Circuit for controlling power supplied to a cathode heater of a cathode ray tube

A cathode heater of a cathode ray tube is rapidly energized either when turning on electric power or when returning to a power-on state mode of the Display Power Management System (DPMS) in an electronic appliance using the cathode ray tube as a video display device. A high voltage generation unit is provided for generating a voltage higher than a rated voltage of the cathode heater of the cathode ray tube. The cathode ray tube enables a stable video display to be presented in a shorter time period due to a rapid heating of the cathode caused when an initial heating unit applies a voltage to the cathode heater which is higher than the rated voltage of the cathode heater, according to a pulse signal which exists for a predetermined time period when turning on electric power or when returning to a power-on state mode of the DPMS. A voltage drop unit is provided to drop the voltage of the high voltage generation unit to the rated voltage of the cathode heater when the predetermined time period elapses so that the cathode heater is thereafter energized with the rated voltage for that cathode heater, in order to maintain the cathode at a normal operating temperature.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to a cathode ray tube, and more particularly 
to a circuit for controlling a cathode heater of a cathode ray tube by 
rapidly warming the cathode heater when an electronic appliance using a 
cathode ray tube is turned on or when such an appliance returns to a 
power-on mode from an operation mode which has cut off the electric power 
supply to a Display Power Management System (DPMS) cathode heater. 
2. Related Art 
A cathode ray tube is widely used as a video display device in electronic 
appliances such as monitors and television sets. An electric power supply 
energizes the cathode heater of an electron gun provided in the neck 
portion of the cathode ray tube. The energized cathode heater causes the 
cathode to be heated and to generate thermoelectrons. The thermoelectrons 
generated from the cathode are controlled, converged and accelerated by a 
plurality of grids provided in the electron gun that they strike the 
fluorescent surface in front of the electron gun, and thereby display an 
image. 
Before the cathode is heated up to a normal operating temperature, 
thermoelectron emission is unstable and the cathode ray tube cannot 
present a stable image. Conversely, after the cathode is heated up to a 
normal operating temperature, the cathode ray tube can present a stable 
image since thermoelectron emission becomes stable. 
A slow cathode warm-up time has been considered undesirable by the public 
because this causes a delay between the moment the cathode ray tube is 
turned on and the moment an acceptable picture appears on the face of the 
cathode ray tube. Several techniques have been attempted to decrease the 
time required to heat a cathode in a cathode ray tube, and to thereby 
decrease the time required to wait for an acceptable picture. 
The Display Power Management System (DPMS) was proposed by the Video 
Electronic Standard Association (VESA) of the United States of America. 
DPMS is an electric power supply management system for reducing electric 
power consumption of a video display system such as a monitor, which is a 
computer peripheral, in accordance with a use state of a computer system. 
According to DPMS, electric power management is performed in four modes by 
selectively outputting and cutting off a horizontal synchronization signal 
and a vertical synchronization signal in the main body of the computer 
system according to a use state, and by corresponding with the horizontal 
synchronization signal and vertical synchronization signal from the main 
body of the computer system in the video display device. The four modes 
are classified as a power-on state mode, a stand-by state mode, a 
suspension state mode and a power-off state mode. 
The video display device is operated in the power-on state mode when both 
the horizontal and vertical synchronization signals are input from the 
main body of the computer system. The video display device is operated in 
the stand-by state mode when only the vertical synchronization signal is 
input. The video display device is operated in the suspension state mode 
when only the horizontal synchronization signal is input. The video 
display device is operated in the power-off state mode when neither the 
horizontal nor the vertical synchronization signals are input. In the 
power-off state mode, electric power consumption must be less that 5 W. 
One technique used to cause the cathode to reach an operating temperature 
quickly requires that the cathode be kept warm all the time, by means of a 
bleeder current. This technique has been called an "instant-on" feature 
and has been provided on some televisions by television receiver 
manufacturers. With this feature, a viewable picture is obtained on the 
cathode ray tube almost instantaneously with the turn-on of the 
television. The bleeder current used to accomplish this feature constantly 
maintains the cathode heater at a near-normal operating temperature. Thus, 
in effect, the cathode ray tube is never completely turned off. One 
example of this "instant-on" feature is disclosed in U.S. Pat. No. 
