Safely Working with Gas Discharge Lamps and
Fixtures
Fixtures for gas discharge lamps may use up to 30,000 V while starting
depending on technology. And, they are often not isolated from the power
line. Neon signs are powered by transformers or electronic ballasts producing
up to 15,000 V or more. Thus, the only safe way to work with these is to
assume that they are potentially lethal and treat them with respect.
Hazards include:
- Electric shock. There is usually little need to probe a live fixture.
Most problems can be identified by inspection or with an ohmmeter or
continuity tester when unplugged.
- Discharge lamps and fixtures using iron ballasts are basically pretty
inert when unplugged. Even if there are small capacitors inside the
ballast(s) or for RFI prevention, these are not likely to bite. However,
you do have to remember to unplug them before touching anything!
Neon signs using iron transformers are also inert when unpowered - just make
sure they are off and unplugged before touching anything!
- However, those using electronic ballasts can have some nasty charged
capacitors so avoid going inside the ballast module and it won't hurt to
check between its outputs with a voltmeter before touching anything.
Troubleshooting the electronic ballast module is similar to that of a
switchmode power supply.
- The pulse starters of some high intensity discharge lamps may produce up
to 30 kV during the starting process. Obviously, contact with this voltage
should be avoided keeping in mind that 30 kV can jump over an inch to
anyplace it wants!
- Nasty chemicals: Various toxic substances may be present inside high
pressure discharge lamps (sodium and mercury) and neon signs (some phosphors).
Contact with these substances should be avoided. If a lamp breaks, clean up
the mess and dispose of it properly and promptly. Of course, don't go out
of your way to get cut on the broken glass! WARNING: Metallic sodium reacts
with water to produce hydrogen gas, an explosive. However, it is unlikely
that the inner tube of a sodium vapor lamp would break by accident.
- Ultra-Violet (UV) light: High intensity discharge lamps generate
substantial UV internally, often the particularly nasty UV-B variety. Unless
designed to generate UV (for medicinal purposes, photoengraving, or
whatever), the short wave radiation will be blocked by the outer
glass envelope and/or phosphor coating. However, should the outer envelope
break or be removed, the lamp will still operate (at least for a while - some
have a means of disabling themselves after a few hours or less of exposure
to air). DO NOT operate such a lamp preferably at all but if you do, at least
take appropriate precautions to avoid any exposure to the UV radiation.
And take care around sharp sheet metal!
Neon Technology
Neon Lights and Signs
Neon technology has been around for many years providing the distinctive
bright glowing signs of commerce of all kinds before the use of colored
plastics became commonplace.
Neon tubes have electrodes sealed in at each end. For use in signs, they
are formed using the glass blower's skill in the shape of letters, words,
or graphics. Black paint is used to block off areas to be dark. They are
evacuated, backfilled, heated (bombarded - usually by a discharge through
the tube at a very high current) to drive off any impurities, evacuated
and then backfilled with a variety of low pressure gasses.
Neon is the most widely known with its characteristic red-orange glow.
Neon may be combined with an internal phosphor coating (like a
fluorescent tube) to utilize neon's weak short-wave UV emissions. A
green-emitting phosphor combines with neon's red-orange glow to make a
less-red shade of orange. A blue-emitting phosphor may be used to result
in a hot-pink color. Neon may be used in tubing made of red glass to
produce a deep red color.
Other colors are usually produced by tubing containing argon and mercury
vapor. The mercury is the active ingredient, the argon produces
negligible radiation of any kind but is important for the "neon" tubing to
work. Clear tubing with mercury/argon glows a characteristic light blue
color.
Such tubing is often phosphor-coated on the inside, to utilize the
major short-wave UV emission of low-pressure mercury. In this way, much
of the "neon" tubes in use are a kind of fluorescent lamp.
Phosphor-coated tubing with mercury can glow blue, blue-green, slightly
white-ish green, light yellow, bright pink, light purple, or white.
Use of mercury vapor with colored tubing (with or without phosphors) can
provide a lime-green or deep blue or deep violet-blue.
Nowadays, nearly all "neon" tubing contains neon or mercury vapor (with
argon), whether with or without phosphors and/or colored glass. Well in
the past, various colors were obtained (generally at reduced efficiency)
by using different gases.
For example, helium can produce a white-ish orange light in shorter
length, smaller diameter tubing. Hydrogen in this case makes a
lavender-hot-pink color. These gases glow more dimly with duller color
shades in larger tubing. Krypton makes a dull greenish color. Argon
makes a dimmish purple color. Nitrogen (generally in shorter length
tubing) makes a grayish purple-pink color. Xenon, which is expensive,
generally glows with a dim bluish gray color, along with the glass tubing
giving a slight dim blue fluorescence from very short wave UV from the
xenon discharge. Krypton also often causes a dim blue glass fluorescence.
