How the Cathode-Ray Tube Works
with the domination of LED, plasma, and LCD displays, there are
still a whole lot of CRTs on the job. Hobbyists workbenches are
filled with them for sure, but design and manufacturing facilities
still have huge inventories of test equipment with CRTs, and a lot
of computer equipment on the production line with CRTs sitting in
racks. LED, LCD, and plasma displays all have their own claims to
genius on the part of their designers, but cathode ray tube designers
- and the designers of the driver circuits - deserve special recognition.
Consider the physics and materials involved: glass, phosphor, magnetics,
thermonics, electrostatics and electrodynamics, relativity (electrons
traveling at relativistic speeds gain mass, requiring stronger deflection
fields). This article from the February 1955 edition of Popular
Electronics provides a look into the CRT from a layman's perspective.
February 1955 Popular Electronics
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
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How the Cathode-Ray Tube Works
This analysis of both electrostatic and magnetic
type tubes provides better understanding of TV picture tube operation
To many, the cathode-ray tube is symbolic of the entire television
industry. There is little question that if it were not for this
tube, the industrial giant of modern television could never have
come into being. However, while it is true that a cathode-ray tube
is used in every TV receiver and that thousands of others are used
in TV broadcast stations, its use is not limited to this field alone.
These tubes are used in radar, in medical electronic apparatus,
in test equipment, in atomic research, in industrial electronic
equipment, in fact, in all phases of present-day industry and science.
To understand the operation of the cathode-ray tube, then, is to
be in a better position to understand the entire field of modern
1. (top) The larger of these electrostatic cathode-ray tubes
is made of glass and measures 5 inches. The smaller is a 1 inch
Fig. 2. (bottom)Construction details of an
electrostatic type cathode-ray tube.
Fundamentally, the cathode-ray tube
(or CRT) is nothing more than a special type of indicating device.
Just as a loudspeaker converts electrical signals into sound vibrations,
and a meter changes electrical energy into the mechanical movement
of a needle pointer, the CRT is an instrument used to change electrical
signals into patterns of light.
Cathode-ray tubes are made
in numerous sizes and shapes and for many special purposes. However,
most cathode-ray tubes can generally be divided into two broad classes
-"electrostatic" tubes and "electromagnetic" tubes. This classification
is based on the means used for deflecting (moving) the electron
beam which "paints" the light pattern on a fluorescent screen. Each
class of tube has certain advantages as well as certain limitations,
and these advantages and limitations have resulted in each type
of tube being used in specific applications.
tubes use an electrostatic field for controlling the electron beam.
This field is built up between a pair of electrodes called "deflection
plates" by the application of moderate a.c. and d.c. voltages. Electrostatic
tubes are generally made in small sizes, with screens from one inch
to about ten inches in diameter. They have a good frequency response
and are widely used in cathode-ray oscilloscopes, medical electronic
equipment, industrial equipment, and in some types of radar work.
Electromagnetic tubes use a magnetic field for controlling
the stream of electrons. Two pairs of coils are used for building
up this magnetic field. The coils are external to the tube proper
and are generally mounted in a single assembly called a "deflection
yoke." Electromagnetic tubes are made with screen sizes from five
inches to thirty inches in diameter. Although they have a comparatively
narrow frequency response, these tubes do permit the formation of
large size, sharply focused, bright images and are used in tremendous
quantities in television receivers and as indicators in radar systems.
Virtually all modern TV receivers employ electromagnetic tubes.
Since most people find it easier to understand the operation of
the electrostatic cathode-ray tube, we shall discuss this type first.
Once a clear understanding of the electrostatic tube has been acquired,
the electromagnetic tube is easy to master. The basic construction
of an electrostatic CRT is shown in simplified form in Fig. 2.
Fig. 3. Photographic illustration of the electron gun assembly
of cathode-ray tube.
Fig. 4. In an electromagnetic type of tube, deflection coils
are placed on the outside.
Fiq. 5. The photograph shows a typical deflection yoke of a
modem TV receiver.
Fig. 6. This industrial multi-gun cathode-ray tube has five
electron gun and deflection assemblies.
a.c. voltage is applied to the filament of the tube, heating it
to a bright glowing red. The filament, in turn, heats the cathode
which is placed close to it. The cathode is covered with materials
which "boil out" electrons when heated, and these electrons gather
in a cloud close to the cathode.
