i recently purchased a Lithonia 70 Watt HPS area lighting fixture,at the same time i also purchased a regent H-38-100-MDX 100 Watt Delux White Mercury Vapor Lamp,i wanted to see how
the mercury vapor lamp would behave on a high pressure sodium ballast,so i screwed it in the socket,the lamp started up red and warmed up to a cool white color,as expected,howeven a minute after reaching full brightness,the arc extinguishes,i,ve observed this closely and the lamp cycles on the 70 Watt HPS ballast,why is this so?

When a MV warms up, it needs 130v across the lamp to stay lit which means the ballast needs an open voltage of over 200v. Your HPS ballast is only 120v open output since HPS lamp only needs 55v across it when fully warmed up. However, if you ever decide to check with voltmeter, be sure to disconnect the ignitor or meter will be damaged!

If I recall, lamp voltage must not exceed 2/3 of ballast open voltage or it will cycle.

does running the Mercury Vapor Lamp in the fixture cause harm to the Ballast?

im already sure if i continue using the lamp,the cycling will kill the ignitor.

the label says the ballast provides 1.6 Amps,what is the opperating current
of a 100 watt mercury vapor lamp?

I doubt running MV will hurt the ballast, but yeah the cycling will wear the ignitor.

The operating current for a 100w MV is around .85-.95 amp fully lit, so yeah it is being overdriven when fired up on 70w HPS.

Thanks,Don!!!

also depending on ignitor, if its a 3 contact ignitor it may damage the ballast from cycling...but 2 contact ignitor will NOT damage the ballast

well its a 3 contact starter,but i only let it cycle 3 times so it probably didnt hurt the ballast or ignitor at all.

i wouldn't do that myself, for fear of expolsion or something bad.....

there's no risk of lamp explosion at all,the only bad thing that will happen is possible damage to the ballast,and certain destruction of the ignitor if i let the lamp continue opperating on the ballast,it was just a test to see how the lamp would look,until i get a  proper replacement bulb.

oh ok. well im not as expercened in sodium vapor as you are. i mostly deal with mercury vapor.

i wouldnt say im the most experienced,as this is my first High Pressure Sodium Fixture,the voltage provided by the hps is simply to low to keep the mercury vapor lamp lit.

Yep,the voltage is too low to stay lit. Are you planning to get a 100W MV ballast or fixture?

Yep,the voltage is too low to stay lit. Are you planning to get a 100W MV ballast or fixture?

i would but 100 watt mercury vapor fixtures seem to be more expensive,only the 175 watt is avaliable around here,and i cant find a 100 watt mercury ballast,i might try metal halide.

If you did not disconnect the ignitor before operating the MV bulb then you probably fried the Starting probe in the bulb. Try it out on a regular ballast to see if it will light anymore. What I do at work is if a ignitor ever goes bad on our 100watt pulse start MH lights I will install a 175watt probe start bulb on the existing 100watt pulse start ballast and it will work fine. Not full brightness but I have had 2 going now for almost a year and no problems. The color is off from the others but same amount of lumens,It's either this or convince them to get replacement ignitors which they don;t they just get new ballast instead, So why waste a $80 ballast cause of ignitor failure? I keep them going. I also tried a MV bulb on a HPS ballast and forgot to take the ignitor off and I cooked the starting probe in the MV bulb so it was useless afterwards without an ignitor.

the starting probe wasnt fried,however during the off periods of cycling,the ignitor would make a weak arc appear outside the arctube.