For general information on neon signs and technology including a neon FAQ, see:
Power Supplies for Neon
Extremely high voltage power supplies are used to power neon signs. In
the past, this was most often provided by a special current limited HV
line transformer called a neon sign or luminous tube transformer. The
output is typically 6,000 to 15,000 VAC at 15 to 60 mA. One such unit
can power 10s of feet of tubing. This transformer acts as its own ballast
providing the high voltage needed for starting and limiting the running
current as well. Warning: the output of these transformers can be lethal
since even the limited current availability is relatively high.
As with everything else, the newest neon sign power supplies use an
electronic AC-AC inverter greatly reducing the size and weight (and
presumably cost as well) of these power supplies by eliminating the
large heavy iron transformer.
Small neon lamps inside high-tech phones and such also use solid state
inverters to provide the more modest voltage required for these devices.
Neon Sign Installation
(From: Clive Mitchell (clive@emanator.co.uk)).
The voltage required to light a run of neon tube is variable according to
diameter, gas type, pressure and number of tubes in circuit.
For a 15 kV transformer and neon gas you could run:
- 33 feet of 10 mm tube,
- 45 feet of 12 mm tube,
- 60 feet of 15 mm tube,
- 78 feet of 20 mm tube,
- 102 feet of 25 mm tube.
- Deduct one foot of tube for every pair of electrodes (tube section).
These figures are based on a chart in "Neon Techniques And Handling"
which is the traditional neon reference.
The larger the diameter of the tube, the lower the voltage required, and
the dimmer it will be. Transformers come with different current
ratings. For larger diameter tubes, you can increase brightness by
using a higher current.
- Don't attempt to run too much tube on a transformer, since it can cause
breakdown of the insulation and destroy the transformer.
- Don't attempt to run too little tube on a transformer, since it can
cause overheating and burn-out.
It is absolutely imperative that proper neon sign cabling and insulators are
used, and that all local regulations are strictly followed. If you are
intending to work with neon tubing, you should learn as much as possible
first, since neon poses both a shock and serious fire risk if installed
incorrectly.
The lengths quoted above may vary according to the transformer you use. The
transformer manufacturers usually provide their own loading charts on request.
Anyone using this information does so at their own risk, and I cannot be held
responsible for any horrible smouldering deaths experienced by incompetent
dabblers, etc.
(From: Kenny Greenberg (kenny@neonshop.com)).
The neon circuit is not so simple. In a standard AC circuit neon acts like a
diac - high breakover voltage followed by fast drop in resistance. Neon sign
transformers are designed to 'leak' and thus self-regulate. You have a
combined resistive and reactive circuit.
But take heart, it's all been figured out. :-)
There are a few variables:
- A 'purely' neon filled tube (generally in the red range) has a higher
voltage requirement than an argon-mercury tube (whose discharge is usually
providing UV for phosphor with a wide range of colors.
- The voltage requirement varies inversely with the tubing diameter. That is
large diameters of a lower voltage requirement than small diameters.
- The voltage requirement varies directly with tubing length.
- The number of units (or pairs of electrodes) increases the voltage
requirement because the electrodes have a voltage drop.
- Wiring methods and length will also contribute to the formula but that's a
whole 'nuther discussion.
You can download a free Neon
Voltage Calculator for Windows.
An old tech method for determining the voltage requirement is to use a
Variac on a large neon transformer. Bring the voltage down to where the neon
just flickers. This should be at a point approximately 78% of the required
voltage.
A better way involves using a milliameter to measure open circuit and closed
circuit current and an rms voltmeter to measure actual operating voltage.
Problems With Neon
These fall into two categories:
- Power supply - like fluorescent ballasts, the high voltage transformers
can fail resulting in reduced (and inadequate) voltage or no power at all.
Since they are already current limited, overheating may not result and
any fuse or circuit breaker may be unaffected. The use of a proper (for
safety if nothing else) high voltage meter can easily identify a bad
transformer. If a high voltage probe is not available, position (with
power off!) the ends of well insulated wires connected to the outputs
of the transformer a fraction of an inch apart (about 1/32" per 1,000 V
of transformer rating) and apply the power from a safe distance. If a hot
arc results, the transformer is likely good (at least when cold).
- Neon tubes - these may lose their ability to sustain a stable discharge
over time as a result of contamination, gas leakage, or electrode damage
(either from normal wear or due to excessive current). Check for obvious
damage such as a cracked tube or cracked seals around the electrodes or
badly deteriorated electrodes. A previously working tube that now will
not strike or maintain a stable discharge on a known good transformer
will need to be replaced or rebuilt.
Comments on Little Neon Bulbs and Tubes
The comments below relate to the little neon bulbs used as indicators, for
voltage regulation or limiting, and other applications in all sorts of
electronic equipment.
(From: Mark Kinsler (kinsler@frognet.net).)
Neon lamps can be used for voltage limiters and oscillator elements and just
about anywhere else that a non-linear element is needed. The tremolo circuit
in the classic Fender guitar amplifier uses a neon lamp relaxation oscillator.
