April 1959 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Before there were CFL light bulbs with complicated electronic
circuits for generating the requisite high voltages without
a transformer, there were just the familiar straight (and sometimes
circular) fluorescent bulbs that use a simple ballast arrangement
and a built-in switch in the bulb base. As with compact fluorescent
(CFL) lights, very few people understood how they worked. Most
knew that the 8-foot-long T-12 bulbs (the large diameter ones
used in commercial buildings) made a really cool implosion sound
when they broke - usually intentionally (by people like me)
since the glass tubes were amazingly tough. I remember many
moons ago, between high school and the time I enlisted in the
USAF, while I was an apprentice electrician working on a renovation
job at a public school (I took electrical vocational courses
for 3 years in HS), a co-gofer and I spent weeks wiring fluorescent
fixtures all throughout the building. We worked atop a tall
scaffolding that was on wheels, pulling ourselves around the
room by grabbing the suspended ceiling grid. When the foreman
wasn't around, we relieved our utter boredom by 'accidently'
dropping the fluorescent bulbs onto the concrete floor. If done
just right so that the bulb hit the floor in a perfectly vertical
position, it would bounce up two or three feet and implode in
mid-air. The acoustics of the big, empty room, devoid thus far
of sound-absorbing ceiling tiles, really enhanced the effect...
but I digress. This article does a nice job of explaining how
the non-curly-Q fluorescent bulbs work - with and without external
The Electronics of Fluorescent Lamps
By Edgar D. Morgan
Fluorescent lamps, once a novelty, are now an accepted part
of the American scene. Their gentle blue-white glow is encountered
everywhere, from the tops of skyscrapers to the depths of the
New York subway system. However, the operating principles of
fluorescent lamps are little understood even by technicians.
Invisible Light. Many people know that fluorescent
lamps are among our most efficient light producers, and that
they operate with a mercury vapor arc. But few realize that
over 80% of the radiation produced by that arc is in the ultraviolet
region and invisible to the human eye, and that every effort
is made to keep as much energy as possible in the invisible
ultraviolet end of the spectrum. Sounds foolish, doesn't it?
The visible light actually comes from chemical compounds
coated on the inside of the glass tube. Called phosphors, these
compounds have the property of emitting visible light when they
are excited by ultraviolet radiation. They have been termed
"light transformers" because of their ability to absorb energy
at one wavelength and radiate it at another.
The fluorescent lamp depends upon ionization for the production
of the necessary ultraviolet arc. Here's how it's done. The
free electrons in the gas are accelerated by an applied voltage,
and each time a collision occurs between an electron and a gas
molecule, one or more additional electrons are displaced. These
electrons in turn, are accelerated enough to repeat the process
on other molecules, and a chain reaction takes place.
Cutaway view of a fluorescent lamp. All fluorescent
lamps are constructed as shown here. Only the glass tube changes
in size and shape from model to model.
As each molecule returns to a stable state, it gives off
its excess energy in the form of radiation. It's the frequency
of the radiation that determines whether visible or invisible
light will be obtained. In commercial lamps, the pressure of
the gas sealed in the lamp is adjusted very carefully so that
nearly all of the radiation occurs at one given ultraviolet
wavelength, 2537 Angstrom units. This frequency is selected
for optimum excitation of the tube's phosphor coating.
Each chemical compound in the phosphor coating radiates light
at a certain wavelength. For instance, zinc silicate releases
its radiation as green light, cadmium borate radiates a predominately
pink color, and calcium tungstate when excited gives off blue
light. By carefully blending these and other compounds, almost
any desired color can be obtained.
Warming to the Job. The electrons required
to facilitate the starting of the arc are provided by coated
tungsten filaments in the ends of the fluorescent tube - similar
to the filament in an ordinary vacuum tube. Once the arc is
achieved, the heated filaments are no longer required and are
automatically switched off. A small amount of argon or krypton
gas present in the tube facilitates the initial arcing, which
also serves to evaporate the globule of mercury in the tube.
From this point on, the arc is basically mercury vapor.
There are two pieces of auxiliary apparatus necessary to
operate a fluorescent lamp. One of these is a starter which
acts as the automatic filament circuit switch mentioned above.
The second additional element required is the ballast, which
serves as a choke coil to regulate arc current as well as an
autotransformer to provide the high voltage kick needed to start
An Inductive Kick. The basic lamp circuit
is shown in Fig. 1. When the switch is closed, the ballast and
the two filament windings are placed in series across the applied
voltage, and the filaments heat. When the switch is released
a moment later, the inductive kick of the iron-core ballast
coil causes a momentary voltage surge across the lamp which
starts the arc. As the ballast is now in series with the lamp,
the arc current is limited by the impedance of the ballast.
This simple circuit is widely used on small desk lamps, but
has too many disadvantages for general lighting use.
