High Pressure Sodium

[bbqbrisket1]

I have read that an hps bulb that keeps going off and then on again every couple of minutes means the bulb is coming to its end of life.
I have purchased a new hps fixture that came with a 100 watt hps bulb. I have put a 70 watt in at it works well. But after installing several 35 watt hps bulbs, they keep going off and on. Could it be that an hps ballast works with only a 100 watt and a 70 watt but not a 35 watt?
thanks

 

[LEDphile]

High pressure sodium ballasts (and other HID ballasts) are typically designed to support a single lamp type and wattage only.

 

[PhotonWrangler]

+ What LEDphile said.

Also I once re-lamped some mercury vapor fixtures with a LP sodium lamp that was advertised as a direct replacement for the MV bulb, but the result was that the sodium bulbs wound cycle on and off continuously. The ballast is designed to maintain the arc for a certain performance envelope, and a bulb that has electrical characteristics that are outside of that envelope might not work properly, even if it's a lower wattage than the ballast is designed to handle.

 

[bbqbrisket1]

Hmmm!, Makes good sense. Will get a lower wattage ballast to check out.
thanks

 

[JoakimFlorence]

I'm not sure and may be wrong, but it might have to do with a negative feedback phenomena. If the bulb becomes too hot and too much of the liquid mercury vaporises, the gas pressure inside will increase to a point where the current voltage level can no longer maintain an arc.
This is the same reason it takes so long for vapor lamps to "restrike", if a vapor lamp has been left on and then suddenly turned off, it will then take longer for it to be able to turn back on, needing some time to cool down.

 

[bbqbrisket1]

Makes sense.

 

[PhotonWrangler]

I'm not sure and may be wrong, but it might have to do with a negative feedback phenomena. If the bulb becomes too hot and too much of the liquid mercury vaporises, the gas pressure inside will increase to a point where the current voltage level can no longer maintain an arc.
This is the same reason it takes so long for vapor lamps to "restrike", if a vapor lamp has been left on and then suddenly turned off, it will then take longer for it to be able to turn back on, needing some time to cool down.

This is likely what's happening here. The lamp becomes an inadvertent type of relaxation oscillator.