The neon lamp is heat-shrinked to a CdS photocell in the volume control
circuit.
Less well-known is the fact that you can make a pretty reasonable computer
logic element out of them: I believe that this was tried sometime in the
1940's.
Another cool use is as a radiation sensor: you bias the lamp so that it almost
turns on, after which any incident radiation: radio waves (as in police
radar), light, or gamma radiation will kick the lamp on. There were various
circuits in the 1950's that used neon lamps to detect uranium, fight nuclear
destruction, or escape the newly-developed police radar guns.
And finally, there's the mystery elevator button. Again, you bias the lamp so
that it almost, but not quite, turns on. If you enclose the lamp properly,
it'll stay off until you touch it. The electric field variation from your
touch will turn the thing on, and it'll stay on. Such lamps are used in some
self-service elevators: once the lamp is fired, the low voltage across it is
sensed by the ancient logic circuits of the elevator controller and it'll send
the elevator to the appropriate floor. These were a lot of fun in the 1960's.
I think the controllers used vacuum tubes.
The problem with neon lamps is that they're not so reliable. Their turn-on
voltage isn't particularly stable. This means that oscillators have a
tendency to drift as the lamps age or when ambient radiation changes. I
suspect that the computers are slow and cranky, and the radiation detector
isn't anything you'd wish to stake your life or drivers' license on.
Still, they're great fun, and I have a fine time with them. One other use:
hang a neon lamp across a telephone line to detect the ring signal. Place it
in series with a piezo beeper, and you've got a reliable telephone ringer.
High Intensity discharge Lamps
High Intensity Discharge (HID) Lamp Technology
These have been used for a long time in street, stadium, and factory lighting.
More recently, smaller sizes have become available for home yard and crime
prevention applications. Like other gas discharge lamps, these types
require a special fixture and ballast for each type and wattage. Unlike
fluorescents, however, they also require a warm-up period.
There are three popular types:
- High pressure mercury vapor lamps contain an internal arc tube made of
quartz enclosed in an outer glass envelope. A small amount of metallic
(liquid) mercury is sealed in an argon gas fill inside the quartz tube.
After the warm-up period, the arc emits both visible and invisible (UV)
light. High pressure mercury vapor lamps (without color correction)
produce a blue-white light directly from their discharge arc. Phosphors
similar to those used for fluorescent lamps can be used to give these a
color closer to natural light. (Without this color correction, people
tend to look like cadavers). Mercury vapor lamps have the longest life
of this class of bulbs - 10,000 to 24,000 hours. The technology was first
introduced in 1934 and was the first of the commercially viable HID lamps.
- Metal halide lamps are constructed along similar lines to mercury vapor
lamps. However, in addition to the mercury and argon, various metal
halides are included in the gas fill. The most popular combination
is sodium iodide and scandium iodide. A few versions of this lamp
have lithium iodide as well. A much less common version has sodium
iodide, thallium iodide, and indium iodide. The use of these compounds
increases the luminous efficiency and results in a more pleasing color
balance than the raw arc of the mercury vapor lamp. Thus, no phosphor
is needed to produce a color approaching that of a cool white
fluorescent lamp with more green and yellow than a mercury vapor lamp
(without correction). Some metal halide lamps have a phosphor that adds
some orange-ish red light, but not much, since the metal halide arc does
not emit much UV.
- High pressure sodium vapor lamps contain an internal arc tube made of
a translucent ceramic material (a form of aluminum oxide known as
"polycrystalline alumina"). Glass and quartz cannot be used since they
cannot maintain structural strength at the high temperatures (up to 1300
degrees C) encountered here, and hot sodium chemically attacks quartz
and glass. Like other HID lamps, the arc tube is enclosed in an outer
glass envelope. A small amount of metallic (solid) sodium in addition
to mercury is sealed in a xenon gas fill inside the ceramic arc tube.
Some versions of this lamp use a neon-argon mixture instead of xenon.
Basic operation is otherwise similar to mercury or metal halide lamps.
High pressure sodium vapor lamps produce an orange-white light and have
a luminous efficiency much higher than mercury or metal halide lamps.
Since hot liquid sodium often eventually leaches through things and can
get lost this way, sodium lamps have a surplus of sodium in them.
Proper lamp operation depends on the sodium reservoir being within
a proper temperature range.
Mercury vapor lamps are roughly as efficient as fluorescent lamps. Metal
halide lamps are much more efficient, generally around 50 to 75 percent
more efficient than fluorescent lamps. High pressure sodium lamps are
roughly twice as efficient as fluorescent lamps.
Unlike fluorescent lamps, HID lamps will give full light output over a
wide range of temperatures. This often makes HID lamps more suitable than
fluorescent lamps for outdoor use.
