Hello,

I recently retroffited Thorn's Piazza II lantern from High pressure sodium to Low pressure sodium lamp. It is a somewhat like an experimental attempt, as low pressure sodium lamps are non existant here in Lithuania. I used Philips gear (BSX 18 ballast and SX26 elecronic series ignitor, and a capacitor) and Philips SOX-E 18 watt lamps I bought from UK. Few days ago I started to operate the lamp, but noticed that there is flickering at one of the electrodes when the lamp is fully warmed up. The other electrode seems fine. What could have caused it? Is it normal? Maybe it is caused by a shabby contact (as the BY22d socket seems not really stable)? Or is it mains voltage that does not fit... it is around 230 volts (differes slightly throigh the day). Since I have no clue what causes it I need an advice (also the flicker is not really rapid, it is somewhat medicore and easily seen).

It could be the arc root "looking for the sweet spot" (there are two points on the electrode surface competing in becoming the arc root). These are quite normal instabilities with new lamps and sometimes happening through the lamp life. After some hour it usually settle.

Sometimes the fluorescents show the arc somewhat swirling in the tube, causing a slow flicker in the light output. This is often related to insufficient mercury pressure and usually settle by itself too, unless the lamp becomes mercury starved at the end of it's life. As LPS is low pressure discharge too, I would expect same effects could happen there as well.
But the LPS would not be sodium starved (as there is no reason to make the process more expensive by dosing only the minimum amount - unlike the mercury, sodium is not poisonous), so I would give it few burning hours to settle.

Generally it is not recommended to burn LPS for only a short time, but always for some hour or so each time it is ignited. Some (older) variants could be even killed by that (the ignition voltage could rise so, they won't start anymore).
So turn it ON only, when you want to light it for at least few hours and do not turn it OFF before.

Well after some time, I can tell that electrodes still flicker, they start flickering during warm up and tend to flicker more or less during the operation of the lamp. Sometimes one electrode would flicker all the time, sometimes both flicker. I have not yet seen it operate without any flicker at all. The arc seems stable though, not flickering, however due to the good optics of Piazza II bulkhead, the light comes out abit uneven. I still hope it is the bulb's fault (having read that Philips makes quite medicore SOX lamps), either way I'm putting link to the retrofit gear, maybe I have connected something wrong. http://www.lighting-gallery.net/gallery/displayimage.php?pos=-82409.
Maybe it is the capacitor (6uF capacitance is higher than required), but I am not sure, as it should correct the power factor, not influence lamp performance.
Also ballast hum is abit audible, but I have no idea how loud they can and should be (I suspect somewhat like fluorescent ones).
Comments are welcomed

There the capacitor indeed can not influence the lamp at all.
If you have a digital meter, try few tests:
First check the mains voltage, if it correspond to ballast rating.
Measure the lamp current as Ac and DC (both using at least 0.4A range of the preferably digital meter). The AC should be 0.35A, the DC zero.
Then measure the AC and DC voltage across the bulb. The AC reading should be between 50..65V, the DC should be zero
If the AC current is higher and voltage way lower and/or current lower and voltage way higher, or when there is any DC component (mainly the current DC component above 50mA) the lamp is likely faulty
If both the current and voltage differ in the same direction, while the current differ way more than the voltage, most likely the ballast is faulty.

If you see no deviation, the instability is really caused only by unlucky electrode shape (not offering only one "sweet spot" for the arc root), otherwise the lamp would be in good form.

And question: How many hours the lamp burned?
I ask, because most lamp designs count on the first 5% of their rated life as a "burn-in" - so a time the lamp need to adopt to the operating condition. And during this "burn-in", these lamps should not be switched ON and OFF too frequently, they have to operate most hours on the operating temperature (so dusk-to-down use is perfectly OK, turning the LPS or HID ON for a hour or less is not OK)

All measurements should be taken after the system thermally stabilize (so at least 30 minutes after power ON)...

The lamp cathodes flicker because you are using `low-loss` gear, which is perfectly normal.
You should see the 35watt lamps on LL gear, these flicker like mad, and you can see the AC `bands` running up and down the tube, you will only get a `perfect` running LPS lamp on autoleak gear, (not including the 140watt SLI/H, wich flickered all along its discharge on its proper autoleak gear), but this was due to Thorns cross section tube design.

I've noticed intermittent, sometimes quite pronounced flickering around the electrodes on my 18W lamps; this appears to be normal behaviour. It would be annoying as a room light but fine for passageways which is what I use them for.


BG