# metal hallide lamp question



## ship (Feb 20, 2008)

Posed this question to one of my vendor reps today. Somewhat discussed as a concept in lamp manuals but not pictured in any troubleshooting guides.

These are HMI 1200w/GS lamps showing more build up on one side than the other and more wear on the opposing side. I see this once in a while with other lamps. What's the cause of it?

Is this ok, non-important or something to be concerned about?


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## icewolf08 (Feb 20, 2008)

This wold make sense to me if it was a DC power supply as it would stand to reason that you would get material migrating from cathode to anode. Off the top of my head I can't think of what would cause that if AC. Makes me want to go look and the old HTI 700W/D4/75 from my PowerSpots which is similar lamp structure to the lamps you pictured and see the distribution of slag.


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## JD (Mar 7, 2008)

It's a temperature thing! On turn-off, one side of the lamp is cooling down first (Direction of ventilation?) causing the metals to condense on the one electrode. I don't think it is anything to be concerned about as all of it vaporizes after its on.


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## ship (Mar 10, 2008)

Not ventilation but yes on turning it off and this not being a problem unless it gets much worse. Further ideas?


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## Pip (Mar 17, 2008)

ship said:


> Not ventilation but yes on turning it off and this not being a problem unless it gets much worse. Further ideas?



Can you flip them every once in a while?


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## ship (Mar 17, 2008)

wouldn't effect it & flip it between uses? Suppose you could with this lamp type but not much so useful in need.


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## Pip (Mar 17, 2008)

ship said:


> wouldn't effect it & flip it between uses? Suppose you could with this lamp type but not much so useful in need.




Well, I was thinking if the + --> - voltage flow was causing buildup on one terminal, that if you switched the lamps after say, a few months of operation, that it would then cause the buildup on the other terminal.


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## ship (Mar 17, 2008)

Yes that's the case, but dependant upon ballast and its switching in the cause of it. 

Given this statement, how would reversing the lamp solve that problem or would that be a random question at all and in general get to where most of my general statements have lead us about this concept? If random and switching - last when switched off, reversing the lamp would serve what purpose? Would a switching AC ballast not already be doing this during normal operation?


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## Pip (Mar 17, 2008)

ship said:


> Yes that's the case, but dependant upon ballast and its switching in the cause of it.
> Given this statement, how would reversing the lamp solve that problem or would that be a random question at all and in general get to where most of my general statements have lead us about this concept? If random and switching - last when switched off, reversing the lamp would serve what purpose? Would a switching AC ballast not already be doing this during normal operation?



I don't really understand what you were asking here:

> or would that be a random question at all and in general get to where most of my general statements have lead us about this concept? If random and switching - last when switched off, reversing the lamp would serve what purpose?



But I definitely see what you mean regarding the switching AC ballast, and I'm not sure about that. I'll go read about it.


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## ship (Mar 18, 2008)

Good thing reading further. All will be interested to know in explination. General idea, the ballast switches at times slower than say a 50/60hz change to a normal incandescent or even moving light lamp and more like a DC based xenon lamp. When switched off as if a DC lamp, some deposits on one end of the electrode might be seen dependant upon what part of the switching between electrodes that ballast/power supply was switched off at remain on that end as a concept shown. Overall, unless such buildups are in the company of other stuff about the lamp or an extreme, it potentially is not a problem in that once the arc is struck again and fixture warmed up in operation, the lamp will go back to normal mode and lamp back to normal operation. This the theory in not being accompaied by other stuff such as pinch/globe crustie coning, pinch/electrode bubbling about the electrode, crusties about the electrode ends, crustie buildups and clouds about the fill pinch along with clouds about it etc. to watch for. This along with normal type stuff such as axial clouding, snow, froted globe etc. due to wear.

Of note is this concept of elelctrode bubbles that normally is even say on a moving light is a way to tell age in general of the lamp but not persay a way to tell on a follow spot - why in ballast type perhaps. Next, that such an observation of electrode bubbles building up on one electrode over another is seemingly something seen on newer lamps and not as much older lamps. Could there be something about the lamp that factors into this given the same power supply?


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