When cold, the metallic mercury or sodium in the arc tube is in its normal
state (liquid or solid) at room temperature. During the starting process,
a low pressure discharge is established in the gases. This produces very
little light but heats the metal contained inside the arc tube and gradually
vaporizes it. As this happens, the pressure increases and light starts being
produced by the discharge through the high pressure metal vapor. A quite
noticeable transition period occurs when the light output increases
dramatically over a period of a minute or more. The entire warm-up process
may require up to 10 minutes, but typically takes 3 to 5 minutes. A hot
lamp cannot be restarted until it has cooled since the voltage needed to
restrike the arc is too high for the normal AC line/ballast combination to
provide.
Problems With High Intensity Discharge Lamps
While HID lamps have a very long life compared to incandescents (up to
24,000 hours), they do fail. The ballasts can also go bad. In addition,
their light output falls off gradually as they age. For some types, light
output may drop to half its original value towards the end of their life.
A lamp which is cycling - starting, warming up, then turning itself off -
is probably overheating due to a bad bulb or ballast. A thermal protector
is probably shutting down the fixture to protect it or the arc is being
extinguished on its own. However, make sure that it is not something
trivial like a photoelectric switch that is seeing the light from the lamp
reflected from a white wall or fence and turning the fixture off once the
(reflected) light intensity becomes great enough!
Sodium lamps sometimes "cycle" when they have aged greatly. The arc tube's
discolorations absorb light from the arc, causing the arc tube to overheat,
the sodium vapor pressure becomes excessive, and the arc cannot be
maintained. If a sodium lamp "cycles", the first suspect is an aging bulb
which should be replaced.
Sodium lamp "cycling" used to be very common, but in recent years the lamp
manufacturers have been making sodium lamps that are less prone to cycling.
If you have more than one fixture which uses **identical** bulbs, swapping
the bulbs should be the first test. If the problem remains with the fixture,
then its ballast or other circuitry is probably bad. Don't be tempted to
swap bulbs between non-identical fixtures even if they fit unless the bulb
types are the same.
Warning: do not operate an HID lamp if the outer glass envelope is cracked
or broken. First, this is dangerous because the extremely hot arc tube can
quite literally explode with unfortunate consequences. In addition, the
mercury arc produces substantial amounts of short wave UV which is extremely
hazardous to anything living. The outer glass normally blocks most of
this from escaping. Some lamps are actually designed with fusable links
that will open after some specified number of hours should air enter the
outer envelope. Thus, an undetected breakage will result in the lamp dying
on its own relatively quickly.
Troubleshooting a Discharge Lamp Fixture
(From: Greg Anderson (a3a30878@bc.sympatico.ca).)
The following applies directly to high pressure sodium lamps. It may also
also be used for metal halide and mercury vapour lamp problems as long as
references to the starter are ignored. (Metal halide and mercury vapour
lamps do not have starters, except for "instant re-light" metal hhalide
lamps.)
The starter produces about 2 to 5 kV spikes to ionize the gas in the
lamp. The starter normally has a triac across the ballast and a diac
trigger cct. When open cct voltage is across the lamp, the diac fires
the triac to short the ballast, the triac then opens. This "kick"
produces the voltage spike. Once the gas ionizes, the lamp impedance
drops then gradually increases as the lamp warms up. The lamp running
voltage is about 1/2 of the open cct voltage
With the lamp removed and power on, you can normally hear a good starter
"ticking".
The open cct voltage is stamped on the ballast and is between about 150
and 350 Vac, depending on lamp wattage and ballast. Also, a capacitor is
often connected in series with lamp to improve peaking and ballast
action.
Steps to follow:
- Bypass the photo cell - It may be bad
- Check connections - water, salt, and bird poop are not good for wiring
- Check the capacitor, if installed - normally they blow-up when bad
- Check for open/shorted ballast.
- Power up and check for starter "ticking"
- REMOVE starter from cct and measure open cct volts
- Check/Replace lamp
- Check/replace lamp socket
- Replace starter
- Replace complete fixture.
- Replace electrician. :)
Repairing a starter is not economically viable and often proves that
electronic devices contain smoke and sometimes fire.
Ballasts and Bulbs Should be Matched!
HID bulbs generally need specific ballasts, and any given ballast can usually
safely and effectively operate only one type or a few types of HID bulbs.
The bulb wattage must be matched to the ballast. A smaller bulb will
usually be fed a wattage close to what the proper bulb takes, and will
generally overheat and may catastrophically fail. Any catastrophic
failures may not necessarily happen quickly. A larger bulb will be
underpowered, and will operate at reduced efficiency and may have a
shortened lifetime. The ballast may also overheat from prolonged
operation with an oversized bulb that fails to warm up.
See The Discharge Lamp
Mechanics Document (rather technical) for why it can be bad to
underpower an arc discharge lamp.
