updated slightly 8/19/2002.
Another is "Electric Discharge Lamps", by John F. Waymouth, available from MIT Press.
After finding these books, you may want to look into others with similar call numbers. Using the Library of Congress system, look into everything with numbers around TK4000.
In a low pressure lamp, the electron and gas temperatures are very different, and the pressure is generally below .05 atmosphere. Power input is generally near or less than 1 watt per centimeter.
In a low-pressure lamp, a variation of this causes the same thing. If you double the current, you usually roughly double the concentration of excited gas atoms and free electrons. The concentration of ions must match that of free electrons but each excited atom is bombarded twice as much by free electrons (remember, there are twice as many electrons around for an excited atom to see). The average kinetic energy of the free electrons must decrease so that ion concentration is also only roughly doubled. To get slower free electrons, the electric field in the discharge (and voltage across the discharge) must decrease.
In either case, it is not a good idea to connect the lamp directly to a voltage source. Once the lamp starts conducting, increasing current will increase the lamp's conductivity, allowing more current to flow. This process does not level off until one of the following happens:
1. A large fraction of easily ionizable atoms are ionized,
2. The concentration of ions/free electrons is so high that more of these somehow impairs mobility of free electrons,
3. The power supply's or wiring's limitations limit the current.
At this point, the current is usually around or over 100 amps or so, and will likely blow fuses/pop breakers, and is certainly not good for the lamp.
The term "negative resistance" refers to a decrease in voltage across the lamp resulting from an increase in current through the lamp.
In a glow discharge, positive ions bombarding the cathode dislodge
electrons from the cathode material. There is a substantial electric field
near the cathode that accelerates ions toward the cathode to make this happen.
The whole process tends to complicate itself, resulting in a double layer
of glow around the cathode, thin dark spaces underneath and between these
layers, and a more substantial dark space between all of this and either
the anode or the main body of the discharge, whichever comes first. In
neon glow lamps, the anode is so nearby that no main discharge body occurs.
"Neon" signs are longer, so a main discharge body occurs. Since these
operate on AC, each end has a significant dark space only half the time,
so these regions are a bit dim rather than dark.
There is generally a natural current density in the cathode process,
generally around a milliamp to .1 amp per square centimeter, depending on
the gases involved, the pressure thereof, and the cathode material. A
glow discharge at this intensity is a "normal glow". Decreasing the current
causes the cathode's glowing layers to cover only part of the cathode. In
this case, the glow often moves around, causing a flickering effect.
If the current is more than enough to cause the cathode to be covered
with glow, (or if the glowing layers are forced into a thinner layer of
space than they normally use), abnormal glow results. The voltage drop of
the cathode process (this voltage is known as the "cathode fall") will be
higher than normal. This causes ions to bombard the cathode harder than
usual. This increases "sputtering", or dislodging of cathode material
atoms. Sputtering effectively "evaporates" cathode material and often
causes darkening of the lamp's inner surface.
Sputtering occurs more easily at higher cathode temperatures. It is generally
recommended to neither have significantly "abnormal" glow nor significant
temperature rise in the cathode, and especially not both of these combined.
The cathode fall of normal glow is usually 50 to 90 volts for neon, argon, krypton, xenon, or mixtures including significant amounts of any of these gases. Some metal vapors may have somewhat lower cathode falls. Nitrogen and some other gases have high cathode falls usually near or even well over 100 volts.
The cathode process in most HID lamps and fluorescent lamps is the
thermionic arc. In this process, at the proper high temperature, some
material in the cathode fails to keep a grip on its electrons. Therefore,
electrons simply flow from the cathode to the gas. The cathode fall is
usually around 10 volts, and the heat dissipated in this process keeps the
cathode hot enough to let electrons flow from it to the gas.
The current density at the cathode process of a thermionic arc is
generally in the tens or hundreds of amps per square centimeter of active
cathode surface, but can occaisionally be as low as around an amp per
square centimeter if a heat source other than the arc heats the cathode.
Another arc process is the cold cathode arc. In this process, ions bombard the cathode material and dislodge electrons from it. This seems similar to the glow discharge, but the effect is quite different. The current density in the cathode process is usually hundreds or thousands of amps per square centimeter. The cathode fall is usually near the ionization potential of the cathode material or the main active gas ingredient, whichever is lower (for a minimum) to twice whichever is higher (for a maximum). Substantial sputtering may occur, especially if the cathode is hot. Cold tungsten is usually reasonably tolerant of this, permitting the use of this process in xenon flashtubes.
