems. I read somewhere that high voltage tends to result in a short failure, but high amperage results in an open failure. There's no circuit protection on these coils (except for the starter).
I'm interested in comments on the relative merits of using diodes vs. bi-directional zeners. Also, the failure modes of both devices. And how to size the bi-directional zeners to minimize the chance of a failure, especially a short failure.
Mike Easley
Lancair Super ES
Colorado Springs
[quote][b]
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echristley(at)nc.rr.com
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| Posted: Mon Oct 12, 2009 4:08 am Post subject: Diodes vs. Bi-Directional Zeners for Contactors |
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mikeeasley wrote:
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I'm interested in comments on the relative merits of using diodes vs.
bi-directional zeners. Also, the failure modes of both devices. And
how to size the bi-directional zeners to minimize the chance of a
failure, especially a short failure.
What would happen if either were fail shorted due to high voltage?
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You'll have a device with small gauge leads laying directly across the
terminals of a battery that should be capable of delivering upwards of
200A. In another context, we would call this a "fusible link". I
suspect that the failure mode due to high current will quickly follow
the short. If you're really worried, install the protection just like a
fusible link (inside a piece of fiberglass sleeve) and be done with it.
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longg(at)pjm.com
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| Posted: Mon Oct 12, 2009 6:15 am Post subject: Diodes vs. Bi-Directional Zeners for Contactors |
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This topic has been back and forth for a long time. It's sort of like the ball bearing analysis we did way back in calculus class. Imagine that, we had to use calculus to sort out the life expectancy of ball bearings.
I would be interested to hear from anyone who has flown either system long enough to distinguish either benefit or failure time between the two. If not, one view of discrimination may simply be marketing.
I suppose I could fly my diode system for 60 hours /week until I'm dead and never determine the supposed Zener benefit.
To keep it in perspective, it's a 12 volt battery, not a warhead. Go out and fly.
Glenn
Do Not Archive
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nuckolls.bob(at)aeroelect
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| Posted: Tue Oct 13, 2009 8:51 am Post subject: Diodes vs. Bi-Directional Zeners for Contactors |
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At 08:56 AM 10/13/2009, you wrote:
Bob N., you are truly to be admired for the limitless energy you have put
into this subject. I don't know how you do it.
See philosophical snippet in my signature . . .
I sell "SnapJacks" bi-directional zeners for putting across the coils of
relays, inductors, and especially contactors--as recommended by Tyco,
Potter and Brumfield, Megavac, Kilovac, and virtually all other relay
engineers who have ever looked at the problem.
But hey, you could be right! One lone voice in the wilderness. I admire you.
Eric my friend. You've missed the point of this discussion
since day-one. This isn't about Bob N. or anyone else being
"right" or "wrong" . . . it's about understanding simple-ideas
and crafting a recipe for success conforming to design goals.
Yes, the folks at all the big houses have the opportunity
to know more about their products than anyone else. Certainly
there was a time that was true. Except for Microswitch, virtually
all of my requests for investigative assistance in a relay or
switch failure over the last 15 years produced a young, wet-behind-
the-ears visitor from the company who was probably a recent graduate.
He/she was assigned support on the legacy products. A sure
bet. No? The legacy products have decades of market history.
Nothing can go wrong . . . go wrong . . . go wrong. They had
never designed a thing, never conducted a failure analysis,
and had no customer or field service experience. They were
seldom accompanied by a learned mentor.
This sad condition is being repeated throughout a once
capable and competent community of suppliers to US
industry. I could have treated these experiences as
either a burden or opportunity. Given that the issues
were costing my employer $millions$, it was good that
I was willing and able to rise to the OPPORTUNITY to
observe, discover, learn and ultimately teach. More often
than not, my discoveries produced facts of performance
that the factory support folks didn't understand.
Further, it was NOT in their best interests to admit
that their capabilities were found lacking. Put yourself
in their shoes . . . you're right out of college. First
job with Mister Big Relays and Contactors, Inc. You
walk back into your office and tell your boss, "The
customer knows more about the product I understand
or was able to offer."
You (and others) have cited the writings by employees
of Mr. Big and (without understanding the significance
of their words) have parroted them back to support
some opinion. Further, you and others have evolved
products base on the same cursory citations.
I have endeavored to test, measure, observe, and
analyze the behaviors switches, relays and contactors
for the purpose of achieving understanding and
offering the best-I-know-how-to-do advice. For
quit a few years, I was well paid by my employers
to carry out such studies.
If you find my offerings lacking, how about offering your
own analysis of the repeatable experiments I've conducted
and/or the data collected therefrom? I'll have to
look back over our writings on this topic but I
can't recall that you have ever offered a data point
discovered by yourself backed up with documentation
on a repeatable experiment.
While I attempt to offer analysis and advice based
on demonstrable fact, you seem content to whack
me about the head and shoulders with Mr. Big's
technical papers that may contain errors of
fact or interpretation. It may be true that the
paper on small mil-spec relays is not directly
related to our discussions about large contactors
used on a OBAM light aircraft.
"One voice in the wilderness" ????? This isn't
a wilderness my friend. It's the universe. The whole
universe runs on physics. Each concept in physics
is a simple-idea. Combining those simple-ideas into
useful recipes for success is what skilled product
development is all about.
I've not said that the latest-greatest spike suppression
techniques do not perform as advertised. What I have
said is that these products are being marketed based
on FEAR of loss of RELIABILITY. I have endeavored to
deflect worries about reliability by encouraging
confidence in failure tolerant design. Confidence
built on understanding that applying the latest-
and-greatest will produce no observable benefit for
the applications we're developing. If you have
a vision about how simple-ideas can be more effectively
used, let's talk about it. If you find any error of
interpretation of the data I've gathered and posted,
nobody would be more pleased to know about it than I.
Please don't remake this into a Friday night wrestling
exhibition between between myself and Mr. Big's
writers . . . or anyone else.
Bob . . .
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