3,767,967 for Instant-On Circuitry for AC/DC Television Receivers issued 
to Luz. This feature might be considered to be wasteful of electrical 
energy since the television is constantly drawing electrical power. Also, 
this feature could present a fire hazard. 
Another technique used to cause the cathode to reach an operating 
temperature quickly requires a modification in the design of the cathode 
heater. Some examples of this are disclosed in U.S. Pat. No. 3,881,126 for 
Fast Warm-Up Cathode Assembly issued to Boots et al., U.S. Pat. No. 
5,424,620 for Display Apparatus for Displaying Pictures Virtually 
Instantaneously issued to Cheon et al., U.S. Pat. No. 3,883,767 for Heater 
for Fast Warmup Cathode issued to Buescher et al. and U.S. Pat. No. 
4,379,980 for Quick Operating Cathode issued to Takanashi et al. Also, the 
cathode design may be modified in order to enable the cathode to reach an 
operating temperature quickly. Some cathode design modifications are 
disclosed in U.S. Pat. No. 3,947,715 for Fast Warm Up Cathode for a 
Cathode Ray Tube issued to Puhak, U.S. Pat. No. 4,675,573 for Method and 
Apparatus for Quickly Heating a Vacuum Tube Cathode issued to Miram et al. 
and U.S. Pat. No. 4,388,551 for Quick-Heating Cathode Structure issued to 
Ray. 
Another technique used to cause the cathode to reach an operating 
temperature quickly is to use a fast-acting heater circuit. This type of 
circuit is typically designed to energize the cathode heater in such a way 
as to cause the cathode heater to become warm quickly. The object of this 
type of circuit is to cause the cathode heater to become warm as quickly 
as possible, thereby decreasing the overall time required to cause the 
cathode to reach a normal operating temperature. 
One example of a fast-acting heater circuit is disclosed in U.S. Pat. No. 
3,886,401 for Apparatus for Accelerating Cathode Heating issued to Berg. 
This circuit includes at least two positive temperature coefficient (PTC) 
resistors. Upon activation of this circuit, the room-ambient resistance of 
the element allows passage of a surge of current which then decreases to 
the normal cathode heater operating level as the temperature of the 
resistive element increases. Because PTC resistors cause heat-related 
energy consumption, this circuit might be considered to be a poor utilizer 
of electrical energy. Also, this circuit might be considered to lack 
precision, due to the PTC resistors. 
A second example of a fast-acting heater circuit is disclosed in U.S. Pat. 
No. 3,982,153 for Rapid Warm-Up Heater Circuit issued to Burdick et al. 
This circuit utilizes a degaussing circuit, a temperature responsive 
resistive element (such as a PTC resistor) and at least two transformers. 
Some might have the opinion that this circuit lacks precision and that 
this circuit poorly utilizes electrical energy, due to the use of the 
temperature responsive resistive element. Others might believe that this 
circuit is undesirable due to the high cost associated with the use of two 
transformers. Still others may dislike the inherent limitation presented 
by the fact that a degaussing circuit is required. 
Cathode and cathode heaters are used in fluorescent light tubes. Some of 
the developments in the rapid warn-up of fluorescent light tubes are 
disclosed in U.S. Pat. No. 4,857,808 for Modified Impedance Rapid Start 
Fluorescent Lamp System issued to Lally et al., U.S. Pat. No. 5,5,010,274 
for Starter Circuits for Discharge Lamps issued to Phillips et al., U.S. 
Pat. No. 5,583,395 for Fluorescent Device Having a Fluorescent Starter 
Which Precisely Controls Heating Time and Absolute Synchronisn of Fire 
Point issued to Lu, U.S. Pat. No. 5,440,205 for Fluorescent Lamp Starter 
Having a Transistor Base Control Means issued to Tahara et al., U.S. Pat. 
