Capacitive ballast?

[ion]

I bought a small mercury discharge tube recently, only to find it impossible to get a suitable ballast locally.  I figured out that an approximately 8H inductor would be suitable for the 5W tube, but not something I can get hold of.

However, a suitably rated 1.5uF capacitor would also provide the required impedance and is easily (and cheaply) available.
Is there any reason ballasts are usually inductive?  Am I missing something that would prevent the capacitor from working?

I suppose even a resistor should work, but the losses would be high.

[David Hess]

An inductive ballast also provides the inductive kickback voltage when the starter releases to ignite the bulb.  Capacitive ballasts have to provide this in some other way.

[ion]

Thanks.  I was hoping the mains voltage would be sufficient to ignite the tube seeing as it runs at around 35V, but I'm not so sure now.  I can't find any data on the required strike voltage for these kind of tubes.

[ion]

I decided to try it out anyway, the setup took less time than I spent trying to find the information online.

And it turns out that 240V mains is not enough to ignite a 5W tube. 
It did seem close to igniting though, it might take less than 1kV if I can find a way to supply it without damaging the capacitor.

[drvtech]

Edit to add: Don't do this, it's dangerous...

Reminds me of an experiment I did as a teenager - I connected up a fluorescent tube in the same way as the cheapo strobe lights you could get for a disco. Rectified mains in series with a fat resistor to charge a capacitor  which was connected directly across the tube. Then a high voltage pulse from a valve audio output transformer applied between one end and an ignitor wire which was wrapped around the same end. The cap charges up and then the pulse creates enough ionised gas to start an arc. The cap discharges through the tube with a nice bright flash and the resistor ensures that the cap doesn't charge again too quickly so that the arc can't be sustained. Risky though. If you work the tube too hard it will explode! But this was 50 years ago and I was young and fearless 🙄

[ion]

That does sound dangerous.  Luckily I won't have to resort to anything that risky - I remembered I had a few old transformers and found one with a high enough inductance on one of the windings to use as an inductive ballast.

Of course, now I need to reduce the inductance because the current is too low, but it's probably better to start a new topic for that.