[PRCo] Re: voltage and frequency

[robert netzlof]

--- Boris Cefer <westinghouse at iol.cz> wrote:

> Why would the light flicker?

With AC, for a brief moment (repeatedly), the current through the
lamp filament is zero. So, for 1/4 cycle, the current, and hence the
filament temperature, increases until the peak current is reached;
then for a 1/4 cycle the current and the temperature decrease. Repeat
during the next half cycle and the next and the next...

Thus, the filament temperature increases and decreases on a cycle at
twice the line frequency; twice, since the filament doesn't care
which way the current flows. (Alternative explanation: power is
proportional to the square of the current and the frequency of sine
squared is twice the frequency of the sine).

Whether the variation in filament temperature is great enough to be
noticable depends in part on how much the filament temperature
changes during a half cycle. The higher the frequency, the less time
there is for the filament to cool before it gets another "jolt" of
current so. Since there's less time to cool, the temperature doesn't
decrease as much as it would using lower frequency current.

The shape and thickness of the filament also play a part. A long,
slender filament would lose heat faster than a short, thick filament.
Low voltage lamps have shorter, thicker filaments than higher voltage
lamps and so one would expect the variation in intensity to be
smaller for lamps running on a low voltage.

Besides all that, how easily can the human eye and brain notice small
changes in light intensity? The higher the frequency the harder it is
to notice the changes. Perhaps some people notice the effect more
than others.

Maybe flicker isn't a good word for the effect, but I can't think of
another. Maybe twinkling?

Bob Netzlof a/k/a Sweet Old Bob



       
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