OK, thanks for warning. I know that overpowered electrical stuff might be
dangerous. Anyway, what makes an essential difference between MH lamp rated
for 400W, 15k hours and with arc size like 25mm, and another one rated for
575W, 750 hours, and arc size like 9mm? Are the materials different, and the
first one would break immediately, when powered to 575W? Or materials are
basically the same, and the major difference is the power density around the
arc, that makes life of the second one very short, compared to the first
one?
It appears to me that the main difference is power density around the
arc. The lamps probably have some significant differences in design, such
as mercury vapor pressure, ratio of arc tube diameter to arc diameter, and
arc voltage and current.
The one with the shorter arc I guess has a shorter life expectancy due
to most or all of the inner surface of the arc tube subject to sputtering
and vaporization of electrode material, although maybe also due to higher
power density.
Maybe the first one would withstand the same power density like the
second one, but it's "underpowered" by design to extend its service to that
15k hours?
There remains a possible problem of increased power to the first lamp
causing the arc tube temperature of the first lamp to possibly exceed that
of the second lamp being used as directed. If this occurs, then the first
lamp could explode a lot sooner than the 750 hour life expectancy
mentioned of the second lamp, and arc tube fragments in the shrapnel could
be even hotter than usual for arc tube fragments in metal halide lamp
explosions! (It appears to me that most metal halide lamps designed to
last around 10K-15K hours have much of their arc tubes operating close to
800 degrees C, and many come in packages advising that when things go bad
be prepared for 1,000 degree C shrapnel - and this is for a lamp being
used as directed as opposed to an overpowered one.)
Also, keep in mind that the large variety of specialty metal halide
lamps have various arc voltages and currents, even among ones of the same
wattage. Since in general ballasts try at least somewhat to regulate
current, use of a lamp on a ballast designed for one with a higher arc
voltage will usually achieve lamp watts less than the wattage of the lamp
that the ballast was designed for. If your substitute lamp has a higher
arc voltage than what the ballast was designed for, then you will usually
get lamp watts more than the wattage that the ballast was intended for, as
well as a chance that the lamp will "cycle" - cut out and restart when
cool enough to do so. That will reduce lamp life greatly!
Now something else about MH lamps: The efficiency, color and color
rendering properties of the lamp will vary with concentration of metal
halide vapors, which will vary with arc tube temperature. Chances are,
all of this has been designed to be optimized when the lamp is used as
directed. Efficiency (or more properly luminous efficacy) is likely to
decrease if the lamp is overpowered or underpowered. Overpowering and
underpowering are also likely to cause both a color shift and a decrease
in color rendering index.
In addition: Overpowering a MH lamp can make the arc wider. Depending
on the optics that use the light from the lamp, this can reduce the
percentage of the lamp's light going where it has to go. Underpowering a
MH lamp, especially undercurrent before warmup is complete, can cause the
arc to become unstable and flicker, move around, or cut out halfway or
most of the way through warmup. Underpowering a MH lamp can make the arc
narrower, and a possible result is a shortage of light in some directions
that light has to go.
Substituting MH lamps with ones of different arc lengths, widths or
center position will probably work against you in a projector or other
system having optics designed for a specific lamp. Moving the arc,
lengthening it or widening it can cause a lot of light to go nowhere
useful. Shortening or narrowing the arc may result in a shortage of light
into some directions where light is needed.
I'm just curious, and my questions my be stupid, as I know nearly
nothing about MH lights.
This is what I actually do, I use HPS 400W ballast, and 400W HID lamp, but
it was specified in Venture Lighting catalog, that this kind of ballast is
suitable for this lamp, and as far as I remember the operating current
should be around 4 amps.
Actually true a 400 watt HPS in "fairly-new" condition wants 4 amps.
400 watt "general illumination purpose" (my words) mercury and metal
halide are different enough to want a little over 3 amps. Let alone
specialty metal halides, which have arc voltages varying and I guess
with many having lower arc voltage (needing more amps) than is the case
for same-wattage metal halides that you can get at Home Depot.
Keep in mind that the ballast can overheat if the lamp has a lower arc
voltage than the ballast was designed for. And the lamp may "cycle"
on-off if its arc voltage is much higher than the ballast was designed
for. Lamps also vary in requirements of voltage of "ignition pulses", as
well as "open circuit" output voltage of the ballast besides
starting/ignition pulses - and this is normally significantly higher than
the worst-case arc voltage and some lamps have more severe requirement of
"open circuit voltage besides ignition pulses" in order to get a spark in
the lamp to "blow up" into an arc that gets sustained.
In fact I built this projector, it's a "public domain" design, that uses
commonly available components, 15" LCD, OHP optics, standard MH bulb. And
the major weakness is lack of decent brightness, so I'm thinking how to
improve that, and I have already tweaked all the optics. I know people
trying 1000W bulbs, but that's too much heat problem, and also 1000W bulbs
have even bigger arcs, so the final gain is less then expected from this
extra power.
Thanks for advices.
My best advice is to get a lamp with arc of similar length and width and
higher design power - and also a ballast for such a lamp. Accomplish that
much, and your biggest remaining task is disposal of the extra heat.
In addition, having any spectacular failure of your higher power lamp
being no more catastrophic than failure of the lamp that the design was
designed for.
Another thing to watch out for: One common specification of lamps is
"light center length". This surely appears to me to be the distance from
the center of the arc (or filament) to the bottom of the lamp, and not the
length of an arc or filament structure.
- Don Klipstein (
[email protected])