nuckolls.bob(at)aeroelect
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 Posted: Thu Oct 27, 2011 5:40 am Post subject: Incandescent lamps in Wig-Wag systems (OOPS) |
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A sharp eyed reader pointed out that I cited the wrong
plot for a close look at inrush on the 55w lamp. The corrected
citation has been substituted in the mini-essay below:
At 10:30 AM 10/26/2011, you wrote:
--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>
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. A slow sweep (1 mS/Div) 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 . . .
(Corrected image citation)
http://www.aeroelectric.com/Pictures/Curves/55W_COLD-START_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 . . . [quote][b]
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