An arc is often not entirely thermionic nor cold-cathode, but one of these processes is usually dominant.
If a hot-cathode lamp is underpowered, the cathode is not as able to emit
electrons by the thermionic process, and significant cold-cathode arc
process may occur. This can cause excessive sputtering. Starting a
hot-cathode lamp also results in some of this as the cathode warms up.
Overpowering a hot-cathode lamp can simply overheat the cathodes.
Because of this, it is generally advised to start fluorescent and HID
lamps as infrequently as practical and to neither overpower nor underpower
them. This makes it difficult to dim fluorescent and HID lamps
significantly without being hard on their cathodes.
There are some special dimming ballasts for some fluorescent lamps.
These dissipate power into the cathodes to maintain a workable thermionic
process when these lamps are dimmed. It is recommended to only dim
fluorescent lamps with appropriate ballasts, and to use these dimming
ballasts only with the lamps they were designed to safely dim.
1. Some is used by the cathode and anode fall mechanisms getting electrons from metal to arc and vice versa. Nearly all of the energy here ends up heating the electrodes. The anode fall is not always significant, the cathode fall usually is. 2. Thermal conduction removes energy from the main body of the arc. This ends up heating the arc's surroundings and any container or arc tube.
3. Whatever energy enters the body of the arc (not lost in electrode falls) and not thermally conducted from the arc is radiated.
Of course, it is desirable to minimize (1) and (2) and to maximize (3).
The electrode falls are generally a fairly constant voltage. Designing the
main body of the arc to have more voltage across it (higher voltage drop)
and use less current reduces the electrode losses.
However, there is a limit to practical arc voltages, since higher voltages
may require complicated equipment to supply them, and also higher pressure.
The thermal conduction loss is a major loss in many high intensity
discharge lamps, especially ones of lower wattages.
This loss varies with arc temperature, gas and vapor type, and is largely
linearly proportional to the length of the arc. However, this loss usually does
not vary much with the arc's diameter nor with the gas pressure. Often,
especially in mercury vapor lamps, the arc temperature is surprisingly constant,
and this leads to a surprisingly constant thermal conduction loss from the
arc, in watts per centimeter of arc length. This loss increases if the arc
tube size and/or gas pressure are great enough for convection to be significant,
and the nearly constant degree of this loss applies to typical general purpose
HID arcs that are many times longer than they are wide. The loss is
different for the nearly spherical arcs in some special HID lamps.
For typical mercury vapor lamps, the thermal conduction loss is generally
around 10 watts per centimeter. For high pressure sodium lamps, this loss is
less constant but generally near 10 watts per centimeter. This loss can vary
with the ratios of the mercury-sodium mix since sodium vapor conducts heat
more than mercury vapor does. For metal halide lamps, this loss is less
constant and generally greater (in watts per cm.) due to convection in the
short, wide arc tubes that are filled to a very high pressure.
The watt/cm. loss could be reduced by:
1. Using a shorter arc. This requires a higher pressure for the same arc voltage. Also, the parts of the arc tube within one tube radius of the electrodes are subjected to being darkened by evaporated/sputtered electrode material, so it may not pay to have an arc length shorter than a few times the arc tube diameter. Reducing the arc tube diameter would help this, but a skinnier arc tube will get hotter from the same watts of heat per centimeter. All of this combined impairs the design of economical miniaturized HID lamps.
2. Fill the arc tube with a less thermally conductive material. Such materials
have larger and/or heavier molecules. Heavier molecules move
more slowly, larger size ones don't go as far between collisions. This
favors use of mercury and xenon as HID lamp ingredients.
Low-heat-conductivity gases and vapors should be gaseous at reasonable arc tube
temperatures, chemically stable or inert at all temperatures from below freezing
to the arc temperature, and not have major infrared or ultraviolet emission
lines that detract from efficiently radiating visible light. This largely
disqualifies polyatomic substances and the vapors of heavier alkali metals.
For more information on this, look in the Elenbaas book mentioned above.
