glowing around support wires in MH

[Pawel Paron]

Hi,

Here is my another experiment with overpowering an MH bulb: Osram HQI-T 250
W/D powered with a 400W coil ballast (using its 240V terminal instead of
230V, so lamp power is slightly less). I run this test yesterday, all was
fine for couple of hours (it's in my DIY projector), nice bright picture and
colours, then I closely inspected the bulb after cooling and didn't notice
any signs of visible wear or damage. But this morning I tried again, and
there was trouble starting the lamp, I saw a weird blue glowing through the
projection lens, so I took a look at the bulb, and this glowing was not
inside the arc tube, but around those wires, that support the arc tube. Like
in the neon tubes, a steady blue light along the wires. I've never seen it
before, although I had some problem lamps, that didn't start correctly, the
striking was always inside the arc tube. But here there is no abrupt
striking, but a steady discharge, a blue light about 1mm thick along all the
lenth of those wires inside the outer envelope. Finaly this lamp started
after a while, and I had no time to test it again, but I wonder if it might
be a sign of a lamp damage, or maybe ignitor? I expect, that ignitor would
generate short pulses, so what would be this steady neon-like glowing coming
from?

Regards
Pawel

ps. yes, I know all the warnings about overpowering gas discharge bulbs

 

[JB]

Hi,

Here is my another experiment with overpowering an MH bulb: Osram HQI-T
250
W/D powered with a 400W coil ballast (using its 240V terminal instead of
230V, so lamp power is slightly less). I run this test yesterday, all was
fine for couple of hours (it's in my DIY projector), nice bright picture
and
colours, then I closely inspected the bulb after cooling and didn't notice
any signs of visible wear or damage. But this morning I tried again, and
there was trouble starting the lamp, I saw a weird blue glowing through
the
projection lens, so I took a look at the bulb, and this glowing was not
inside the arc tube, but around those wires, that support the arc tube.
Like
in the neon tubes, a steady blue light along the wires. I've never seen it
before, although I had some problem lamps, that didn't start correctly,
the
striking was always inside the arc tube. But here there is no abrupt
striking, but a steady discharge, a blue light about 1mm thick along all
the
lenth of those wires inside the outer envelope. Finaly this lamp started
after a while, and I had no time to test it again, but I wonder if it
might
be a sign of a lamp damage, or maybe ignitor? I expect, that ignitor would
generate short pulses, so what would be this steady neon-like glowing
coming
from?

Regards
Pawel

Sounds like you've got an arctube leak, probably caused by overpowering the
lamp. Not surprising really. The ignitor does produce pulses, but at a very
high repetition rate.

JB

 

[Pawel Paron]

Why in the world would you want to destroy a perfectly good HQI-T/250W/D?
These lamps are wonderful!

I don't want just to vandalize it But it's a pretty cheap lamp, so I
decided to risk it for my experiment. It has a very good colour spectrum, as
I see that colour rendering is noticeably better than I had with previous
Venture bulb.

Here is this weird phenomenon I observe:

http://acn.waw.pl/pawelp/projektor/zdjecia/hqi250w.jpg

Today I checked three times, and the lamp still works, it just takes longer
to start than the first time, but eventually it starts every time, after
maybe 15-20 seconds, and this startup time did not extend, at least for this
few trials. So I doubt it's a leak, as the gases would finally spread evenly
inside the whole envelope, and the lamp would probably stop working, or at
least work improperly. Or maybe this process of leaking is so slow, that it
will take days or weeks? I'll see.

Anyway, I have observed the "outer glow" discharge with many different
types of lamps. Usually hpm or hps, when I attempted to re-strike them via
an OSRAM 3.5kV StE ignitor, but never with HPI ignitors. Because the
re-strike voltage while the lamp is still hot is in the order of tens of
kVs, a discharge forms around the support wires instead.

I don't know what ignitor it is, some electronic one, and there is a label
saying 4.5V max pulse voltage. And I don't try to hot restrike this lamp,
this blue glowing happens in a cold lamp. Pulsing frequency is rather slow,
I can hear it, and it's visible as little strikes inside the arc tube, but
this outer glowing is steady, so I guess it might be powered just with some
capacitancies (wires, and lamp itself), charged to high pulse voltage? It's
definitely not a blinking with each pulse, but steady glowing, which
disappears when a solid arc in the tube is ignited. Just interesting, as
I've never seen it before.

Thanks everybody for responses.

Pawel

 

[Pawel Paron]

I wonder if the inner discharge tube has fractured and leaked mercury
vapour into the outer tube or if the high voltage is stimulating
discharge in the outer vacuum.

I just posted link to a photo. I don't know the lamp technology, is it
supposed to be a vacuum around the arc tube, or some gas? I know people on
DIY sites sometimes strip such bulbs of this outer shell, to fit the arc
tube into the optics, and it works, at least for some reasonable time.