No. 5,319,281 for Fluorescent Tube Heating and Starting Circuit issued to 
Roth, U.S. Pat. No. 4,661,745 for Rapid-Start Fluorescent Lamp Power 
Reducer issued to Citino et al. and U.S. Pat. No. 3,731,142 for 
High-Frequency Fluorescent Tube Lighting Circuit with Isolating 
Transformer issued to Spira et al. 
SUMMARY OF THE INVENTION 
Accordingly, it is therefore an object of the present invention to provide 
an improved circuit for controlling a cathode heater of a cathode ray 
tube. 
It is another object to provide a circuit which rapidly energizes the 
cathode heater at the initiation of operating mode, thereby raising the 
temperature of the cathode quickly, with the result being that a viewable 
video display is provided in a short time period. 
It is yet another object to provide a circuit for controlling a cathode 
heater which operates in a cost effective, reliable, and precise manner. 
It is still another object to provide a circuit which has no power drain 
when the cathode ray tube is not in an operating mode, and therefore 
conserves electrical energy and eliminates any fire hazard from 
continuously partially energized circuitry. 
These and other objects of the present invention can be achieved by an 
apparatus that consists of a high voltage generation unit, an instant 
heating signal generation unit, an initial heating unit, and a voltage 
drop unit. The high voltage generation unit is used to generate a high 
voltage to be applied to a cathode heater of a cathode ray tube. The 
aforementioned high voltage is higher than the cathode heater's rated 
voltage. The instant heating signal generation unit is used to generate a 
driving pulse signal during a predetermined time period in order to 
facilitate the application of a high voltage to the cathode heater to warm 
up the cathode either when turning on electric power or when changing to a 
DPMS power-on state mode. The high voltage generated from the high voltage 
generation unit is applied through an initial heating unit to the cathode 
heater so that the cathode can be rapidly heated during the driving pulse 
signal. A rated voltage generated by dropping the high voltage of the high 
voltage generation unit through a voltage drop unit is applied to the 
cathode heater when the driving pulse signal is cut off after the 
predetermined time period elapses. The rated voltage is applied to the 
cathode heater when the driving pulse signal is cut off. 
Therefore, according to the present invention, a cathode is rapidly heated 
at the initiation of operation mode because a high voltage is applied to 
the cathode heater, said high voltage being higher that the rated voltage 
for the cathode heater. Thereafter, when the cathode is heated to a normal 
operating temperature, the rated voltage is applied to the cathode heater. 
In other words, according to the present invention, a cathode is rapidly 
heated by applying a high voltage exceeding the rated voltage when the 
cathode heater is first energized, upon the initiation of operating mode. 
Later, when the cathode reaches its normal operating temperature, the 
voltage applied to the cathode heater drops down to the rated voltage. 
The present invention is more specifically described in the following 
paragraphs by reference to the drawings attached only by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings and particularly to FIG. 1, which illustrates 
a circuit for controlling a cathode heater of a cathode ray tube. A 
reference numeral AC denotes an AC voltage of an AC electric power source. 
The voltage AC is supplied to a primary winding of a power transformer T1 
through power switches SW1 and SW2. A secondary winding of the power 
transformer T1 is connected to a ground capacitor C1 through a diode D1, 
and the juncture of the diode D1 and the ground capacitor C1 is connected 
to a cathode heater 2 of a cathode ray tube 1 in through a resistor R1. 
In the circuit in FIG. 1, the voltage AC is applied to the primary winding 
of the power transformer T1 through the power switches SW1 and SW2 to 
induce an AC voltage across the secondary winding when the power switches 
SW1 and SW2 are turned on. The induced AC voltage across the secondary 
winding of the power transformer T1 is rectified through the diode D1 and 
smoothed through the ground capacitor C1 so as to be converted into a 
first DC voltage of about 8 volts. The first DC voltage is supplied to an 
electronic appliance as an operational power supply voltage. Then the 
first DC voltage is dropped to a second DC voltage of 6.3 volts, which is 
the rated voltage of the cathode heater 2, through the resistor R1 which 
is used for controlling an input voltage. The second DC voltage is applied 
to the cathode heater 2 in order to energize the cathode heater 2 and 
thereby cause the cathode heater 2 to become warm. 
