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Don Klipstein Don Klipstein is offline
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Default Replacing fluorescent bulb

In article , sinister wrote
in part:

Aside from the point you make, changing the wattage won't mess up or
otherwise be incompatible with the "ballast" thing-ee?


It is normally a "Bad Idea" to use bulbs of wattage that the ballast is
not specifically rated for. In some cases, this may even be a fire
hazard.

However, if a 40 watt 4-foot bulb appears to run OK on a ballast for
same size 25 watt bulb/"lamp", you won't overheat anything, at least not
worse than anything that would happen with a 25-watter. The 40-watter
may have shortened life from the filaments running too cool to work
properly as electrodes.

One thing to check: Open the fixture and read the ballast label. Not
only see what "lamps" (bulbs) the ballast is rated for, but also see if
the ballast is one of those el-cheapo stool specimen shop light ones that
require the fixture to be suspended in mid-air for the ballast to reliably
not overheat.

Also see if the ballast is for running more lamps than the fixture takes
- this requires replacing the ballast, otherwise the ballast can overheat.
Most ballasts for 25 watt lamps are for 2 of them.

Maybe you might find the ballast was for 40/34/30, 40/34 watt lamps or
40 watt lamps only. In that case the ballast could have overheated on the
25-watter.

Ballasts for 2 4-footers usually have "Class P thermal protection" - a
"bulb" with a bimetal switch. If the ballast overheats, it cycles on/off
every several minutes or a couple to a few times an hour. However, I
suspect the bimetal switch could "get stuck" if this goes on too long
without the situation being fixed.
I have also seen this switch appear to me to not be sensitive enough:
My experience includes a dual-40-watt rapid start ballast with a shorted
output series capacitor. That makes the ballast run much hotter than it
should, but I did not see that thermal switch cut in. The main visible
symptom was that the ballast "eats bulbs" (life only a couple thousand
hours). If you have a clamp-on AC ammeter and put it around one ballast
input lead, you will see obviously abnormally high current if that
capacitor is shorted - not only does the fixture take more watts, but also
the power factor goes down. But I digressed from the point of that
thermal switch not always being what I think it should be.

- Don Klipstein )