Fluorescent tube ballasts: do I need to fit every lamp?

[rs20]

I'm thinking of buying a Osram Electronic Ballast which takes 2x36W T8 fluorescent tubes, but for now I only want to use a single 36W T8 tube*. It seems feasible to me that I should be able to safely just install one of the tubes; because presumably in a normal 2x 36W setup, when one of the tubes fails in service the ballast will keep the other one running. An absent tube is just a special kind of failed tube, right?

However, I'd like anyone with experience in these matters to confirm whether this is likely to work, or whether it might somehow damage the ballast. I've checked the datasheet for the ballast, and it doesn't specify anything about how it behaves under these circumstances. Any advice appreciated.

* Why not just buy the 1x36W ballast then, you ask? Well, I may want to double the output power of this thing in the future, and don't want to buy a whole new ballast when that day comes.

[Zero999]

It depends on the ballast.

Quite often the tubes are wired in series so both tubes need to be fitted, otherwise it won't work.

This is an electronic ballast though so might be different. Ask the seller.

EDIT:
You could also read the datasheet:
http://www.osram.com/media/resource/hires/493509/3323088/Technical%20application%20guide%20QUICKTRONIC%20Intelligent%20DALI%20DIM%20%28GB%29.pdf

[rs20]

It depends on the ballast.

Quite often the tubes are wired in series so both tubes need to be fitted, otherwise it won't work.

This is an electronic ballast though so might be different. Ask the seller.

EDIT:
You could also read the datasheet:
http://www.osram.com/media/resource/hires/493509/3323088/Technical%20application%20guide%20QUICKTRONIC%20Intelligent%20DALI%20DIM%20%28GB%29.pdf

Thanks, but that datasheet doesn't seem to definitively answer the question (I can't promise to have read every page; although the majority of the 60 pages seem to be about all the dimming and control functions). The wiring diagram shows that the lamps are in parallel, so that's necessary but not sufficient to establish that it would work.

[Zero999]

I haven't read it either.

The lamps aren't connected in parallel, just one side is. Fluorescent lamps can't be connected in parallel, otherwise only one would light.

I suspect it would work perfectly well with only one tube fitted but if you're not sure, you should ask the manufacturer.

[rs20]

I contacted Osram and got a very clear answer within a few hours. Just copying it here for whoever comes across this thread in 2 years' time:

All our 2x36 ballasts have a serial connection internally, so they cannot be operated with only 1 tube.

Thanks Hero999 for your input as well! Good to get some context and to share of the suspicion that it would work with a single tube, even if we both turned out to be "wrong" in that respect  . Evidently it's easier to connect them in series (I guess this takes care of equalizing the currents in the two tubes in a very straightforward manner!).

[Richard Head]

If they are connected in series the design only requires one inductor.

[Zero999]

If they are connected in series the design only requires one inductor.

These aren't simple inductive ballasts. They're switched mode power supplies, in which case a capacitor is often used to limit the current.

I contacted Osram and got a very clear answer within a few hours. Just copying it here for whoever comes across this thread in 2 years' time:

All our 2x36 ballasts have a serial connection internally, so they cannot be operated with only 1 tube.

Thanks Hero999 for your input as well! Good to get some context and to share of the suspicion that it would work with a single tube, even if we both turned out to be "wrong" in that respect  . Evidently it's easier to connect them in series (I guess this takes care of equalizing the currents in the two tubes in a very straightforward manner!).

It's good they responded. I suppose connecting them in series makes it easier to keep the current through both of the tubes constant. If each tube were connected to a different winding of the transformer, then they'd have monitor the current through each of the tubes separately which would make the design more complicated.

[Richard Head]

Hero999

I am aware that they are not normal iron ballasts. I designed a range of electronic ballasts about 12y ago.
The electronic ballast also has an inductor (ferrite and much smaller) to limit the current through the tubes. Since the switching frequency is much higher (+/- 50khz) it can be tiny in comparison to the iron one.
For 230VAC mains two 4 ft tubes are wired in series (with a resonant cap across them via the filaments). The resonant capacitor is series resonant with the inductor and upon switch-on the voltage across the resonant capacitor climbs with each cycle until the tubes break down and ignite. If a tube is removed (or filament breaks) the current path is interrupted and neither will light.
I remember that it was a tough market to be in due to the super cheap Asian imports.