I will soon be in the possession of a 20w itty-bitty LPS lamp. It is a spectral source, and has a tubular envelope and an octal vacuum tube base, which I do have a socket for.

The arc tube has 2 tungsten-wire-bundle electrodes (like an HID lamp) instead of emissive beehive filaments like in commercial LPS lamps. This lamp also contains a starting probe electrode connected with an internal resistor, just like any other probe-start lamp. The arc tube proportions are very similar to an MV or MH lamp, but it is not made of quartz, probably (hopefully) just 2 ply typical LPS glass.

The running arc voltage is  very low, 15V, and the lamp is supposed to be used with a specialized (probably electronic) ballast that costs over $100. This is a limiting factor for me, so I want to find a way to run it with what I have handy.

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I have the following questions:

Considering the low running voltage in conjunction with the starting probe, do you think this lamp would strike reliably on 120V OCV? I would think so but I just wanna make sure.

I know that a 120V choke ballast with a 15V lamp would have astronomical ballast losses compared to the lamp power itself, but I was wondering if I could just use my spare variac as an adjustable choke so I can dial in the right current. Do you think this would work? This would be around 1.3A line current for a 20w lamp lol, but it wont be running all day.

Is there a better way of running these other than the proper ballast?

Thanks!
 

There are multiple kinds of these lamps. For 3-electrode ones it is expected for them to be able to start from about 220V OCV. So in 220-230V world we just run them with a simple choke of some suitable impedance like somewhat overloaded 80W fluorescent choke.

In 120V world you will likely need some step-up transformer with a choke or a leak transformer, both with enough OCV for the lamp to start and an impedance to regulate the current to 1.2-1.3A typical for these spectral lamps.

 

There are multiple kinds of these lamps. For 3-electrode ones it is expected for them to be able to start from about 220V OCV. So in 220-230V world we just run them with a simple choke of some suitable impedance like somewhat overloaded 80W fluorescent choke.

In 120V world you will likely need some step-up transformer with a choke or a leak transformer, both with enough OCV for the lamp to start and an impedance to regulate the current to 1.2-1.3A typical for these spectral lamps.

So obviously these lamps aren't optimized for efficiency if they need 220V to strike but just 15V to run. I do have a 120V to 240V autotransformer, but maybe I could run it with a simple 120V choke ballast if I could get it to start with a fluorescent starter. Do you think this might give it the voltage it needs to strike, or does it need the continuous 240V supply to be able to properly warm up?

I just fired up my GP20Na and tested it. At about 110v a glow discharge will appear at the starting electrode, but it needs a whole 180-190v to actually strike. If it strikes, current skyrockets to 2a (which creates a rather frightening bright blue arc inside the lamp) with a 100w 230v ballast in series (rated for 1a). I know the spectral source at work does it too, but it looks less extreme. Over there it's run on a biiiiiig choke that all spectral sources are run from.

Turning down the *input* voltage to about 170v will set the current at 1a which is underrunning the lamp slightly but otherwise in a stable way.

In short: yes, you do need the high voltage. I cannot tell for how long you need it, but it seems like once struck, you can lower the input voltage. This means you may get away with a choke (i cannot tell you how many Henry it should be - the one i'm using is 0,4H) and a 110v starter for a fluorescent tube. I might try that out at a later point in time.

EDIT:
i don't have a 110v starter sitting around, but manually shorting out the lamp to create the inductive kickback from the choke ballast, will strike the arc at 110v input voltage. The current will be insufficient to run the lamp properly but you can try out multiple ballast combinations to make it work.

Edit2: the lowest input voltage on which i can get it to strike is 110v. The aux electrode already needs to have a glow discharge or it will not strike.

That's enough fooling around with exposed mains wriring on the bench. Keep in mind that experimenting with mains operated stuff is risky and you should not fool around with it, if you are not 100% confident in your ability to see your own mistakes coming before you make them.

So obviously these lamps aren't optimized for efficiency if they need 220V to strike but just 15V to run. I do have a 120V to 240V autotransformer, but maybe I could run it with a simple 120V choke ballast if I could get it to start with a fluorescent starter. Do you think this might give it the voltage it needs to strike, or does it need the continuous 240V supply to be able to properly warm up?

These are not optimized for efficiency at all. As they are lab sources, they need to have desired spectral properties, but no one cares about the power consumed until they are run in a portable or airborne instrument.

