I'm attempting to reproduce a little science demo where you can get a sodium flame to cast a shadow when illuminated by a low pressure sodium light. At first I thought this would be pretty easy since all I'd need to buy is a low pressure sodium light, but it turns out that's far more difficult than I'd realized.

I found a few new old stock and used bulbs, but they were all rather expensive and plenty of them were sold as-is. Eventually while being dimmer than I'd like I settled on these 20W "Gp20Na-1" bulbs that appear to be designed for polarimeters. As far as I can tell these are the only LPS bulbs still being produced so they're not that expensive (~$35 / each).

I found plenty of evidence that people were successfully powering LPS bulbs with the Fulham Workhorse ballasts so I ordered a WH3-120-L to power one bulb and a WH5-120-L to potentially power two bulbs if the one isn't bright enough.

However, I've had no luck actually getting the bulbs to work. With the WH3 and one wire connected I see an arc strike and over several minutes it turns more orange, but the initial pink glow never fully goes away. I measured what the ballast was drawing from the wall and it was only around 8W so it seems like it's substantially underdriving the bulb.

I tried attaching both red wires from the WH3 and this improved things marginally. The bulb got a bit brighter and it was drawing 10-11W, but still far less than the 20W it's supposed to be getting.

In both cases the current draw doesn't noticeably change after 10m of running.

One of the eBay lists for this bulbs has the following specs:
Power: 20W
Working voltage: AC15±5V
Working current: AC1-1.3A

They also mention as a precaution: "The lamp is ignited at the rated voltage. When igniting, the ballast must meet the requirements in series. Do not directly insert the power supply". I'm not really sure what that means and if it's relevant.

Does anyone have any idea how I can get this bulb to light up properly?

Right now the next step I can think of is to build my own power supply to send it a square wave where I can adjust the voltage/current. My hope is that is I can feed it the working voltage/current in the listing it will just work, but I don't get why this bulb won't work with the Fulham ballasts like the more standard LPS bulbs I've seen people mention here.

The Fulham ballast you are using is not delivering sufficiently high current for  GP20Na lamp.  I think it will probably only provide about 0.4A, whereas you lamp needs minimum 1A /‘d better 1.3A.  Other smaller low pressure sodium lamps require only about 0.3-0.6A and could run up further on the ballast.  So I think you simply need to use a ballast having a higher output current.

Interesting lamp. I found a power supply for one here but it isn't cheap.

I suggest you just look around for ordinary SOX lamps, they aren't as cheap as they once were, and on your side of the water they're not as common as over here, but I think it's your best bet.

Another thing to look for would be a SOX wallpack such as the Canadian-produced Philips Goldeye 18W fixture.

Regular low pressure sodium lamps are still pretty simple to find on alibaba

Does anyone know what sort of open circuit voltage these lamps need?  A 35-50w hps ballast might work?


Try using all 4 red wires of the wh5 ballast and if you want to run two lamps connect them in series.

I think the warning is just saying not to try running them without a ballast.

It's not that simple. The current is the important thing with any discharge lamp, which for a SOX lamp is usually 0.6 or 0.9A and the OCV/striking voltage is approximately 500V.

This is a spectral lamp though and all bets are off with these, it depends on the manufacturer's specifications.

I did briefly try running the one bulb with the WH5. At first I tried with just two of the red wires and it initially seemed promising. The power started at 14W and slowly climbed. However, after it hit around 20W it quickly jumped to 50W and the ballast started making a loud buzzing sound like something might be arcing. I quickly unplugged it.

I tried again with just one red wire with similar results. I'm not sure exactly what was going on, but I'd rather not destroy either the ballast or the bulb.

Alibaba does have a couple options that look better than I remembered so I'll check if someone will reasonably ship me just one or two bulbs. I'd much prefer to buy something that's still being produced though so I have a chance of being able to get replacements in the future.

I think the striking voltage must not be too different because the ballast seems to start the arc just fine, I just can't provide enough current.

I found a picture of the original ballast and it appears to just  be a series choke/inductor running off 220v AC. It also  looks like the lamps have starting electrodes.

If you're lucky it might start off 120v, try using a 150w incandescent bulb in series as a ballast. If it doesn't start try letting it warm up on the WH3 then switch to the incandescent ballast.

https://www.ebay.com/itm/354839049965?  I have one of these from a different seller and mine contains a CWA ballast, with the ignitor disconnected mine puts out 200v open circuit and 1.3A at ~20V.

It's not that simple. The current is the important thing with any discharge lamp, which for a SOX lamp is usually 0.6 or 0.9A and the OCV/striking voltage is approximately 500V.

This is a spectral lamp though and all bets are off with these, it depends on the manufacturer's specifications.

I know, a 35w hps ballast will put out something like ~1.2A into 20V (but it would probably get pretty warm) and a 50w hps ballast will put out something like ~1.3A into 40v (although it might not start 2 lamps in series even with the ignitor)

I think these lamps are designed for 220v open circuit.

Alibaba does have a couple options that look better than I remembered so I'll check if someone will reasonably ship me just one or two bulbs. I'd much prefer to buy something that's still being produced though so I have a chance of being able to get replacements in the future.

I think the striking voltage must not be too different because the ballast seems to start the arc just fine, I just can't provide enough current.

there are NEW production SOX lamps from China on Alibaba and NOT "NOS" OR "runout" lamps
so you SHOULD be able to order them at a later date
they also have fixtures with ballast gear on Alibaba

Soviet DNAS18 spectral lamps run well on 80W fluorescent chokes (ballast being slightly overheated). Prescribed lamp current is about 1.2A, too. That is, on Eurasian 230V line voltage. 'Black candle' experiment with LPS lamp is quite impressive! If you look at bright sodium lamp through a sodium-doped flame, the flame naturally looks black, very unusual to see!

Looks like GiselaSchumacher got one of those to run off 140v with an incandescent ballast.  https://www.lighting-gallery.net/gallery/displayimage.php?album=6580&pos=0&pid=195105