Pawel

 

[Clive Mitchell]

Here is my another experiment with overpowering an MH bulb: Osram HQI-T
250 W/D powered with a 400W coil ballast (using its 240V terminal
instead of 230V, so lamp power is slightly less). I run this test
yesterday, all was fine for couple of hours (it's in my DIY projector),
nice bright picture and colours, then I closely inspected the bulb
after cooling and didn't notice any signs of visible wear or damage.
But this morning I tried again, and there was trouble starting the
lamp, I saw a weird blue glowing through the projection lens, so I took
a look at the bulb, and this glowing was not inside the arc tube, but
around those wires, that support the arc tube. Like in the neon tubes,
a steady blue light along the wires. I've never seen it before,
although I had some problem lamps, that didn't start correctly, the
striking was always inside the arc tube. But here there is no abrupt
striking, but a steady discharge, a blue light about 1mm thick along
all the lenth of those wires inside the outer envelope. Finaly this
lamp started after a while, and I had no time to test it again, but I
wonder if it might be a sign of a lamp damage, or maybe ignitor? I
expect, that ignitor would generate short pulses, so what would be this
steady neon-like glowing coming from?

I wonder if the inner discharge tube has fractured and leaked mercury
vapour into the outer tube or if the high voltage is stimulating
discharge in the outer vacuum.

 

[Don Klipstein]

I wonder if the inner discharge tube has fractured and leaked mercury
vapour into the outer tube or if the high voltage is stimulating
discharge in the outer vacuum.

The way that glow looks to me does not look like that of getting a glow
in a practical vacuum. The way the glow looks to me, I suspect the
pressure within the bulb is in/near the 1 torr ballpark. I suspect
leakage - not sure whether from the arc tube or if the outer bulb leaked
in some air, although I think more likely the arc tube.

If the arc tube is leaky, the lamp's electrical characteristics will
change as the arc tube loses mercury.

Meanwhile, I am surprised that a metal halide lamp would have had a
vacuum. I thought they had a fill gas between the arc tube and the outer
bulb.

- Don Klipstein ([email protected])

 

[Clive Mitchell]

Here is this weird phenomenon I observe:

http://acn.waw.pl/pawelp/projektor/zdjecia/hqi250w.jpg

Is this one of the home-made video projectors that uses a standard metal
halide discharge lamp and a full size LCD monitor for the image?

If it is then it's a pretty neat idea. Bulky, but very cheap and
serviceable. The LCD monitor has the back and light panel removed so it
can be used as a huge slide. The large image area makes the optics less
critical and it's also easier to keep the LCD panel cool.

 

[Pawel Paron]

Is this one of the home-made video projectors that uses a standard metal
halide discharge lamp and a full size LCD monitor for the image?

Yes, it is:

http://acn.waw.pl/pawelp/inne/projektor/

Pawel

 

[Pawel Paron]

If the arc tube is leaky, the lamp's electrical characteristics will
change as the arc tube loses mercury.

I can hardly imagine such a tiny leak, that wouldn't quickly damage lamp
performance, especially that pressure must be significantly different during
on and off conditions. And, if there is really a vacuum in the outer shell,
any leak would quickly suck out the gas content of the arc tube, leaving the
lamp dead. Maybe under extremely high temperatures, the glass starts some
tiny leaks, like developing some microscopic pores, that will slowly let the
gas out? Or there was a temporary leak during first hours of "overpowered"
usage, that magically sealed itself after a while? Just an idea, but I've no
clue about physics inside MH lamp, so please don't laugh at me. Yesterday in
the evening this lamp was still fine, I didn't notice any visible change in
brightness or colour, just the same trouble with the ignition.

Pawel

 

[Pawel Paron]

Or another idea (can be also stupid): maybe due to this high temperature,
some substance evaporated or reacted in other ways inside the outer glass,
and created this "glowing gas"? Something from metal wires, or from the
glass, or some chemical deposits, left inside during the manufacturing
process?

Pawel

 

[JB]

Or another idea (can be also stupid): maybe due to this high temperature,
some substance evaporated or reacted in other ways inside the outer glass,
and created this "glowing gas"? Something from metal wires, or from the
glass, or some chemical deposits, left inside during the manufacturing
process?

Very ,very unlikely in an Osram lamp. There is also a reactive metal 'flag'
in the lamp called a 'getter' to absorb any microscopic traces of water
vapour and oxygen which may be left after sealing the outer envelope.

JB

 

[Clive Mitchell]

Yes, it is:

http://acn.waw.pl/pawelp/inne/projektor/

Looks good. One thing to be careful of is the lamps reflector
arrangement. If the reflector focusses the light in a manner that causes
a hot-spot it can cause seal failure.

 

[Pawel Paron]

Looks good. One thing to be careful of is the lamps reflector
arrangement. If the reflector focusses the light in a manner that causes a
hot-spot it can cause seal failure.

Unfortunately it's the only reasonable solution for a DIY projector. There
are only spherical reflectors easily available, as parabolic or elliptical
one would be enormously huge and expensive. And a spherical reflector has to
be focused on the light source, thus rising heat in the lamp. At least I
found a dichroic one, so some infrared is not reflected back on the arc
tube.

Anyway, this is proven to work by many people, at least with not overpowered
lamps, and I also tested it for months with a 400W lamp, overpowered to
about 550W, no problems. By the way, yesterday my 250W lamp overpowered to
400W was still fine.

Pawel