Hi all,

If you've seen my latest photo upload of the GE 1.5W daylight fluorescent night light, you may have read that I plan to harvest the lamp out of it and try it on an incandescent ballasted preheat setup. I have already done that, and it was successful - photo to come later.

I need some help figuring out the setup. While the incandescent lamp I use as the ballast (7.5 watts) powers the lamp perfectly, it doesn't provide nearly enough power to adequately preheat the cathodes. They don't glow at all, and the lamp is impossible to start. I find that with an incandescent ballast equating to about 15 watts, the lamp will start, although it's difficult, and the cathodes still don't visibly glow at all. I find that with a 25 watt incandescent ballast, the cathodes will glow a dim orange, and the lamp starts reliably. Of course, I have to quickly switch back to the 7.5 watt lamp once the lamp starts, since it's severely overdriven by the 25 watt ballast.

Is there a way I could set up an incandescent ballasted system, such that the 25 watt lamp serves as the ballast in the preheating phase, and then the 7.5 watt lamp serves as the ballast after the lamp has started? I've attached a diagram below that I *think* will work, although it would be kind of cumbersome with two starters.

Thanks!
Trent

EDIT: I just realized the setup in my diagram wouldn't work. If the starter between the cathodes opens before the starter after the power source, it will be bye-bye lamp. I've updated the design (attached as take 2.jpg), and I think it would prevent this problem, although now it would be even more cumbersome, with three incandescent lamps in the setup.

First a question" Why is there the lamp in series with the starter? It just lowers the preheating current, nothing else, while you have just the opposite problem...

Second such two starters would never operate in sysnc. The tube one will have to close first, only then the second one start to warm up. But in the meantime the first one make already few cycles.
So you will need a rely sensing really the arc current (so two windings) or a manual start dpst starting switch

These small lamps are designed for cold starts, therefore the filament is way thicker than correspond to the rated current.
The lamp tends to rectify from time to time, what makes them very unstable on an inductive ballast.
Even the high current crest factor of the capacitor ballast have it's function here: It extend the bright area at and a bit behind the electrodes, so allow for a bit more light from so short tube (the tube design count on this effect).

You haven't given me many details to identify which starter and diagram you're referring to, but assuming you're talking about the starter near the power source in the second diagram, it's to prevent the problem I described in the edit, which is the lamp being directly connected to power with no ballast in between, if the starter between the cathodes opens before the starter after the power source. A 40 watt ballast in series with that starter protects from this, and I've compensated for the reduced current by changing the ballast in between the cathodes to be 40 watts, as well.

I mean the top left one, between the cathodes. It is connected in the same way on both pictures, the top one with 25W lamp, bottom with 40W lamp. That lamp appear to just add an extra resistance in series with the heaters during preheat, so lowers the heating current.
The one bypassing the 7,5W ballast is clear, the top one will cause short circuit via an unballasted discharge, once the right hand side starter closes...


But the closing order will be all the time the same:
First, when you apply the power, there is no discharge, so no significant current in the circuit.
So only the left starter sees the voltage, the right one is bypassed by the 7.5W ballast lamp.
That mean just the left starter closes as it heats up, so after about 1..3 seconds.
Only when it closes, the right hand side starter will become exposed to a voltage, so can start to heat up. That provided, there will be sufficient voltage across the 7.5W ballast.
But during this time the left one cool down (it's contacts are closed, so no discharge there) and disconnect sooner, than the right one have even a chance to warm up.

Most starters have their timing about:
- 1..3 second initial warm up till it closes the first time after power ON
- ~0.5..1 second ON time and then switches OFF. Mostly at this moment the lamp already starts.
- ~0.05..0.5 seconds open before reclosing the next time (it is already preheated)

That mean the first starter won't give as much chances during it's ON time for the right one to pass through the initial heating phase...

Regarding the first picture, there has to be some sort of current limiting during preheating, or else the cathodes would immediately self-destruct.

You're right regarding the whole dual-starter setup - it would be too finicky to work reliably.

If there's no automatic way to temporarily increase current during preheating, I suppose I could use a manual switch...

Regarding the first picture, there has to be some sort of current limiting during preheating, or else the cathodes would immediately self-destruct.

But the current limit is required for the arc as well. Otherwise after warming up, the lamp will ignite like on a RS ballast, but with no limit on the arc current. So the filament current may be limited, the arc one not.
And once you add some form of current limit for the arc (second picture on the right side), it will limit the filament current as well.

If there's no automatic way to temporarily increase current during preheating, I suppose I could use a manual switch...

I see only one possibility: A NC relay with two windings sensing the arc current (the preheat current will cancel each other, so the relay does not activate, while when ignited, it will flow through the coil). But find such relay would be hard and the setup will become quite complex...

I would guess the original circuit will remain the simplest automatic starting arrangement for this lamp...

But the current limit is required for the arc as well. Otherwise after warming up, the lamp will ignite like on a RS ballast, but with no limit on the arc current. So the filament current may be limited, the arc one not.
And once you add some form of current limit for the arc (second picture on the right side), it will limit the filament current as well.

Yes, hence having two different sets of ballasts. In the second diagram (ignore the first; I know it won't work), the two 40 watt lamps serve as the ballast during the preheating phase, and the 7.5 watt serves as the ballast during the running phase. I think there's some miscommunication going on between us, but it doesn't matter - I know it would be too cumbersome and finicky to try and get working. All I'd like to do is rig this 1.5 watt lamp up in a preheat system, so I think the easiest way to do that is implement a manual switch that switches between 7.5 watt and 25 watt lamps for the ballast. I agree with you, though, that the lamp was clearly designed for an instant start system, hence the starting probes and very high resistance cathodes.