MadSci Network: Engineering |
Greetings:
Reference: F. E. Terman, Electronic and Radio Engineering,
McGraw-Hill, NY, 1955
Your question is related to the design of cathode-ray tubes
(CRT) and the circuits used for operating them in television
(TV) sets. The same problem occurred in old black and
white TV sets and in oscilloscopes used for viewing electrical
signals in the laboratory. I’ll first describe the problem in
black and white TV sets which used CRTs with a single electron
gun and then add the extra complications encountered in color
CRTs with 3 electron guns.
A CRT consists of three basic components:
1) an arrangement for producing and focusing an electron beam,
and controlling the beam intensity with the TV signal, called an
electron gun; 2) a means for deflecting the beam either
electrostatically or magnetically to form the picture scan lines
and the pictures height and width; and 3) a fluorescent screen
on which the electron beam is focused to form a fine spot and
which gives visible light when bombarded by electrons.
Secondary electrons emitted from the screen reach a carbon coating
inside the bell of the tube to complete the circuit. Depending on the
length of the CRT the accelerating electrode and the carbon
coating are held between +10,000 and +20,000 volts (10kv to 20kv)
relative to the cathode. This large potential difference gives
the electrons a large energy tobombard the phosphors
for a brighter viewing screen.
In most TV applications the circuits that produce the horizontal
deflection voltages of the picture are also used to generate the
10 to 20kv required to accelerate the electrons. If a high
intensity electron beam stays focused on one spot on the
fluorescent screen for a long period of time, it can heat the
phosphor to the point of destruction leaving a burnt spot in the
screen. This is why screen saver circuits are suggested for
computer screens, so that fixed patterns of high intensity light
are not burned into the viewing screen. A cathode also produces a
few heavy negative ions which pass through the deflection system
with out being scanned. More about this later.
Normally if the TV set is turned on, the deflection circuits scan
the beam horizontally and vertically so that burning is not a
problem. However; when many older TV sets are turned off the
horizontal and vertical scanning circuits stop scanning instantly,
yet the high voltage circuits that are used to accelerate the beam
may stay charged up for several seconds to minutes. Also, even
though the filament that heats the cathode to produce electrons
is turned off, the cathode mass cools slowly (it’s in a vacuum) and it
continues to emit electrons while it cools. The result is a bright
spot in the middle of the CRT. The spot slowly fades as the
cathode cools and the high voltage leaks off the electrodes. Often
the spot will grow in diameter as the focusing voltages also
discharge. In later CRTs a very thin aluminum film was placed inside
of the screen. Electrons easily pass through this film while larger
heavier ions are stooped by the film before reaching the
screen. The conductive film also helps bleed off the high voltages
very quickly when the set is turned off.
The first color TV sets used three electron guns, a red gun, a
green gun and a blue gun. The viewing screen was composed of
hundreds of thousand of triads of red, green and blue dots of
phosphor. A special metal plate called a shadow mask was placed in
front of the viewing screen and it had many hundreds of thousand
holes in it which are placed over each triad of phosphor dots.
The shadow mask was aligned so that only electrons from the red
gun hit the red dots of phosphor, only green gun electrons hit
the green dots of phosphor and only blue gun electrons hit the
blue phosphor dots. The shadow mask was (is)not very efficient and
most of the electrons from the 3 guns hit the shadow mask instead
of the phosphor dots. So when the TV set is turned off any of the
3 guns may cause a red, green or blue color spot such as that
produced by the black and white CRT; however, there is a high
probability that the electrons will hit the shadow mask and not
the phosphor dots. As the 3 cathodes cool and the voltages start
to decrease, each beam may move a very small distance and
cross a hole in the shadow mask causing a colored spot on the screen.
Many modern CRTs, such as the Trinitron, use red green and blue
phosphor stripes and in place of dots and have eliminated the
shadow mask. These CRTs have more complex electrode structures
to give brighter pictures and to eliminate many problems in
early color CRTs.
Best regards, your Mad Scientist
Adrian
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