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nuckolls.bob(at)aeroelect
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 Posted: Wed Oct 26, 2011 7:33 am Post subject: Incandescent lamps in Wig-Wag systems |
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A few weeks ago I was called to task for some
demonstrably sloppy data gathering when we were
discussing inrush limiting on incandescent lamps
used in wig-wag recognition systems. The plot I'd
published at:
http://www.aeroelectric.com/Pictures/Curves/Wig_Wag_Currents.jpg
. . . was properly identified as somewhat 'fuzzy' and
not properly supportive of some estimates of the cold
start inrush values. I ran across my little po' boy's
solid state switch yesterday so I drug it to the bench
and set up the following experiment:
http://aeroelectric.com/Pictures/Schematics/Test_Set-Up_Picture.jpg
http://aeroelectric.com/Pictures/Schematics/Test_Set-Up_Schematic.jpg
I ran some data plots with a 1 ohm resistor-load in place
of the lamp. Here we observe the expected current flow
plotted at scan speeds that will display 3-4 wig-wag cycles.
http://www.aeroelectric.com/Pictures/Curves/1-OHM_250mS.jpg
Then I zoomed in on an exemplar ON transition . . .
http://www.aeroelectric.com/Pictures/Curves/1-OHM_50uS.jpg
. . . and we see that the 'probe' connection is a tad
bit over-damped as indicated by the rounding at the upper
corner of the rise.
I then replaced the load resistor with a 55w halogen
head lamp bulb and produced this plot of wig-wag
currents:
http://www.aeroelectric.com/Pictures/Curves/55W%20WIGWAG.jpg
Here we see the effects of lamp temperature on the inrush
and running currents. At turn on, the lamp was cool enough
to produce about a 9A spike. The measured current just
before turn-off was on the order of 5A.
Inrush current from a cold start is determined by total
loop resistance of the wiring, battery source impedance,
and lamp cold resistance. The lamp has a measured cold
resistance of 145 milliohms. If driven by a zero-ohms
source at 12v, the theoretical inrush would be
12/.134 = 83 amps. The slow sweep peek at a cold start
in this test setup yielded the following trace:
http://www.aeroelectric.com/Pictures/Curves/55W_COLD-START_1mS.jpg
Here we see a measured inrush on the order of 42 amps.
This suggests that wiring and battery resistances added
another 140 milliohms or so to the loop resistance.
Zooming in for a closer peek at the cold start . . .
http://www.aeroelectric.com/Pictures/Curves/1-OHM_50uS.jpg
we see the same 42A displayed value for peak current.
Further, there is still more rounding of the plot as
compared with the almost purely resistive plot cited
above.
Exploration of lamp behavior during turn-on could not
be conducted with a mechanical switch or relay. Contact
bounce during turn-on would badly contaminate the data.
The solid state switch driven by a function generator
suffers no such shortcomings.
The closer look confirms that my original suppositions
for inrush on the trashy trace were correct. The earlier
plot was made with longer wires than for this experiment
hence we saw a 32A cold start inrush as opposed to 42A
in this case. It also confirms that the dynamics for rate of
rise and peak currents are limited by system wiring
(resistance and inductance) and there were no erroneous
observations limited by oscilloscope sample rates.
This experiment supports my original assertions that
turn-on transitions during wig-wag operations are a small
fraction (about 25%) of the initial cold-start value.
Bob . . .
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skywagon
Joined: 11 Feb 2006
Posts: 184
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| Posted: Wed Oct 26, 2011 8:25 am Post subject: Incandescent lamps in Wig-Wag systems |
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Bob,
Your further investigation of the halogen 55W inrush current is enlightening
and hints at why the early failures.
I am ignorant concerning "modern" filaments used in these brighter lamps.
A start-up of 42 amps seems high for a design that nominally floats at 4 - 5
amps. I am wondering what that inrush thump does to a captured spring
round, mechanical filament. There must a some mechanical jolting going on
for those few msecs.
When I observed the filament under a 6X magnifier, the filament windings
appeared to have little spacing between turns. You could observe some
spacing but, it was the same or less than the diameter of the filament wire.
Possibly they have an alloy now that does not expand much from heat and
magnetics of a huge initial inrush of current.
David
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nuckolls.bob(at)aeroelect
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| Posted: Wed Oct 26, 2011 9:39 am Post subject: Incandescent lamps in Wig-Wag systems |
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At 11:22 AM 10/26/2011, you wrote:
| Quote: |
Bob,
Your further investigation of the halogen 55W inrush current is
enlightening and hints at why the early failures.
I am ignorant concerning "modern" filaments used in these brighter lamps.
|
I'm not sure there's much difference in modern vs.
legacy tungsten filaments. Both lamps us the same
material. The major differences are the surrounding
gasses within the glass envelope that permit operation
at higher temperatures.
| Quote: | A start-up of 42 amps seems high for a design that nominally floats
at 4 - 5 amps. I am wondering what that inrush thump does to a
captured spring round, mechanical filament. There must a some
mechanical jolting going on for those few msecs.
|
If you wired your landing light system with say,
10AWG wire, you could get more than a 10X increase
of inrush vs. running current. Yes, the thermal stresses
during cold-start turn on are impressive. Power
dissipated in the exemplar filament at cold start . . .
W = R x I^2
.145 x 42 x 42 is 255 watts
if you wired with short runs of fat wire
that produced an 82A inrush we get
.145 x 82 x 82 = 974 watts.
Of course this tapers off rapidly. Referring
to the 'fuzzy' plot
http://www.aeroelectric.com/Pictures/Curves/Wig_Wag_Currents.jpg
We see that it takes about 100 milliseconds
for the max inrush value to fall to normal
operating levels. But for the first 20 milliseconds
or so, that little chunk of tungsten is being
bullied rather severely.
| Quote: | When I observed the filament under a 6X magnifier, the filament
windings appeared to have little spacing between turns. You could
observe some spacing but, it was the same or less than the diameter
of the filament wire.
Possibly they have an alloy now that does not expand much from heat
and magnetics of a huge initial inrush of current.
|
I think it's mostly a rate of temperature rise combined
with the manner in which conductors burn in two.
If you hang a length of wire out in space and hit
it with a current expected to burn it in two, it
will open up close to the center of the span.
This is because resistance goes up with temperature
and material at the center of the span gets energy
pumped into it both locally and from both sides. I.e.
temperature AND resistance goes up fastest in the
middle thus making it the most likely point to reach
destructive temperatures.
Evaporation of filament material causes localized
increase in resistance. This aggravates the rate
of rise for temperature during the inrush transient
and causes failure to happen in the filament's weakest
location.
I believe there are two forces at work in the failure
of lamps. Vibration while cold and d-temp/d-seconds
rate during inrush. Keep-warm offers some mitigation
of both effects . . . inrush limiting can only
mitigate turn-on transients.
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Wed Oct 26, 2011 10:33 am Post subject: Incandescent lamps in Wig-Wag systems |
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<nuckolls.bob(at)aeroelectric.com>
At 11:22 AM 10/26/2011, you wrote:
| Quote: |
A start-up of 42 amps seems high for a design that nominally floats
at 4 - 5 amps. I am wondering what that inrush thump does to a
captured spring round, mechanical filament. There must a some
mechanical jolting going on for those few msecs.
|
I went back and got this plot
http://www.aeroelectric.com/Pictures/Curves/55W_COLD-START_25mS.jpg
This shows that current draw for lamp under examination
in this test configuration goes from 44A down to 10A in
about 37 mS. It probably takes another 150-200 mS to
settle out on the steady state current measured at 5.2 amps.
Bob . . .
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