Today's Message Index:
----------------------
1. 05:58 AM - Re: Strobe switches (racerjerry)
2. 06:29 AM - Re: Re: Strobe switches (Rene Felker)
3. 07:08 AM - Re: Strobe switches (Eric M. Jones)
4. 07:44 AM - Re: Re: Strobe switches (Robert L. Nuckolls, III)
5. 07:45 AM - Re: Strobe switches (Robert L. Nuckolls, III)
6. 08:06 AM - Re: Re: Strobe switches (Robert L. Nuckolls, III)
Message 1
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| Subject: | Re: Strobe switches |
I am very reluctant to throw in my two cents here, as there are much greater minds
than mine on this list. I figger that strobe power pack current characteristics
may be causing switch arcing; slowly damaging the switch contact material,
increasing contact resistance to the point where local heating and a snowball
type failure occurs. There are quite a few different materials used for switch
contacts. Depending on switch requirements and UL ratings; all contacts
have their own characteristics and ability survive differing harsh environments.
Ol tungsten lamps are similarly known for their high initial current inrush
characteristics and switches require a T rating from UL, but these are AC
switches and any arc is self extinguishing at the zero crossing point of the AC
wave. DC switches are a different animal. DC switches require a quick snap
action which is independent of operator manipulation.
One trick for switch survival in such instances is to use a double pole switch
with both sets of contacts wired in series to double arc distance and contact
separation speed. (Thank you Mr. Tibolla, wherever you are).
Again, if I were to guess, I would first suspect the initial current draw of the
large strobe capacitor would be the cause. Any contact bounce on switch closing
would definitely aggravate things and hasten failure. I too would want to
check the current / voltage profile of that strobe. Carling generally makes
darned good switches.
--------
Jerry King
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=417461#417461
Message 2
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| Subject: | Re: Strobe switches |
Jerry, thanks for your input. I learned a lot. I do not have this problem in
my airplane, but a better understanding of this failure mode will help me with
future projects.
Do not archive
Rene' Felker
N423CF
801-721-6080
-----Original Message-----
From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of racerjerry
Sent: Thursday, January 23, 2014 6:58 AM
Subject: AeroElectric-List: Re: Strobe switches
--> <gki@suffolk.lib.ny.us>
I am very reluctant to throw in my two cents here, as there are much greater minds
than mine on this list. I figger that strobe power pack current characteristics
may be causing switch arcing; slowly damaging the switch contact material,
increasing contact resistance to the point where local heating and a snowball
type failure occurs. There are quite a few different materials used for switch
contacts. Depending on switch requirements and UL ratings; all contacts
have their own characteristics and ability survive differing harsh environments.
Ol tungsten lamps are similarly known for their high initial current inrush
characteristics and switches require a T rating from UL, but these are AC
switches and any arc is self extinguishing at the zero crossing point of the AC
wave. DC switches are a different animal. DC switches require a quick snap
action which is independent of operator manipulation.
One trick for switch survival in such instances is to use a double pole switch
with both sets of contacts wired in series to double arc distance and contact
separation speed. (Thank you Mr. Tibolla, wherever you are).
Again, if I were to guess, I would first suspect the initial current draw of the
large strobe capacitor would be the cause. Any contact bounce on switch closing
would definitely aggravate things and hasten failure. I too would want to
check the current / voltage profile of that strobe. Carling generally makes
darned good switches.
--------
Jerry King
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=417461#417461
Message 3
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| Subject: | Re: Strobe switches |
Here's my guess:
The only key difference in strobe supplies is their really big capacitors. When
caps are flat (perhaps after long storage), their current demand is gigantic
for a short time. Good design would put a peak current limiter like a CL-11 surge
suppressor in the input circuit.
An O-scope would show this, but maybe only on the first start after long storage.
Furthermore the test design for this is tricky because the current you are
trying to capture is very high and very short.
A solid state relay, or any other relay, probably isn't the solution to the problem.
--------
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=417468#417468
Message 4
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| Subject: | Re: Strobe switches |
At 07:57 AM 1/23/2014, you wrote:
I am very reluctant to throw in my two cents
here, as there are much greater minds than mine
on this list. I figger that strobe power pack
current characteristics may be causing switch
arcing; slowly damaging the switch contact
material, increasing contact resistance to the
point where local heating and a snowball type failure occurs.
I agree that some stress of this nature is in
play. A stress that seems to be unique to a
sub-set of the family of strobe products. I can't
speak to the contact style of switches pre-Carling-
rockers on the Cessnas. Without a doubt,
the mechanical features of these switches went
un-changed for decades and, perhaps, persist
to this day. Here's a breakdown of the electron-
pathway through a fast-on-tabbed Carling switch . . .
http://tinyurl.com/qcmyt4e
There are quite a few different materials used
for switch contacts. Depending on switch
requirements and UL ratings; all contacts have
their own characteristics and ability survive
differing harsh environments. Ol tungsten
lamps are similarly known for their high initial
current inrush characteristics and switches
require a T rating from UL, but these are
AC switches and any arc is self extinguishing at
the zero crossing point of the AC wave. DC
switches are a different animal. DC switches
require a quick snap action which is independent of operator manipulation.