Fig. 1. Basic fluorescent lighting circuit
in general use only on small desk lamps. A manual switch performs
the starting function.
One of the disadvantages of the single lamp circuit is its
poor power factor. As the circuit is primarily inductive, due
to the ballast, the current and voltage have a phase relationship
which makes for inefficiency in light output compared to current
drawn. A partial solution to this problem is the circuit shown
in Fig. 2.
Fig. 2. This circuit is sometimes used to
correct the poor power factor in the circuit of Fig. 1. The
ballast is shown here functioning as autotransformer as well
as choke coil.
Here the starter is an automatic device although its function
is the same as the switch in Fig. 1. The power factor in this
unit is improved by a shunt capacitor. As a capacitor and a
coil have an opposite effect on power factor, one offsets the
other; and the resistor serves to bleed off any charge which
remains on the capacitor. The circuit of Fig. 2 has been drawn
to indicate that the ballast is also serving as an autotransformer.
The number and size of the lamps used determines whether this
is necessary or not.
Factors and Flickers. Another objectionable
feature of the single lamp circuit is its flicker, due to the
60-cycle line. Since incandescent lamp filaments operate at
a very high temperature, there isn't time for them to cool sufficiently
from cycle to cycle for the variation of light to be seen. The
fluorescent lamp must extinguish and restrike its arc 120 times
per second as the voltage reverses polarity. This causes a disturbing
stroboscopic effect around machinery with cyclical motion.
Probably the most common circuit in use today is shown in
Fig. 3. This is a two-lamp circuit and corrects several of the
disadvantages inherent in single lamp setups. The lamp indicated
as inductive is connected in the same way as the lamp in Fig.
1. The other lamp, however, has a capacitor in series with its
ballast. This serves to change its phase relations so that its
current leads rather than lags the voltage, and corrects the
overall power factor. Thus, the two lamps operate more efficiently
as a unit.
Fig. 3. For the control of two lamps, this
circuit is in wide-spread use and has excellent power factor
The addition of this series capacitor, though, produces another
problem. When starting, the capacitive circuit sometimes limits
the current required to preheat the filaments. This effect is
overcome by adding another compensating coil in series with
the starter on this lamp only.
Use of this type of two-lamp circuit is also beneficial in
overcoming flicker. Because the lamps operate out of phase,
they reach their peaks of illumination at different times and
the combined light is relatively free from disturbance.
The Starter Story. One of the most fascinating
pieces of auxiliary equipment is the fluorescent starter. Its
task is to close the circuit containing the filaments when voltage
is first applied, and after a preheating period of several seconds,
to open the filament circuit and keep it open as long as the
lamp remains on. A bimetallic strip is used which bends when
heated, and serves as sort of a time switch.
The most common type of starter is the glow starter. The
entire unit is sealed within a small glass envelope containing
neon or argon gas and connected as shown in Fig. 4. A bimetallic
strip controls a contact which is normally open.
Fig. 4. Typical glow-type starting circuit
used in home-type fluorescent circuits.
When first turned on, the full line voltage is applied across
the glow lamp, causing the gas to ionize and conduct. The heat
of the ionized gas is sufficient to cause the bimetal strip
to close the filament circuit. The contact closing shorts out
the glow discharge and the bimetal begins to cool. After cooling
(which takes long enough to preheat the filaments satisfactorily),
the contact within the starter opens and the fluorescent lights.
The starter is now across the lamp voltage, which is not enough
to ionize the neon gas, and so the unit is inoperative as long
as the lamp remains on.
The glow-type starter consumes no energy from the circuit
after the starting period is over. Its timing is not accurate,
however, and as it is difficult to maintain the proper gas pressure
over a long period, sometimes the timing tends to become very
How a Glow Starter Works
The line voltage
produces a glow discharge between the bimetallic strip and the
fixed contact (A); the heat from the glow actuates the bimetallic
strip, the contacts close and the filament preheating begins
(B); this shorts out the glow discharge, the bimetallic strip
cools and the contacts open (C). The resulting inductive kick
from the ballast then starts the tube.
A great many of our present-day starters incorporate modifications
such as a manual reset. If the lamp does not light after repeated
attempts by the starter, it ceases functioning until the trouble
is corrected and the starter is reset by pushing a spring-loaded
button. There are others that use different contacts for restarting
so that it isn't necessary to wait for a bimetal strip to cool
completely before it can recycle.
The starter shown in Fig. 4 also contains a capacitor across
the lamp contacts, which acts to suppress radio interference.
Manufacturers of fluorescent lamps publish handbooks which
elaborate on these principles. They also give many more lighting
circuits and their applications. The very fact that fluorescent
lamps are so readily accepted and so seldom studied in detail
is in itself a fine tribute to their efficiency and dependability.
Posted November 11, 2013