While thoroughly browsing my ongoing lamp data sheet, I discovered that 50W American HPS lamps have the same current as 100W MH, but at half the voltage. Ideally this would mean that a 100W MH ballast could run two 50W HPS lamps in series, but this may not be possible.

Would the ignitor pulse created by a 100W MH ignitor be enough to strike two 50W HPS lamps in series? Would I need a small starting compensator capacitor across one of the lamps if I ever decide to do this?

Why not to try? HPS lamps are typically easier to ignite than MH due to cleaner internal atmosphere (no halides, just inert gas and trace of mercury vapor)


You may try to add a capacitor of some 1000-10000pF with 5kV rating or so.

@RRK
I can't try it right now because I don't have any 50W HPS lamps, never mind two of them lol.

I did not know that HPS lamps were easier to ignite, that is very interesting to know.

I probably do have some capacitor around that range, I would have to check my collection.

The problem could be with arctube temperature stability: One tube may warmup faster a bit, so get higher voltage drop, so more power, so creating even greater temperature difference.
Normally these HPSare designed for the ballast reducing the current as the lamp gets hotter so its arc voltage higher,creating a kind of thermal stabilization. Now for the whole combination this may be the same for the 100W pulse MH, but not when speaking about how the voltage so power distributes across the two series lamps.
So it will very likely ignite without any problem, but the light ofboth lamps may end up very uneven and one of them overloaded...

@Medved
Wow, I never considered that but that definitely does make sense. Do you think it would runaway or maybe would it eventually stabilize?

The problem could be with arctube temperature stability: One tube may warmup faster a bit, so get higher voltage drop, so more power, so creating even greater temperature difference.
Normally these HPSare designed for the ballast reducing the current as the lamp gets hotter so its arc voltage higher,creating a kind of thermal stabilization. Now for the whole combination this may be the same for the 100W pulse MH, but not when speaking about how the voltage so power distributes across the two series lamps.
So it will very likely ignite without any problem, but the light ofboth lamps may end up very uneven and one of them overloaded...

I don't believe this will happen as 100W ballast will have roughly 2x higher impedance so the whole circuit will work in a more current stabilizing manner. What may or may mot happen is when HPS lamp ages it increases the voltage so the circuit with higher than prescribed impedance is at risk of lamps color shifting and cycling prematurely. But this certainly does not apply to a short  experiment with a couple of new lamps.

While thoroughly browsing my ongoing lamp data sheet, I discovered that 50W American HPS lamps have the same current as 100W MH, but at half the voltage. Ideally this would mean that a 100W MH ballast could run two 50W HPS lamps in series, but this may not be possible.

Would the ignitor pulse created by a 100W MH ignitor be enough to strike two 50W HPS lamps in series? Would I need a small starting compensator capacitor across one of the lamps if I ever decide to do this?

As a matter of fact, I have actually ran a series pair of North American low wattage high pressure sodium lamps before using a single ignitor to get them started.

@RRK
Interesting, that is what I thought would happen.

@WorldwideHIDCollectorUSA
Nice to know, I will keep that in mind.

You can see it documented here:

https://www.lighting-gallery.net/gallery/displayimage.php?album=search&cat=0&pos=54&pid=191131