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nuckolls.bob(at)cox.net
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 Posted: Wed May 21, 2008 3:53 pm Post subject: Coil Suppression Techniques (With Corrected Link) |
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At 05:47 PM 5/21/2008 -0500, you wrote:
<nuckolls.bob(at)cox.net>
At 07:42 AM 5/21/2008 -0700, you wrote:
I have this weird Deja Vu feeling that we have covered this subject before.
And I will save you the trouble Bob....I know you'll never agree, and
you'll retort with a long quasi-techno piece and then stumble TO EXACTLY
THE WRONG CONCLUSION.
Coil suppression and relay contact arcing have been well studied: See:
http://relays.tycoelectronics.com/appnotes/
Basically (ESPECIALLY with contactors) Do not use diodes for coil
suppression unless you're hoping for greatly reduced relay/contactor life,
so you can get stuck in a remote area and have an adventure. I know people
like this but they usually don't build airplanes. Gotta'-Have-Drama-Dammit!
You've cited that document before. And I've
read it several times both before you cited it
and after you cited it.
Kindly point out to me where the authors offer DATA
supporting an assertion that the plain vanilla
diode coil suppression has a profound effect on relay life.
They correctly asserted and I confirmed that
diodes do indeed extend the time from switch
opening until energized contacts begin to move.
This is opening delay. They went on to extrapolate
that opening delay translates directly into slower
contact spreading velocity and extrapolated further
that this translated to increased contact wear.
I did the experiments and published the results
that argue against their extrapolations. If you
have some data to the contrary, please share it
with us.
Just because you've read some words under the
letter head and over the signatures of persons
in high places does not make their words golden
unless they're supported by data from and
understanding of repeatable experiments.
Here's what sane people have figured out:
. . . are you suggesting I am less than sane??
Mechanical relays and contactors depend upon magnetism generated by an
electric current running through a wire coil. When the current stops, the
magnetic field collapses. But the relay does not know the difference
between a wire coil moving in a magnetic field (as in a generator) or a
magnetic field moving in a wire coil (as in a collapsing magnetic field).
Thus a large voltage—1000V to 1500V typically—is induced in the coil.
This current goes the same direction the original current did—so it slows
the contact opening—allowing arcing, chatter, bouncing, contact welding
and even re-closure! Yikes!
Go to your workbench, measure it, document
it and share it with us. Show me where my
data and interpretation of my data is wrong.
The important feature of relay and contactor
operation that you're overlooking is the extreme
relationship between magnetic force and air-gap.
Air is an exceedingly poor conductor of magnetic
lines of force. So while a diode does indeed slow
the rate of drop in coil current (hence increased
delay) once the armature comes unstuck from its
seated condition, the effect of increasing
air-gap is many times more influential than rate
of decay in coil current.
Went back to the workbench to look at the S704-1 in
more detail. Here's the test setup:
http://aeroelectric.com/Pictures/Schematics/Relay_Test_Setup.pdf
Relay response with no coil suppression looks
like this:
http://www.aeroelectric.com/Pictures/Curves/S704-1_Drop-Out_Delay_without_Diode.jpg
where we see the high voltage spike on the coil
trace and a 2.5 mS dropout delay.
WITH a coil suppression diode, we get
relay response like this:
http://www.aeroelectric.com/Pictures/Curves/S704-1_Drop-Out_Delay_with_Diode.jpg
Dropout Delay is increased to 12.5 mS or about 5x
longer than with the diode. This was the feature
pointed out in the article you cited . . . where
the authors extrapolated this into a commensurate
slowing in contact spreading velocity (longer fires).
However, when we take the diode off and look at
transition time . . .
http://www.aeroelectric.com/Pictures/Curves/S704-1_D-E_Transition_NoSuppression.jpg
From the time the contacts FIRST open until they
first contact the opposite side is 0.6 mS. Let's
put the diode back on and we get . . .
http://www.aeroelectric.com/Pictures/Curves/S704-1_D-E_Transition_Diode_Suppression.jpg
Hmmm . . . transition increases to 0.75 mS, about
a 25% increase NOT A 500% INCREASE.
Let's go back an look at the traces I took where
we were observing the arc in a spreading set of
contacts with no diode . . .
http://www.aeroelectric.com/Pictures/Curves/704-1OpeningTimeNoDiode.gif
Here I could see about 0.21 mS of "fire" as compared
with . . .
(Here's the right trace)
http://www.aeroelectric.com/Pictures/Curves/704-1OpeningTimeWithDiode.gif
about 0.23 mS arcing when the diode was in place. Hmmm,
there was an increase but not a very big one. In both
cases, observed arcing times were about 1/3 the total
transition time.
Now let us consider another feature of relays and contactors
that REALLY drives service life issues. Take a look at:
http://www.aeroelectric.com/Pictures/Curves/S704-1_Contact_Bounce_with_Diode.jpg
Where we see that after the first time the contacts
touch, really get with the high-tempo hat-dance
for perhaps several dozen closure and re-opening
events. This means that for every operation of the
switch on the panel, the contacts are getting
5-25 times more activity than the single switching
event might suggest. Now look at:
http://www.aeroelectric.com/Pictures/Curves/S704-1_Contact_Bounce_without_Diode.jpg
Well fooey . . . even with the diode off the contacts
do the cat-on-a-hot-tin-roof routine.
Let us further consider that when folks like those Tyco
engineers evaluate service life, they're working in
the laboratory test environment and evaluating products
where service life is measured in the tens of thousands
of operations . . . 50,000 typical. 250,000 is not
unusual. Let us suppose that their paper was based on
real statistical studies of dozens of relays with
various coil suppression techniques and yes, there
was an observable increase in mean operations between
failures from 45,744 to 49,666 by "optimizing" the
coil suppression. Hmmm . . . 10% . . . that IS significant
to Tyco and probabably most of their customers.
They didn't speak to this kind of study in the paper
you cited and I'd like to believe they've done their
homework.
Let us assume their undocumented assertions
WERE correct on the scale I suggested. How does this
affect the OBAM aircraft builder who's switches,
relays and contactors probably won't see 5,000
operations over the lifetime of the airplane?
Further, environmental stresses will be root cause
for most replacements of such devices in personally owned,
non-revenue generating light aircraft, not electrical
stresses. In any case, the 5x increase in drop-out delay
DOES NOT extrapolate into a proportionate drop in contact
life.
The common palliative is a diode AND zener in series, or better yet, a
bidirectional zener across the coil. They call these Transils™,
Surmetics™, Transorbs™, TranZorbs™, TransGuards™, Mosorbs™; the
list is endless. (Over-paid executives dream up these names.) They are
generic P6KE18CA bidirectional zeners.
Jeeeeeze........
Eric,
Of all the contributors to this list I expect
more of you. We had some substantive discussions
on the inner technical workings of various products
and ideas in Plymouth a couple of years ago. You
struck me as one who appreciates understanding
and having a handle on the simple-ideas that go
into your recipes for success.
Please don't wave anyone's documents in the air as
justification for an extrapolation of my infirmities.
Let's not make this about you or me. May I suggest
we explore, understand and then explain the
physics. Make my day. Show me were I'm wrong.
Bob . . .
--
Date: 5/20/2008 4:45 PM
incoming mail is certified Virus Free.
Checked by AVG.
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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john_rv10(at)yahoo.com
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| Posted: Thu May 22, 2008 9:49 am Post subject: Coil Suppression Techniques (With Corrected Link) |
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Great data Bob.
Thanks,
John
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