At 03:45 PM 6/27/2006 -0700, you wrote:
>I've come upon this interesting paragraph on piclist.com (don't I already
>know what is written there? Well, no, in fact now that there is about 3GB =
of
>text on the site, I find more and more often that I have no clue were stuff
>came from. Rather fun actually!) and I was curious about the unanswered pa=
rt
>of it: How does one know how much voltage to put through the lamp to heat
>the filament without producing light?


Hi, James:-

I like using math, but I think in this case the easiest and fastest way wou=
ld
be to plug it into a variac and test it empirically!  A light dimmer could =
be
used too, but only with a true-RMS  meter.

- for a given lamp construction, the resistance varies over a 10-15:1 range =

depending on filament temperature

- the light output in the visible spectrum, total light output, and peak
   emission wavelength, and the visible color vary with the filament =

temperature.

http://www.answers.com/main/ntquery;jsessionid=3Dfiogdhcsscfap?tname=3Dwien=
-s-displacement-law&sbid=3Dlc05b

   A filament at 400=B0C will look black to the eye, but will be emitting a=
 fair
   bit of IR.

- there are at least two quite different constructions for line voltage lam=
ps-
   vacuum used for low wattage bulbs and gas filled used for higher wattage=
s.
   As you might expect, the vacuum 'filled' bulbs increase in resistance =

faster,
   since it's only being cooled by radiation (4th power of temperature, IIR=
C)
   vs. convection (ignoring loss through the filament support structure).
   (the effect from the gas is strong enough to be exploited in vacuum gaug=
es)

   Here are a few data points for gas filled:
   Voltage    Resistance (+/- maybe 3%)
   0%         8%
   10%       27%
   20%       43%
   30%       55%
   40%       65%
   50%       72%
   60%       80%
   70%       85%
   80%       90%
   90%       95%
  100%      100%

There are plenty of data and formulas on *light output* vs. RMS voltage, but
usually only in a range near the nominal voltage.

BTW, there's a classic circuit for low distortion Wien bridge oscillators =

which is
amplitude-stabilized by the nonlinear resistance of an incandescent pilot =

lamp bulb
in the feedback path. The original form was used by Bill Hewlett in HP's =

first product
(made in the famous Palo Alto garage)-- the HP200A and bought by (among =

others) Disney for
Fantasia- one of their first big orders for 8 pieces at $54 each. I have =

it's descendant
the  HP204D for low-distortion audio testing in conjunction with an HP =

distortion analyzer.

http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0002/00=
02history.html =




Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff



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