Can a fluorescent tube be used in series?
How much voltage do you need to get through the two 20-watts series, and can a 40-watt ballast provide that high voltage? I don't think so.
Because 20 watts bulb needs about 450 v high voltage to the breakdown, 40 tile tubes need about 550 volts can be breakdown. Usually ballasts provide high voltage is slightly more than, but not more than a lot.
The second floor says: "theoretically can light up". Can you say the theory? In theory, I think, it's not bright.