A high positive voltage
is placed on the accelerator anode and this voltage attracts the
negatively charged electrons, causing them to move in a stream toward
the front of the tube. The electron stream passes through a narrow
hole in a cylindrical shaped electrode called the grid. Although
not shaped like a conventional grid, it is given this name after
the grid in an amplifier type vacuum tube because it serves the
same function... it controls the number of electrons which can pass
The electron stream next passes through a group
of two or three cylindrical electrodes which have different d.c.
voltages applied to them. The accelerating anode may be one of this
group. The d.c. voltages applied as well as the shape and size of
these electrodes set up an electrostatic field which narrows the
electron stream and focuses it into a sharp beam. Because of this
action, one of the electrodes may be termed the focusing anode.
Next, the sharply focused electron beam passes through a
pair of flat electrodes arranged in a horizontal plane with respect
to the tube. If a d.c. voltage is applied to these plates, the electron
beam will be attracted toward the more positive plate and repelled
from the negative plate, bending either up or down, depending on
how the d.c. voltage is applied. If an a.c. voltage is applied,
the beam will move up and down alternately. Since the beam moves
in a vertical direction, these electrodes are called the vertical
defection plates. The horizontal defection plates are a similar
pair of electrodes, but arranged at right angles to the vertical
plates, and serve to move the beam either to the left or right.
After passing through the deflection plates, the electron
beam goes on to strike the front of the tube, which has been covered
with chemicals which glow when struck by the electrons. This action
is termed "fluorescence" and, therefore, the film of chemicals on
the face of the CRT is called a fluorescent screen. The choice of
chemicals used in making the fluorescent screen determines the color
of the glow and how long the glow continues after the electron beam
strikes the screen. This latter characteristic is called the persistence
of the screen.
Most of the cathode-ray tubes used in oscilloscopes
have a medium persistence green screen. Zinc orthosilicate is frequently
used for such screens. Television receivers generally employ a medium
persistence white screen. Various mixtures may be used to produce
a white screen, including a combination of zinc sulphide and zinc
beryllium silicate. For high speed photography, a CRT using a short
persistence blue screen is desirable. Such screens may be made from
Since the purpose of a cathode-ray tube
is to obtain a pattern of light on a screen, the fluorescent screen
is often considered one of the most important parts of the tube.
This is shown by the fact that a tube's size is given in terms of
its screen diameter. A "seven inch tube" has a screen with a diameter
of approximately seven inches.
In a cathode-ray tube, the
assembly of electrodes which produces the stream of electrons, not
including the deflection plates, is called the "electron gun." The
electron gun of a typical CRT is shown in Fig. 3.
CRT is somewhat simpler. in construction than an electrostatic tube
since it does not have deflection plates. However, it still has
an electron gun assembly, although the focusing anodes may be missing.
The beam of electrons sent out by the electron gun is deflected
by a magnetic field set up, in turn, by two pairs of curved coils
mounted around the neck of the tube close to the bulge of the "funnel."
See Fig. 4. The coil assembly, or deflection yoke, of a typical
television receiver is shown in Fig. 5. Note how the two sets of
coils are mounted at right angles to each other.
cathode-ray tubes which do not have focusing electrodes in their
electron gun assembly employ an external magnet to produce a magnetic
focusing field. Such magnets may be either electromagnets or permanent
magnets, or a combination of both. Thus, electromagnetic tubes may
be subdivided into two smaller classes; those employing electrostatic
focusing and magnetic deflection and those employing both magnetic
focusing and magnetic deflection.
The shape of the light
pattern formed on the screen of a CRT depends on the type of electrical
signals applied to the deflection elements (whether coils or electrostatic
plates). If steady d.c. voltages are applied, a dot will appear
on the screen, with its exact position determined by the relative
sizes of the applied deflection voltages. If a.c, signals are applied,
a line or pattern will be formed, with its shape and size determined
by the electrical waveforms and amplitudes of the a.c. signals.
In a television receiver, the light pattern forms a raster made
up of a series of horizontal lines.
Varying the voltage
applied to the grid electrode in the electron gun assembly will
change the number of electrons that can pass through and strike
the screen and hence the instantaneous brightness of the glow. In
a television receiver, the video signal is applied to the grid-cathode
circuit of the CRT and changes the evenly glowing raster into a
pattern of light and dark segments which, in turn, makes up the
The vast majority of present day cathode-ray tubes
employ a single electron gun assembly and a single set of deflection
elements. However, tubes have been made with a number of electron
guns, including some of the tubes designed for color television.
Cathode-ray tubes have also been made with several complete electron
guns plus deflection elements. Such tubes arc virtually several
independent cathode-ray tubes with a single screen. One such tube
is shown in Fig. 6.
Special purpose cathode-ray tubes, color
television picture tubes, TV camera tubes, and other types of cathode-ray
tubes will be discussed in other issues of POPULAR ELECTRONICS.