Gas Discharge Lamps

Gas Discharge Lamps
Gas Discharge Lamps, Ballasts, and Fixtures
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Sam's and Don's D-Lamp FAQ
Gas Discharge Lamps, Ballasts, and Fixtures
Principles of Operation, Circuits, Troubleshooting, Repair
Version 1.35
Copyright (C) 1996,1997,1998,1999
Samuel M. Goldwasser Donald L. Klipstein
--- All Rights Reserved --Corrections or suggestions to: [email protected] or [email protected]
Reproduction of this document in whole or in part is permitted if both of the
following conditions are satisfied:
1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.
Table of Contents
Authors and Copyright
Gas discharge lamp basics
Safely Working with Gas Discharge Lamps and Fixtures
Neon Technology
Neon Lights and Signs
Power Supplies for Neon
Neon Sign Installation
Problems With Neon
Comments on Little Neon Bulbs and Tubes
High Intensity discharge Lamps
High Intensity Discharge (HID) Lamp Technology
Problems With High Intensity Discharge Lamps
Troubleshooting a Discharge Lamp Fixture
Ballasts and Bulbs Should be Matched!
Operation of Discharge Lamps on DC
Special purpose HID lamps such as xenon and HMI
HID Automotive Headlights
Substitution of Metal Halide Lamps?
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Low Pressure Sodium Lamps
Back to Discharge Lamp FAQ Table of Contents.
Authors and Copyright
Authors: Samuel M. Goldwasser and Donald L. Klipstein
Corrections/suggestions: [email protected] or [email protected]
Copyright (c) 1996,1997,1998,1999
All Rights Reserved
Reproduction of this document in whole or in part is permitted if both of the following conditions are
1.This notice is included in its entirety at the beginning.
2.There is no charge except to cover the costs of copying.
We will not be responsible for damage to equipment, your ego, county wide power outages,
spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or
worse that may result from the use of this material.
Back to Discharge Lamp FAQ Table of Contents.
Gas discharge lamp basics
The use of electrically excited gas discharges significantly predates the invention of the incandescent
lamp. Physics labs of yesteryear as well as today have use of a variety of gas filled tubes used for
numerous purposes involving light generation including spectroscopy, materials analysis, studies of
gas dynamics, and laser pumping. Look through any scientific supply catalog and you will see many
different types of gas filled tubes in all shapes and sizes.
Gas discharge lamps are used in virtually all areas of modern lighting technology including common
fluorescent lighting for home and office - and LCD backlights for laptop computers, high intensity
discharge lamps for very efficient area lighting, neon and other miniature indicator lamps, germicidal
and tanning lamps, neon signs, photographic electronic flashes and strobes, arc lamps for industry
and A/V projectors, and many more. Gas discharge automotive headlights are on the way - see the
section: "HID automotive headlights".
Because of the unusual appearance of the light from gas discharge tubes, quacks and con artists also
have used and are using this technology as part of expensive useless devices for everything from
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curing cancer to contacting the dead.
Unlike incandescent lamps, gas discharge lamps have no filament and do not produce light as a
result of something solid getting hot (though heat may be a byproduct). Rather, the atoms or
molecules of the gas inside a glass, quartz, or translucent ceramic tube, are ionized by an electric
current through the gas or a radio frequency or microwave field in proximity to the tube. This results
in the generation of light - usually either visible or ultraviolet (UV). The color depends on both the
mixture of gasses or other materials inside the tube as well as the pressure and type and amount of
the electric current or RF power. (At the present time, this document only deals with directly excited
gas discharge lamps where an AC or DC electric current flows through the gas.)
Fluorescent lamps are a special class of gas discharge lamps where the electric current produces
mostly invisible UV light which is turned into visible light by a special phosphor coating on the
interior of the tube. See: Fluorescent Lamps, Ballasts, and Fixtures for more info.
The remainder of this document discusses two classes of gas discharge lamps: low pressure 'neon'
tubes used in signs and displays and high intensity discharge lamps used for very efficient area and
directional lighting.
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. See the document: Notes on the
Troubleshooting and Repair of Small Switchmode Power Supplies
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
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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!
Back to Discharge Lamp FAQ Table of Contents.
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.
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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:
The Internet's Neon Shop
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 ([email protected])).
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
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
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insulation and destroy the transformer.
Don't attempt to run too little tube on a transformer, since it can cause overheating and burnout.
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 ([email protected])).
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:
1. 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.
2. The voltage requirement varies inversely with the tubing diameter. That is large diameters of a
lower voltage requirement than small diameters.
3. The voltage requirement varies directly with tubing length.
4. The number of units (or pairs of electrodes) increases the voltage requirement because the
electrodes have a voltage drop.
5. Wiring methods and length will also contribute to the formula but that's a whole 'nuther
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:
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1. 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).
2. 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 ([email protected]).)
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 selfservice 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.
Back to Discharge Lamp FAQ Table of Contents.
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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 warmup 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 warmup 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.
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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 warmup 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 ([email protected]).)
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
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
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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:
1. 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.
2. 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.
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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.
3. 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.
4. 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
5. 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
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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
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
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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
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
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
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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
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
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 warmup time. As noted in the section: "High
intensity discharge (HID) lamp technology", common HID lamps require a warmup 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 warmup 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
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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 ([email protected]).)
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
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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
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!
6. Back to Discharge Lamp FAQ Table of Contents.
Low Pressure Sodium Lamps
(Portions from: Bruce Potter ([email protected]))
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
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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.
7. Back to Discharge Lamp FAQ Table of Contents.
-- end V1.34 -Back to Don's Home Page.
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