Even if the ballast and bulb wattages match, substitutions can be
limited by various factors including but not limited to different
operating voltages for different bulbs. Examples are:
- Pulse-start sodium lamps often have a slightly lower operating voltage
than metal halide and mercury lamps of the same wattage, and ballasts
for these sodium bulbs provide slightly more current than mercury and
metal halide ballasts for the same wattage would. The higher current
provided by the pulse-start sodium ballast can overheat mercury and
metal halide lamps. Mercury and metal halide lamps may also "cycle"
on and off in lower voltage sodium ballasts, such as many 50 to 100
watt ones.
- Metal halide lamps have an operating voltage close to that of mercury
lamps in many wattages, but have stricter tolerances for wattage and
current waveform. Metal halides also usually need a higher starting
voltage. Most metal halide lamps 100 watts or smaller require a high
voltage starting pulse around or even over 1,000 volts.
175 to 400 watt metal halide lamp ballasts can power mercury lamps of
the same wattage, but the reverse is not recommended. Mercury lamps
50 to 100 watts will work on metal halide ballasts, but hot restriking
of mercury lamps 100 watts or smaller on metal halide lamps may be hard
on the mercury lamp since the starting pulse can force current through
cold electrodes and the starting resistor inside the mercury lamp.
- 1,000 watt mercury lamps come in two operating voltages, one of which
is OK for 1,000 watt metal halide ballasts. A few wattages of pulse-start
sodium (150 watts?) come in two voltages.
A low voltage lamp in a high voltage ballast will be underpowered,
resulting in reduced efficiency, possible reduced lamp life, and possible
ballast overheating. A high voltage lamp in a low voltage ballast will
usually cycle on and off, operate erratically, or possibly overheat.
This will usually result in greatly reduced lamp life in any case.
- One class of sodium lamps is made to work in mercury fixtures, but
these only work properly with some mercury ballasts, namely:
- 'Reactor' (plain inductor) ballasts on 230 to 277 volt lines.
- 'High leakage reactance autotransformer' ballasts, preferably with
an open circuit voltage around 230 to 277 volts. NOT 'lead',
'lead-peak' nor any metal halide ballast!
These sodium lamps may suffer poor power regulation and accelerated aging
in the wrong mercury ballasts, especially after some normal aging changes
their electrical characteristics. Also, these lamps may overheat and
will probably have shortened life with pulse-start sodium ballasts.
- Many sodium lamps require a high voltage starting pulse provided only
by ballasts made to power such lamps.
Operation of Discharge Lamps on DC
Sometimes, one may want to run a discharge lamp on DC. There are two
possible reasons:
- Only DC power is available.
- To reduce flicker. Sometimes, the lamp performs differently for
electricity flowing in one direction than the other. In addition, the
positive and negative ends of the arc can make different amounts of
light, resulting in a flicker rate equal to the AC frequency rather than
twice the AC frequency.
However, end flicker is usually not significant. In HID lamps, the
total arc size is generally small. Only if the fixture has a reflector
that causes some areas to receive light from only one end of the arc
should end flicker be significant. In most multi-tube fluorescent
fixtures, the tubes are usually in series pairs with the two tubes in any
pair oriented in opposite directions. This generally reduces end flicker
effects, especially in fixtures with diffusing lenses.
Bulbs should perform close enough to identically in both directions, unless
the bulb is near the end of its life. In such a case, one electrode
deteriorates enough to affect performance before the other does. However,
this generally indicates a need to replace the bulb rather than to attempt
to make it flicker less.
If you want to rectify the AC to provide the bulb with DC, use a bridge
rectifier after the ballast. Most ballasts, including all "iron" types,
require AC of the proper voltage and frequency to work. Do this only if
only two wires feed the bulb. Otherwise, diodes in the bridge rectifier may
short parts of the ballast to each other, at least for half the AC cycle.
Problems can also occur with fluorescent ballasts with filament windings.
Only fully isolated filament windings or separate filament transformers
should be used if you rectify the output of a ballast with filament windings.
Also, the bridge rectifier must withstand the peak voltage provided by the
ballast.
If the power supply is DC of adequate voltage, you need a resistor ballast
or an electronic ballast specifically designed to run your lamp from the
available DC voltage. "Iron" ballasts only limit current when used with
AC. Preheat fluorescent lamps operated from DC supplies and without
special ballasts need both the usual "iron" ballast to provide the
starting "kick" and a resistor to limit current.
In addition, most discharge lamps are only partially compatible with DC, and
some are not compatible at all.
Mercury vapor and fluorescent lamps generally work on DC. However, the life
may be shortened somewhat by uneven electrode wear.
Fluorescent lamps may get dim at one end with DC. Since the mercury vapor
ionizes more easily than the argon, some of it exists as positive ions. This
can cause the mercury to be pulled to the negative end of the tube, resulting
in a mercury shortage at the positive end. This is more of a problem with
longer length and smaller diameter tubes.
Some fluorescent fixtures made for use where the power available is DC have
special switches to reverse polarity every time the fixture is started. This
balances electrode wear and reduces mercury distribution problems.
