Re: A bang or a whimper?

NetMax wrote:

"George Pontis" <gpontis@xxxxxxxxxxx> wrote in message
news:43f3e33d$0$95937$742ec2ed@xxxxxxxxxxxxxxxxx
Bill Stock wrote:

CF bulb expiry that is.

I was just wondering if Compact Fluorescent bulbs generally fade
out of existence or go out with a pop? I was watching the TV on
the computer when the speakers started making snapping sounds.
Great, the computer has a problem. Then about 30 minutes later,
the light on the aquarium went out. I was thinking the ballast
must be fried, with all that noise on the power line. But
switching the bulbs around proved it was just a dead bulb.

I was surprised that the bulb didn't just fade away (although it
probably has somewhat) and also by the amount of noise these
things put out. What's your experience been? Do you let your CF
bulbs die or replace them at regular intervals?


The light output for the lamp goes down with old age, and for that
reason it is often recommended to replace CF once a year.

Also, as the lamp gets older the voltage drop across it increases.
With an electronic ballast, that means that the power dissipated in
the bulb goes up. So now the ballast is putting out more power and
getting hotter, and the lamp is running hotter too. Sometimes
things break badly due to the extra stress of temperature and
power. The best electronic ballasts have circuitry to prevent this
runaway condition, and it is usually implemented by not starting
the lamp or shutting it down when the power goes up too much.

The economical and generally troublefree Fulham Workhorse ballasts
do not have this protection. A 2x55W JBJ fixture (that I had to
repair) also did not have this protection. I don't know if any of
the usual aquarium lighting vendors do provide it, but it is a
worthwhile safety if you have a choice. It is sometimes called "End
of Life" protection.

George


OT, and at the risk of sounding like I know what I'm talking about
(cause I'm not familiar with electronic ballasts), I don't see the
power increasing because the voltage dropped (P=E^2/R, less voltage =
less power). If the resistance increased, then the voltage might
increase from the lower current, but that would also decrease the
power (P=I^2R). If the resistance decreased, then the current would
increase, and the power would increase, but the voltage would have
dropped (or remained the same, not increased). So not I'm just
curious, what is the non-critical failure mode direction (change in
resistance?).

Along the same lines (and perhaps the real source of my curiosity) is
that I was always taught that conventional fluorescent bulbs should
be changed at between 6 and 12 months of operation, but the higher
cost CFs lasted years. It was never apparent why this should be so.
Sorry for all the questions and going a bit OT.


The voltage _increases_ as the lamp ages. The electronic ballast
maintains a fairly constant current even as the voltage increases, so
power (V * I) goes up.

The older magnetic ballasts were not able to maintain the current as
the lamp aged, so they were not prone to the same type of failure.
Intead, they would have trouble starting the lamp because of the
increased voltage requirement. If it started then the current would be
lower than nominal, and the brightness would drop off more quickly with
age. In addition, the current through a magnetic ballast follows the AC
line which means that the peaks are higher than the average. This wears
out the electrodes faster than the steady current of a good electronic
ballast.

The best T8 fluorescent lamps specify a lifetime of 20000 hours. That
would be reached with a good ballast and few starts, as in a modern
office with good fixtures. But power compact lamps (and T5) that we use
for aquaria operate at a higher power density and probably will not
last quite as long. Cheap knockoffs and some common lamps may be closer
to 5000 hours. So with a good ballast, operating 12 hours a day, the
one year figure seems reasonable and conservative. When they become
hard to start, or have a lot of blackening around the tube ends, then
that would be a good time to replace.

Geo.

--

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