I am trying to calculate the proper choke inductance to use with a F40T12 tube in a preheat ballast configuration.

I read somewhere that these fluorescent tubes operate at around 100V across them at around .45A (I may be wrong).

That would mean (at 120V) the choke would have to drop 20V across it at .45A.

This calculates to an impedance of about 44 ohms that would have to be in series with the tube.

An inductor with the value of 120mH (.12H) would have an impedance of 45 ohms (at 60Hz), which would (approximately) drop the 20 volts at .45 amps that I need to drive this tube.

All of my electronics knowledge is signaling to me that this is correct, but IDK, I just want a sanity check. Does anyone know anything about this kind of stuff? I am doing this simply to entertain myself, so not urgent. Am I missing something crucial?

Thanks so much for your help!
 

A F40T12 is 430ma current

120v isn't high enough for a simple preheat choke/ballast setup running a F40. (you may be able to get it to light briefly, but to sustain the arc, you need something more (I don't remember what that voltage is))

Closest you could come to such a setup is what was in the old LOA (lights of america) shoplights ... that consisted of a small choke-type ballast, a capacitor, & a simple electronic starter. I'm not sure how it worked, if they effectively built a 'voltage doubler' circuit with those few parts, or if between the capacitor & choke they raised the voltage "just enough" while also limiting current..

Those weren't the best lights as far as quality goes (and only ran the lamps at around 30w), but they did work ... I think if you could improve on that design, you'd have a pretty cool setup.