Mercury vapor lamps generally work OK with DC, but some may only reliably
work properly if the tip of the base is negative and the shell of the base is
positive. This is because the starting electrode does its job best when it
is positive.
In addition, if the nearby main electrode is positive, it may cause a thin
film of metal condensation that shorts the starting electrode to the nearby
main electrode. This may make some brands, models, and sizes of mercury
lamps unable to start after some use. The negative main electrode will not
release as much vaporized electrode material, since the electrode material
easily forms positive ions making the electrode material vapor tend to
condense on the electrode rather than condense on nearby parts of the arc
tube.
Metal halide and sodium lamps should not get DC. Use these only with
ballasts that give the bulb AC. In metal halide lamps, ions from the
molten halide salts can leach into hot quartz in the presence of a DC
electric field. This can cause strains in the quartz arc tube. At the
ends of the arc tube, electrolysis may occur, releasing chemically
reactive halide salt components that can damage the arc tube or the
electrodes. The arc tube may crack as a result.
There are a few specialized metal halide lamps that are made to work on DC.
These often have asymmetrical electrodes and/or short arc lengths. These
lamps often also must be operated only in specific positions, and only with
the type of current they were designed for in order to achieve the proper
distribution of active ingredients within the arc tube and to achieve proper
electrode usage. For example, some of these lamps may go wrong in some way
or another with AC.
In high pressure sodium lamps, which contain both sodium and mercury, the
sodium forms positive ions more easily than the mercury does and drifts
towards the negative electrode. The positive end can go dim from a lack
of sodium. In addition, if any part of the arc tube is filled with a
mixture containing excessive sodium and a lack of mercury, heat conduction
from that part of the arc to the arc tube will increase. Furthermore, the
hot arc tube may suffer electrolysis problems over time in the presence of
sodium ions and a DC electric field.
Low pressure sodium lamps should not get DC for the same reasons. The sodium
is likely to drift to the negative end of the arc tube, and hot glass will
almost certainly experience destructive electrolysis problems if exposed to
hot sodium or sodium ions and a DC electric field.
Special purpose HID lamps such as xenon and HMI
The usual general purpose HID lamps are mercury vapor, metal halide, and
high pressure sodium. You can get these at home centers, although usually
only in wattages up to 400 watts. These versions of HID lamps are
optimized for high efficiency, long life, and minimized manufacturing
cost.
However, the arc surface brightness of these lamps is roughly equal to the
surface brightness of incandescent lamp filaments and general purpose
halogen lamp filaments. For some applications such as endoscopy and movie
projection, it is necessary to have a much more concentrated light source.
This is where specialized HID lamps such as short arc lamps and HMI lamps
come in.
Short arc lamps consist of a roughly spherical quartz bulb with two heavy
duty electrodes spaced only a few millimeters apart at the tips. The bulb
may contain xenon or mercury or both. Mercury short arc lamps have an
argon gas fill for the arc to start in.
In a short arc lamp, the arc is small and extremely intense. The power
input is at least several hundred and more typically a few thousand watts
per centimeter of arc length. The operating pressure in the bulb is
extremely high - sometimes as low as 20 atmospheres, more typically 50 to
over 100 atmospheres. These lamps are an explosion hazard!
Mercury short arc lamps are used when a compact, intense source of UV is
needed or where one cannot have the high voltage starting pulses needed
for xenon short arc lamps. Mercury short arc lamps are slightly more
efficient than xenon ones. The pressure in a mercury short arc lamp does
not need to be as high for good efficiency as in a xenon one, but is still
tremendous.
Xenon short arc lamps are more common than mercury ones, since they do not
require time to warm up the way mercury lamps do and have a daylight-like
spectrum. A disadvantage of xenon is the requirement of a very high
voltage starting pulse - sometimes around 30 kilovolts!
Xenon short arc lamps are used for movie projection and sometimes for
searchlights. Lower wattage ones are used in specialized devices such as
endoscopes.
HMI lamps are metal halide lamps with a more compact and more intense arc.
The arc is larger and less intense than that of a short arc lamp. Typical
power input is hundreds of watts per centimeter of arc length, but gets to
a few kilowatts per centimeter in the largest ones.
HMI lamps are used in some spotlights. They are used in some endoscopes
and projection applications where the intensity of the HMI arc is adequate
since they cost less and last longer and are more efficient than true
short arc lamps.
There are all sorts of HMI and similar lamps, including HTI lamps and the
lamps used in HID auto headlights.
HID Automotive Headlights
First there were gas lamps, then there were electric bulbs, then sealed
beam, then halogen. Now, get ready for - drum roll please! - high intensity
discharge lamps with sophisticated controllers. High-end automobiles from
makers like BMW, Porsche, Audi, Lexus, and now Lincoln are coming equipped
with novel headlight technology. No doubt, such technology will gradually
find its way into mainstream automobiles - as well as other applications
for mortals.