The circuit shown in FIG. 1 energizes the cathode heater 2 by applying the 
rated voltage to the cathode heater 2 at the moment that the power 
switches SW1 and SW2 are turned on. One of the disadvantages of the 
circuit shown in FIG. 1 is the long wait needed prior to being able to 
view a stable image on the cathode ray tube. It requires approximately 
10-11 seconds for the cathode ray tube 1 to display a stable image since 
it takes that amount of time for the cathode heater 2 to heat up to the 
normal operating temperature. 
Turning now to FIG. 2, which illustrates another circuit for controlling a 
cathode heater of a cathode ray tube. The voltage AC is applied to 
activating terminals of power switches SW11 and SW12. The first fixed 
terminals a11 and a12 of the power switches SW11 and SW12 are connected to 
a primary winding of a power transformer T12. The second fixed terminals 
b11 and b12 of the power switches SW11 and SW12 are connected to a primary 
winding of a power transformer T11. A secondary winding of the power 
transformer T11 is connected to a ground capacitor C11 and a cathode 
heater 12 of a cathode ray tube 11 through a diode D11. A secondary 
winding of the power transformer T12 is connected to a ground capacitor 
C12 through a diode D12, and the juncture of the diode D12 and the ground 
capacitor C12 is connected to the cathode heater 12 of the cathode ray 
tube 11 through a diode D13 and a resistor R11. 
In FIG. 2, the power switches SW11 and SW12 will be in the OFF position 
when the activating terminals of those power switches are connected to the 
second fixed terminals b11 and b12. When the power switches SW11 and SW12 
arc in said OFF position, voltage AC is applied to the primary winding of 
the power transformer T11 through the power switches SW11 and SW12. The 
voltage AC applied to the primary winding of the power transformer T11 is 
induced to an AC voltage across the secondary winding of the power 
transformer T11. The induced AC voltage across the power transformer T11 
is rectified through the diode D11 and smoothed through the ground 
capacitor C11 so as to be converted to a DC voltage of about 3-4 volts. 
The DC voltage of about 3-4 volts is applied to the cathode heater 12 of 
the cathode ray tube 11 to keep the cathode heater 12 energized and warm 
while the power switches SW11 and SW12 are in the OFF position. 
In the circuit in FIG. 2, the power switches SW11 and SW12 will be in the 
ON position when the activating terminals of those power switches are 
connected to the first fixed terminals a11 and a12. When the power 
switches SW11 and SW12 are in said ON position, the voltage AC is applied 
to the primary winding of the power transformer T12 through those power 
switches, to thereby induce an AC voltage across the secondary winding of 
the power transformer T12. The voltage induced across the secondary 
winding of the power transformer T12 is rectified through the diode D12 
and smoothed through the ground capacitor C12 to output a DC voltage of 
about 8 volts. 
In FIG. 2, the DC voltage of about 8 volts is applied to a load as an 
operational power supply voltage and dropped to a voltage of about 6.3 
volts through the diode D13 and the resistor R11. The voltage of about 6.3 
volts is applied to the cathode heater 12 to cause it to be heated up to 
the normal operating temperature. A voltage which is lower than a rated 
voltage is applied to the cathode heater 12 to keep the cathode heater 12 
energized and warm while the power switches SW11 and SW12 are in the OFF 
position, whereas the rated voltage is applied to the cathode heater 12 
when the power switches SW11 and SW12 are in the ON position. 
