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.

120v isn't high enough for a simple preheat choke/ballast setup running a F40

Oh. My. Gosh. I feel so stupid. Thats why I can't find any 120v 40w chokes on Ebay. I literally had no idea! (embarrassing)

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.

Very interesting, if anyone else knows anything about how these work, let me know. 

I do have access to 208v, 240v, and 277v 60Hz, so if I recalculated the inductance for one of those higher voltages (and the correct .43A) do you think that would work? Thanks again!

Any of those voltages above 200 should be fine.

Your best option is: if you can find one, a UK 240v/50hz F40 choke should work fine on 240v/60hz!
(note: if using a capacitor with one, you'd need to lower the capacitance value they list due to 50/60 hz difference .. but most don't need a capacitor)


As a side note, you could also get a US 120v F40 preheat ballast (hard to find these days, but they do show up on eBay occasionally), those are basically a transformer to raise voltage combined with a ballast to limit current. While its more than just a simple choke, its a good way to go if you simply want a preheat F40 on 120v. This is the way I went.
(I have a couple of the old LOA shoplights too.. but those were simply got as cheap lights for the garage, nothing more)

Your best option is: if you can find one, a UK 240v/50hz F40 choke should work fine on 240v/60hz!

Do you think the decrease in current due to the increase in supply frequency through the choke would be enough to notice a brightness decrease? As far as I am aware I would have to give it a more than 240V at 60Hz to get the same lamp current as 240V at 50Hz (which is okay from an electronics and safety standpoint, but not very practical).

Last night I was excessively bored, so I just used the ~.9H primary of a little 120V transformer as a choke for an almost EOL 40W lamp and used a toggle switch as a starter on 240V. Well, the primary of that 120V transformer was overvolted enough to pass enough current to light the lamp almost perfectly (only a little dim). I was pretty excited when this happened, but then the primary of that puny little transformer primary started to sizzle and smoke from the excess voltage dropped across it, so I called it quits. Still pretty exciting that my calculations of ~.9H were roughly correct! I wonder how this value compares to a 50Hz choke.

Proper impedance for 40W tube at 220V is 390 Ohms. 0.9H choke at 60Hz is 339 Ohms, so it is a bit low. There is also an active resistance part to account, of course.

Proper impedance for 40W tube at 220V is 390 Ohms. 0.9H choke at 60Hz is 339 Ohms, so it is a bit low. There is also an active resistance part to account, of course.

Yes I did forget to mention that the puny little transformer primary did indeed have quite a bit of resistance that probably brought the total impedance well past 390 ohms.