I will assume that the lamp needs >= 220V open circuit voltage to start successfully. Since you have 120 -> 240V transformer you can combine some chokes to make ~1.2A source. Just do not run 120V chokes on 240V as they surely will saturate. As arc voltage is so low the short circuit current without a lamp will be almost equal to the working one.

i don't have a 110v starter sitting around, but manually shorting out the lamp to create the inductive kickback from the choke ballast, will strike the arc at 110v input voltage. The current will be insufficient to run the lamp properly but you can try out multiple ballast combinations to make it work.

That's enough fooling around with exposed mains wriring on the bench. Keep in mind that experimenting with mains operated stuff is risky and you should not fool around with it, if you are not 100% confident in your ability to see your own mistakes coming before you make them.

Thanks so much for trying that out!!! This is what I will do in conjunction with my spare Variac to try and run this lamp. I don't have any starters either but I will just use a light switch for testing. Once I get it all figured out I will get an enclosure and a starter+socket and try and find the proper choke. Maybe I will wind my own with one of my spare cores, IDK.

I once tried to make a 6w fluorescent fixture on a wooden board and I must have forgotten to unplug it at least 10 times while testing different chokes. That was a very "fun" day. I have definitely had my fair share of mains work (and shocks), whether or not that is a good thing I don't know.

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These are not optimized for efficiency at all. As they are lab sources, they need to have desired spectral properties, but no one cares about the power consumed until they are run in a portable or airborne instrument.

I will assume that the lamp needs >= 220V open circuit voltage to start successfully. Since you have 120 -> 240V transformer you can combine some chokes to make ~1.2A source. Just do not run 120V chokes on 240V as they surely will saturate. As arc voltage is so low the short circuit current without a lamp will be almost equal to the working one.

If my choke+starter setup doesn't end up working I will definitely try this, but I don't have any 240V chokes around so I will have to figure something else out. We will see.

So you sure can connect two identical 120V chokes in series for 240V input. Or more realistically, you can dig up some mercury/MH HX transformer (in about 70-100W range) that measures ~ 1.2...1.3A short circuit current at secondary. That likely will run your lamp okay, though may become a bit hot.

So you sure can connect two identical 120V chokes in series for 240V input. Or more realistically, you can dig up some mercury/MH HX transformer (in about 70-100W range) that measures ~ 1.2...1.3A short circuit current at secondary. That likely will run your lamp okay, though may become a bit hot.

Bold of you to assume I have a vast collection of ballasts lol 

I do have a 100W MH ballast, I will check the secondary short-current to see if it matches. Now that I think about it that might be a really good idea because of the this:
 
https://www.lighting-gallery.net/index.php?topic=17802.0

If I could get that to work, that would be MV, MH, HPS, and LPS all from one ballast which would be even darn cooler! I would need to make a mogul to octal adapter though... I will figure it out eventually

If your MH ballast of the HX autotransformer type for probe start MH lamp it may actually work rather well

It is meant for pulse-start metal halide, but if I leave the ignitor disconnected it should be fine… right? It has over 240V OCV. Or do they have different characteristics?

Should drive the lamp at around 1.5 Amps by a quick head calculation. Should be okay. Keep an eye on the tempreature of the ballas as the same will have likely around 35% more power dissipation in its windings. However that should most certainly be fine, especially when run intermittently and not over long time. These ballast need to withstand certain overloadiing due to phenomena on Lamp EOL.

Should drive the lamp at around 1.5 Amps by a quick head calculation. Should be okay. Keep an eye on the tempreature of the ballas as the same will have likely around 35% more power dissipation in its windings. However that should most certainly be fine, especially when run intermittently and not over long time. These ballast need to withstand certain overloadiing due to phenomena on Lamp EOL.

I know that capacitive ballasting is usually bad... but maybe a capacitor in series with the lamp might help drive it with less current?

I know that capacitive ballasting is usually bad... but maybe a capacitor in series with the lamp might help drive it with less current?

The ballast label says 1.35A - 1.70A short circuit, which is promising, but I just measured it and it is on the dot 1.70A. So that sucks.

Anybody know if a capacitor would be a good idea in this situation?

Id try it without capacitor and monitor the lamp current. The lamp would run slightly overloaded at around 1.5-1.6 amps, however it should be able to handle that.
It is certainly possible to limit the current even more with a capacitor in series with the lamp, however i calculated that capacitor to be 88µF. The current spikes from that thing at lamp reignition would damage the electrodes much more then then the slightly overload...
If you really want to run the lamp perfectly, it may be best to reduce the input voltage with a variac.
It would also be possible to reduce the input voltage with a choke (reactor ballast for the americans) however, which one to choose would be a wild guess when not all ballast dat is know (winding ration, main and stray impedance of the transformer etc.)

I might try to wind a small choke to put in series with the lamp for some current limiting, but I will have to measure the current to see if it is really necessary.