Among the potential advantages of HID headlights are higher intensity,
longer life, superior color, and better directivity:
- Light intensity - HID lamps are about 3 times as efficient as halogen
lamps. Thus, even when the efficiency of the DC-DC converter is taken
into consideration, the lower power input can actually result in much
brighter headlights than are possible with halogen bulbs. This reduced
power also leads to cooler operation and less drain on the battery and
alternator.
- Lifespan - an HID lamp can be expected to last 2,700 hours or more and
thus covered under the bumper to bumper warranty for 100,000 miles. As
a practical matter, the HID lamp may outlast the automobile. Since
warranty replacement of headlights turns out to be a significant expense,
there is strong incentive to see this long lived technology take off.
- Spectral output - the light from the HID lamp is richer in blue (and more
like daylight) than halogen bulbs. This turns out to enhance reflectivity
of signs and road markings.
- Beam pattern - the small arc size of the HID lamp permits the optical
system to be optimized to direct light more effectively to where it is
needed and prevent it from spilling over to where it is not wanted.
In order to make this practical - even for a $40,000 Lexus - special DC-DC
converter chips have been designed specifically with automotive applications
in mind. These, along with a handful of other basic electronic components,
implement a complete HID headlight control system.
The HID bulb itself is similar in basic design to traditional HID lamps:
Two electrodes are sealed in a quartz envelope along with a mix of solids,
liquids, and gasses. When cold, these materials are in their native state
(at room temperature) but are mostly gases when the lamp is hot. Starting
of these lamps may require up to 20 KV to strike an arc but only 50 to 150 V
to maintain it. Lamps may be designed to operate on either AC or DC current
depending on various factors including the size and shape of the electrodes.
A unique set of ballast operating parameters must be matched to each model
HID bulb.
Of all the problems that had to be addressed for HID headlights to become
practical (aside from the cost), the most significant was the warm-up time.
As noted in the section: "High intensity discharge (HID) lamp technology",
common HID lamps require a warm-up period of a few minutes before
substantially full light output is produced. This is, of course, totally
unacceptable for an automotive headlight both for cold start (imagine:
"Honey, I have to go cook the headlights") as well as when they need to be
blinked. The warm-up problem was solved by programming the controller to
deliver constant power to the lamp rather than the more common nearly
constant current that would be provided by a traditional ballast. With
this twist along with a special lamp design, the lamp comes up to at least
75% of full intensity in under 2 seconds. The controller also provides
'hot strike' capability for blinking (recall that HID lamps typically
cannot be restarted when hot). Thus, restarting a hot lamp is absolutely
instantaneous.
While this technology is just beginning to appear, expect inroads (no pun
intended) into household, office, store, factory, and other area and work
lighting. The combination of high efficiency, long life, desirable spectral
characteristics, small size, and solid state reliability should result in
many more applications in the near future. The nearly instant starting
capability addresses one of the major drawbacks of small HID lamps.
If you have some time and money to spare:
(From: Declan Hughes (hughes@aero.tamu.edu).)
Check out: OSRAM Sylvania Products Inc.
They have a "sample" for sale at $250.00 for one lamp including the 12 VDC
electronic ballast. 42 W total power, 35 W light power, 3,200/2,800 lm output
(there are two types, D2S and D2R), 2,000 hours rated lifetime, 91/80 lm/W
luminous efficacy, 4,250/4,150 K color temperature, 6,500 cd/cm^2 average
luminance, 4.2 mm arc length, burning position horizontal +/- 10 deg., luminous
flux after 1 sec. = 25%, max. socket temp. = 180 deg C, any errors are mine.
For more info, look in Don
Klipstein's Automotive HID Lamp File.
Substitution of Metal Halide Lamps?
The following was prompted by a request for info on replacing an (expensive)
250 watt metal halide lamp in a video projector with something else.
I would not substitute this lamp, for many reasons below:
The metal halide lamp requires a ballast. The ballast should only run a
250 watt metal halide lamp of the same arc voltage. You will have to
measure the arc voltage yourself after the lamp warms up, and do this
without exposing yourself to the nasty UV that some of these things emit
but which does not pass through glass. Arc voltages of many specialized
metal halide lamps are not widely published and may or may not be
available from the lamp manufacturer.
WARNING: The strike voltage on these may be several kV which will probably
obliterate your multimeter should the arc drop out and attempt to restart
while you are measuring it! Either the operating or strike voltage may
obliterate you should you come in contact with live terminals!
(Special metal halides probably usually only need a couple to a few kV.
Xenon metal halide automotive lamps need 6 to 12 kV to strike and 15 to 20 kV
for hot restrike. The worst are short arc xenon that may use up to 30 kV or
more.)
Most metal halide lamps are AC types and some are DC and you can only
use AC lamps on AC output ballasts and DC lamps on DC output ballasts.
Different metal halide lamps may have different requirements for
starting voltage also.
