How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 736

I have seen some new 48" fluorescent tubes rated at 25W compared to the standard 40W. I was told I could use these in my existing fixtures without doing anything to the ballast. What effect will replacing a 40W bulb with these 25W bulbs have on my fixtures and ballasts? - ST
I would guess that the lower wattage tubes will work fine in your existing fixtures, but I am not expert enough to be certain. The 25W tube itself is evidently built so that a smaller current flows through it than through a normal 40W tubes when the two are exposed to similar voltages. The ballast's job is to prevent a catastrophic rise in that current by adjusting the voltage across the tube dynamically during each half cycle of the power line and to keep the tube operating even as a half cycle is coming to an end. Although the 25W tube will draw less current than the ballast expects, the ballast should behave pretty well. I would expect that the tube designers have anticipated this situation and have built the tube to operate with the standard ballast. If a reader knows better, please let me know.
         

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