Hello all,
One of our guys went to service a fixture and noticed that someone had put a 250W bulb in a fixture which had a 400W ballast, now the guys here are arguing about whether or not the bulb or ballast will have a shortened life expectancy.
What do you think? If a 250W bulb is put in a Holophane fixture with a
400W Metal Halide ballast, is the 250W bulb going to die prematurely? What about the ballast?Remember that we are not just talking about just a purely resistive load here.
Thanks.
It is certainly not a good thing for either of them. John
maybe not but i work on old GE building that is filled with the proto type HPS some three wire igniters are hand built then clear coated with epoxy others look more typical. seems the GE electricians didn`t care what bulb went where. i find 250 watt and 400 watt mixed and have replaced them with 250`s. the bulb life didnt seem to be affected but the ballasts are really large, much more copper than they use these days. some seem to be around 30 years old original ballasts
Since this is an "Engineering" forum, I'd hoped for a bit of engineering insight... no one here does engineering work in this ballast/bulb industry?
I have seen it lots of times. The only difference is the lamp is acting like a current limiting fuse. The 400 watt ballast has more amps than is needed for the lamp. I have never really paid much attention to this, I just replaced the lamp with one that matches the ballast when I find it.
You need to post this in sci.engr.lighting. You will get very good answers. Many have in-depth ballast design knowledge. John
This is not entirely true. The ballast behaves as a constant current device. The lamp will take as much current as it can before it melts down or explodes without a ballast (not considering starting issues, of course). Consider the nature of electric arcs. The ballast keeps the current stable for the arc. The current and, perhaps the voltage characteristics are different for 250 and 400 watt MH lamps. Runing a 250W bulb with a 400W ballast will certainly shortent the life of the bulb. It could effect the ballast as well but to a lesser extent. John
Exactly the info I needed. While the guys here at the shop will not care the slightest about what's at issue with this, I want to know just for the sake of satisfying my curiosity. Thanks John.
Nah!
The ballast usually acts just a "reactance!"
The lamp acts like a "constant voltage" sink.
The ballast supplies so much current for a given voltage drop. A high wattage ballast will have less inductance.
The interesting question (which I can't answer) is "How does the arc voltage on a lamp change when the lamp runs 'hot?'" Since the 250 watt lamp didn't self-destruct I suspect that hotter arcs run at slightly higher voltages.
I think its the other way around. Higher plasma temps have lower 'resistance' (or higher currents). That's why arc type lamps need ballasts in the first place.
For a given bulb chemistry, operating at its design temperature, the arc voltages will be similar across different wattage bulbs. The variable is arc current (higher for higher wattage bulbs), so a ballast is designed to regulate a certain arc current, depending on bulb rating. Put in a smaller wattage bulb and the (too high) arc current will erode the electrodes and shorten the bulb's life.
You have many misconceptions about how an arc lamp operates. The ballast behaves as a constant current source. Not a simple resistance! A hotter arc is more conductive. This is main reason there is a ballast! Without the ballast, the lamp would draw so much current it could explode or trip current protection devices.
John
Another example of the constant current property of a ballast - ballasts used in flourescent lamps for signs are made for multiple tubes and are rated for a range of total tube lengths (like 12-16 feet). The total voltage across the tubes (they are essentially wired in series) is proportional to the length. The ballast has to put out an essentially constant current over the rated lengths.
--------------------- Quite a while ago I read about a lighting design for an athletic field where the lamps (metal halide?) were intentionally a lower wattage than the ballasts. The lamps put out more than their rated light but their life was shortened. This was considered good economy because the lamps were not used many hours per year and had a long practical life.
JohnR66 wrote:
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