If you match arc voltage, AC/DC type, and the ballast will start the
lamp, you might be in business but good chance not. Many projector lamps
have specific cooling requirements and some have specific burning position
requirements. Metal halide lamps may prematurely fail (possibly violently!)
if they overheat, in addition to being off-color. If overcooled, they are
more like mercury lamps and will be off-color and have reduced light output.
In addition, some metal halide lamps have a halogen cycle in them to keep
the inner surface of the bulb clean, and that may not work if the lamp is
overcooled and not enough of the chemicals in the bulb get vaporized.
This could also even make the lamp fail.
If you get the alternate lamp to operate satisfactorily, the arc may be
in a different location from that of the original lamp. The arc may be of
a different shape or size than that of the original lamp. This can
affect your projection. Your projection may not get much light or may
have illumination of only part of the picture.
The arc may have a different color or spectrum, which can affect the
color rendering of what's being projected. Metal halide arcs are often
not of uniform color, and if the alternate lamp has a less color-uniform
arc than the original lamp then your pictures may have strange color tints
in them.
As for using a halogen instead of metal halide? You will get less
light, as well as problems from the filament having a different shape or
size than the original metal halide arc does. Most likely, the filament
is larger or longer than the arc and this will reduce the percentage of
the light being utilized. Should you try a halogen lamp hack, you will
almost certainly have to bypass the metal halide ballast. And halogen
lamps emit more infrared than metal halide lamps of the same wattage - you
might overheat the source of your image (e.g., LCD panel or transparency).
I would not recommend substituting a projector lamp for all of these
reasons. This should only be tried at your own risk and only by those
that are very familiar with all of the characteristics of the lamps in
question - including being familiar with burning position requirements,
cooling requirements, shape and size of the light-emitting region, etc.
Projector lamps in general, and especially specialized HID lamps, should
be used only in equipment made specifically to use the particular lamps in
question, or by those who know about these things well enough to make
their own ballasts and know the other messy things about these lamps. And
you may not save much by using a different lamp - specialized metal halide
lamps are all expensive.
And for anyone shopping for any sort of projector - look into price,
availability, and life expectancy of lamps!
Low Pressure Sodium Lamps
(Portions from: Bruce Potter (s60231@aix2.uottawa.ca))
Low pressure sodium lamps are the most efficient visible light sources in
common use. These lamps have luminous efficacies as high as 180 lumens per
watt.
A low pressure sodium lamp consists of a tube made of special sodium-resistant
glass containing sodium and a neon-argon gas mixture. Since the tube is
rather large and must reach a temperature around 300 degrees Celsius, the tube
is bent into a tight U-shape and enclosed in an evacuated outer bulb in order
to conserve heat. As an additional heat conservation measure, the inner
surface of the outer bulb is coated with a material that reflects infrared but
passes visible light. This material has traditionally been tin oxide or
indium oxide.
The electrodes are coiled tungsten wire coated with thermionically emissive
material, and somewhat resemble the electrodes of fluorescent lamps. Unlike
most fluorescent lamps, low pressure sodium lamps have only one electrical
connection to each electrode and the electrodes cannot be preheated.
The gas mixture is a "Penning" mixture, consisting mainly of neon with a small
amount of argon. Depending on who you listen to, this mixture is .5 to 2
percent argon, 98 to 99.5 percent neon. More argon-rich mixtures around 98-2
may be favored today since hot glass has some ability to absorb argon from a
low pressure electric discharge. Ideally the mixture should be only a few
tenths of a percent argon, in order to ionize most easily and do so much more
easily than pure neon or pure argon.
A significant surplus of sodium is contained in the glass arc tube since the
glass may absorb or react with some of the sodium. The sodium vapor pressure
is controlled by the temperature of the coolest parts of the arc tube. When
the arc tube reaches a proper temperature, further heating is reduced by the
lamp's efficiency at producing light instead of heat.
The arc tube has dimples in it, which are normally slightly cooler than the
rest of the arc tube. This causes the sodium metal to collect in the dimples
instead of covering a larger portion of the arc tube and blocking light.
The low pressure sodium lamp usually requires 5 to 10 minutes to warm up.
The light of low pressure sodium consists almost entirely of the orange-yellow
589.0 and 589.6 nM sodium lines. This light is basically monochromatic
orange-yellow. This monochromatic light causes a dramatic lack of color
rendition - everything comes out in an orange-yellow version of
black-and-white! This can cause some confusion in parking lots since cars
become more alike in color.
Some basically red and reddish color fluorescent inks, dyes, and paints can
fluoresce red to red-orange from the yellow sodium light and these will stand
out in sodium light with color differing from that of the sodium light.
Another disadvantage of low pressure sodium light is that many objects will
look darker than they would with an equal amount of other light. Red, green,
and blue objects look dark under low pressure sodium light. Most other
sources of light of sodium-like color such as "bug bulbs" have significant red
and green output and will render red and green objects at least somewhat
normally.
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