Page 161 - The Art and Science of Analog Circuit Design

Jim Williams

UTC# LS-52
AUDIO
TRANSFORMER
BALLAST
I CAPACITOR

h—{ PHOTOMETER

Figure 11-3.
to optical emission. Lamp conversion efficiency seemed independent of Variable frequency
high-voltage test
frequency over a fairly wide range. I did, however, notice that higher
setup for evaluating
frequencies tended to introduce losses in the wiring running to the lamp.
lamp frequency
These losses occurred at all frequencies, but became pronounced above sensitivity.
about 100kHz or so. Deliberately introducing parasitic capacitances from
the wiring or lamp to ground substantially increased the losses. The les-
son was clear. The lamp wiring was an inherent and parasitic part of the
circuit, and any stray capacitive path was similarly parasitic.
Armed with this information I returned to the computer displays. I
modified things so that the wire length between the inverter board and
display was minimized. I also removed the metal display housing in
the lamp area. The result was a measurable decrease in inverter drive
power for a given display intensity. In two machines the improvement
approached 20%! My modifications weren't very practical from a me-
chanical integrity viewpoint, but that wasn't relevant. Why hadn't these
computers been originally designed to take advantage of this "free" effi-
ciency gain?

Playing around with Light Bulbs

I removed lamps from the displays. They all appeared to have been in-
stalled by the display vendor, as opposed to being selected and purchased
by the computer manufacturer. Even more interesting was that I found
identical backlight boards in different computers driving different types
of lamps. There didn't seem to be any board changes made to accommo-
date the various lamps. Now, I turned my attention to the lamps.
The lamps seemed to be pretty complex and wild animals. I noticed
that many of them took noticeable time to arrive at maximum intensity.
Some types seemed to emit more light than others for a given input
power. Still others had a wider dynamic range of intensities than the rest,
although all had a seemingly narrow range of intensity control. Most
striking was that every lamp's emissivity varied with ambient tempera-

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