Question about fluorescent light bulbs

 Here's a question I've been wondering about for a while.  When a
fluorescent light bulb get old and starts to dim, does it draw less
power?  For example, if I had a fluorescent bulb that drew 14 Watts
when new, should I expect it to draw 12 wattage maybe 3 or 4 years
from now, or would it still use the same amount of power even though
it puts out less light?  Would it matter if it's a compact fluorescent
or a light tube?

 
No, it consumes the same amount of power as it dims.
Just like a mercury vapor HID outdoor light that get dim with age,
(which basically is the same as a fluorescent light in most respects)
it still pulls the full rated power.

A mercury vapor HID light and fluorescent are basically
a constant current device.
The quartz glass may blacken (HID), or the phosphor may lose
it's effectiveness with age (fluorescent), but the current going
through the mercury arch, and the mercury it's self, does not change.
Thus, power consumption will be constant


No change.
The same forces affect both systems.

 Brian wrote:

It actually uses a little more as the gas is converted and conducts
less. The extra is caused by heat in the ballast because of increased
resistance across the tube.

 

The gas is converted to what?


The ballast is a series choke, no? It would seem that increased
tube resistance would lower the total power.

Nick

 
Things that can happen to the mercury vapor.
Get contaminated by byproducts of the deteriorating light.
Get trapped in the materials in the light (lowering gas pressure)
And leak out microscopic cracks in the glass (no container is perfect.)

All of the above will generally lead to an
elevated arc voltage/reduced conduction.


Some are series chokes.
Most are constant current transformers.

In the series choke type.
With a shorted lamp, the ballast will pull very little power,
and a good amount of current.
The current will be 90 degrees out of phase with the voltage.

At normal operating voltage, the current will be moderately less
than under shorted lamp condition, and the current will
be more in phase with the voltage, and power will be dissipated in the lamp.

As the voltage increases in the light, the current will drop just slightly.
But the voltage across it will increase, and power will stay about the same.
Or even increase in many conditions.
The total current used by the system will drop slightly, but the current
will be more in phase with the voltage, so the power factor will improve.

On a constant current transformer, the change in lamp voltage will
cause no noticeable affects to lamp current within the operating
range of the transformer, and any increase in lamp voltage
will cause a direct increase in total consumed wattage.

All in all, I generally assume that the power consumption will stay the
same.
A simple rule of thumb.