One trick for switch survival in such instances
is to use a double pole switch with both sets of
contacts wired in series to double arc distance
and contact separation speed. (Thank you Mr. Tibolla, wherever you are).
Keep in mind that we've seen failures across
the entire spectrum of metal-to-metal interfaces.
http://tinyurl.com/96yqmg
http://tinyurl.com/8zzkfbb
Failures of terminals are almost always a
failure to achieve gas-tight joining. Failure
of the rivet joints are probably precipitated by
degradation of the plastic housing material that
relieves mate-up pressures between the tab and
the contact-rivet. Degradation rooted in elevated
temperatures at a slowly failing junction.
Again, if I were to guess, I would first suspect
the initial current draw of the large strobe
capacitor would be the cause. Any contact
bounce on switch closing would definitely
aggravate things and hasten failure. I too would
want to check the current / voltage profile of
that strobe. Carling generally makes darned good switches.
Carling's demonstrated service history has
offered no incentives for folks in the airplane
business to boot them off the airplanes. At
the same time, tens of thousands of these
same switches have endured the test of time
as controls for strobe lighting systems.
Some years ago, a company that employed
my services banned a particular brand of
relay from the fleet of products . . . they
purged stock and replaced with Brand-M
based on anecdotal information that Brand-M
demonstrated a 'longer service life' in the
same abusive slot on the airplane. Brand-M
STILL stuck but only 1/3rd as often.
It took several years of P&P (play and ponder)
to finally deduce root cause . . . an effect
that was not covered in any of the relay engineering
manuals but repeated demonstrated on my workbench.
The 'fixes' I recommended based on findings
were never implemented. The failure rate had
dropped down enough to fall off the ten-worst-
problems list and the bean-counters lost
interest.
Observations of anecdotes here on the List
suggest that what ever differences
exist across the family of strobe
lighting systems, there IS a
characteristic that will bring out the
worst joint in any bus-voltage pathway to the
strobe supply. The 'worst' has manifested
mostly in failures in and around the strobe
switch. But Listers have told us that the
fast-on terminal to their fuse-block has
proven to be the weak-link that succumbed
to the ravages of whatever weapon their
strobe brought to the fight.
I think we've observed and analyzed enough
history to suggest that an investigation
into weaponry is in order. Hence my decision
to acquire tools of investigation which,
with the help of others here on the List,
may peel back the layers of the onion
and make all secrets known.
Our work here on the strobe switch failure
mystery has little to zero chance
of causing anyone to change their designs.
The best we can hope for is to identify
root cause and craft tools to identify its
recurrence so that some brands of strobes
can be avoided.
Ever hear of "too-little, too-late"? By the
time we figure this out, xenon strobes may
be on their way out of the marketplace and
the 'problem' will simply go away. But there
are simple-ideas and science to be identified
that will give foundation for any future investigation
into mysterious failures of perfectly good
switches and terminals.
Bob . . .
Message 5
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| Subject: | Re: Strobe switches |
At 07:38 PM 1/22/2014, you wrote:
>
>Yes I still have the problem system and would be happy to send to you.
>Just let me know the address and it's on it's way. No return required.
Sure.
P.O. Box 130, Medicine Lodge, Ks, 67104-0130
Thanks!
Bob . . .
Message 6
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| Subject: | Re: Strobe switches |
At 09:07 AM 1/23/2014, you wrote:
Here's my guess:
The only key difference in strobe supplies is their really big
capacitors. When caps are flat (perhaps after long storage), their
current demand is gigantic for a short time. Good design would put a
peak current limiter like a CL-11 surge suppressor in the input circuit.
But we've seen failures in switches that are
always left on. Initial inrush events, while
most severe, occur only at turn-on . . . what
we're seeing appears to be a manifestation of
persistent RMS heating . . . i.e. a recurring
condition during operation.
An O-scope would show this, but maybe only on the first start after
long storage. Furthermore the test design for this is tricky because
the current you are trying to capture is very high and very short.
Actually, no worse that for getting
a 100w landing light lit up from a cold
start. Such inrush currents are generally
limited by ship's wiring. Here's an exemplar
plot for starting a 55W lamp.
http://tinyurl.com/mvugala
Cold resistance of the lamp was measured
at 150 mOhm, wiring added another 165
mOhm for a total of 315 mOhm and a predictable
inrush on the order of 14/.315 or 44A. If
the bulb had a monster capacitor across it
then the absolute max inrush would still
be limited by a/c wiring to 14/.165 or
84A.
This is in the ballpark for expected tungsten
lamp inrush values cited when switches are
rated in the lab. Ratings that assume the
system integrator expects a service life in
the thousands of cycles.
We're witnessing failures in under 1000
cycles . . . perhaps as few as 200 cycles.
The failures include devices that don't have
to switch . . . only carry the load.
It may be that initial inrush is a factor
but I'm guessing it's not the dominate
effect. The fact that many strobe systems
do not exhibit the problem while one
system can duplicate the problem several
times suggests that root cause can be
chased down by comparing problem children
with well behaved children . . .
A solid state relay, or any other relay, probably isn't the solution
to the problem.
A solid state relay will eliminate the failures
with roots in contact resistance . . . but there
are other metal-to-metal joints at risk. You're right,
there's more going on here than can be explained
by failure to observe switch ratings from the
catalogs.
Bob . . .
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