1. Mercury has major ultraviolet lines at wavelengths shorter than those most favored by typical mercury arc temperatures. Increasing the temperature shifts the spectral output towards shorter wavelengths, causing radiation at these wavelengths to increase by more than the 4th power of the temperature.
2. Except in the shortest two major ultraviolet lines, the emissivity of
mercury vapor in its spectral lines increases with temperature. This
causes the mercury to radiate almost as well as a blackbody at its main
emission wavelengths if the temperature is high enough, and not radiate
nearly as well as a blackbody at lower temperatures. The result:
radiation varying more dramatically than temperature to the 4th power.
Why emissivity in these spectral lines varying with temperature? Because
the lower of the two electron orbits (energy levels) used to radiate these
lines are elevated orbits, not the "ground state" (unexcited state). The
mercury atom must be excited just to elevate an electron into the lower
level of transitions responsible for all major spectral lines except two
short-wave ultraviolet ones. This is also why mercury vapor tends to have
no absorption lines except the two shortwave UV ones.
With radiation increasing very dramatically with a slight increase in arc temperature, and decreasing dramatically with a slight decrease in arc temperature, it is easy to see why mercury arcs have a nearly constant temperature in most cases. This temperature is around 5500-5900 Kelvin.
Many other substances do the same thing, but typically don't regulate the arc temperature to the degree that mercury vapor does. For example, sodium's main yellow emission line is at a wavelength slightly shorter than most favored by a typical sodium arc temperature, and high pressure sodium lamps also have significant spectral lines resulting from transitions between elevated electron orbits. High pressure sodium arcs are not as constant in temperature as mercury arcs, with arc center temperatures generally in the low to mid 4,000s degrees Kelvin.
Suppose you have the lamp and the choke in series, powered by a variable current source, as opposed to a variable voltage source. Suppose you had optimum cathode heating regardless of current through the lamp. Due to the "negative resistance" characteristic most gas discharges have, the voltage across the tube will increase as current is decreased. In fact, there will be a point at which the combined tube-ballast voltage is minimized. This has some voltage across the choke ballast, meaning the voltage across the lamp is less than the line voltage.
The minimum line voltage to work at all is typically approx. 1.2 to 1.25 times
the lamp voltage. For reasonably good, stable and reliable operation, the voltage
across the lamp should generally not exceed approx. 2/3 of the line voltage.
With a "high-leakage-reactance transformer" ballast, the lamp voltage needs to
be correspondingly less than the "open circuit" (no load) output voltage
of the transformer.
Here is the explanation. These fluorescent fixtures have starters. The starter is usually of the "glow switch" type. The glow switch starter is a glow lamp with a bimetal strip for one electrode. The bimetal strip changes shape as it heats up, and contacts the other electrode and temporarily becomes a short circuit. For more information on fluorescent lamp circuits, go to the Fluorescent Lamp General Info Page.
But why the light sensitivity? The glow lamp in the starter may be hard to ionize at normal line voltage. Sometimes light hitting the electrodes of the glow lamp can help via the photoelectric effect. Light hitting the electrodes can dislodge or at least loosen a few electrons of the electrode material's atoms. Some starters have holes in them, which let stray light in to help.
Some starters have been made with easier-ionizing gas in them, but have been prone to ionizing too easily. They ionized instead of the fluorescent tube at times when the tube should have struck. Other starters have had radioactive material in them to assist starting, but many people do not like radioactive things around them.
Some neon lamps do this also, normally when they have aged past their expected life. This is usually a characteristic of "high intensity" neon lamps such as NE-2H, which have a reddish orange color and are filled with pure neon. The electrodes are coated with material favorable to a glow discharge, but the electrodes wear out and either higher voltage or the photoelectric effect is needed to make them start. These lamps often flicker when there is light and stay out when it is dark. Neon lamps with a neon-argon mixture start more easily, but are dimmer. They have a non-reddish orange color. They usually work reliably until they are too dark from sputtered electrode material to be useful.
If you replace a neon lamp, be sure that the dropping resistor for an NE-2H is at least 33K ohms for use with 120 volts AC. It is common to use 22K for more light, but this compromises the life of the NE-2H. If you replace an NE-2H with an NE-2 (easier starting), be sure to use a much higher dropping resistor, at least 150K ohms and preferably at least 180K for use with 120 volts AC. Light output will be low.
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