The circuit in FIG. 2 has several characteristics which might be considered 
to be disadvantages. For example, it utilizes a second transformer to keep 
the cathode heater energized, so that the cathode stays warm. Since 
transformers are expensive, the product cost is increased by the use of 
this second transformer. Also, a large amount of electrical energy is 
consumed when the cathode ray tube 11 is not being used, while the power 
switches SW11 and SW12 are in the OFF position. This could be considered 
wasteful. Or it could be considered a fire hazard. Also, it might be very 
difficult to meet the electrical power consumption limitations of DPMS. 
According to the DPMS, electrical power consumption must be less than 5 
watts in the power-off state mode. Therefore, since electrical power 
consumption is about 3.6 watts while the cathode heater is being kept warm 
when said power switches are in the OFF position, an electronic appliance 
must be designed so that less than 1.4 watts is consumed in all other 
components. However, it is difficult to meet the limitation of electric 
power consumption proposed by the DPMS since electrical power over 1.4 
watts is consumed in loads other than the cathode heater, for example in 
components such as the microprocessor. 
Turning now to FIG. 3, which illustrates a circuit for controlling a 
cathode heater of a cathode ray tube according to one preferred embodiment 
of the present invention. As shown in FIG. 3, there is a high voltage 
generation unit 20, an instant heating signal generation unit 21, an 
initial heating unit 22, a voltage drop unit 23, a cathode heater 24, and 
a cathode ray tube 25. 
In FIG. 3, the high voltage generation unit 20 is used for generating an 
operational voltage higher than a rated voltage of a cathode heater 24 of 
a cathode ray tube 25 with an input of an AC power supply when power 
switches SW21 and SW22 are in the ON position. Power switches SW21 and 
SW22 are connected to a primary winding of a power transformer T21, and a 
secondary winding of the power transformer T21 is connected to a ground 
capacitor C21 through a diode D21. The operational voltage is output to a 
load from the connection point between the ground capacitor C21 and the 
diode D21. When the power switches SW21 and SW22 are turned to the ON 
position, the power supply voltage AC is applied to the primary winding of 
the power transformer T21 of the high voltage generation unit 20 through 
the power switches SW21 and SW22. The power supply voltage AC applied to 
the primary winding of the power transformer T21 causes an AC voltage to 
be induced across the secondary winding of the power transformer T21. The 
induced AC voltage is rectified through the diode D21 and smoothed through 
the ground capacitor C21, to thereby output a DC voltage of about 8 volts. 
In the circuit in FIG. 3, the instant heating signal generation unit 21 is 
used for generating a driving pulse signal for a predetermined time period 
at the beginning of an application of voltage to the cathode heater 24. At 
the moment when the power switches SW21 and SW22 are turned to the ON 
position, that is, when beginning to energize the cathode heater 24, the 
instant heating signal generation unit 21 outputs a driving pulse signal 
of high voltage for a predetermined time period of, for example, about 3-4 
seconds. The instant heating signal generation unit 21 may be a 
microcomputer, for example. In the case of the cathode ray tube 25 being 
used in a television set, the instant heating signal generation unit 21 
generates the driving pulse signal when turning the power switches SW21 
and SW22 to the ON position. In the case of the cathode ray tube 25 being 
used in a monitor, the instant heating signal generation unit 21 performs 
an electric power supply voltage management according to a horizontal 
synchronization signal and a vertical synchronization signal input from 
the main body of a computer system, turns on an application of a voltage 
to the monitor or cuts off an application of a voltage to the cathode 
heater 24 according to the horizontal and vertical synchronization signals 
input from the main body of the computer system, and controls the power 
switches SW21 and SW22 to be connected in case of converting to the 
power-on state mode with both the horizontal and vertical synchronization 
signals input as well as outputting a as driving pulse signal of high 
voltage for a predetermined time period. The driving pulse signal output 
from the instant heating signal unit 21 is applied to the initial heating 
unit 22. 
In FIG. 3, the initial heating unit 22 is used for applying the operational 
voltage of the high voltage generation unit 20 to the cathode heater 24 
during the driving pulse signal of the instant heating signal generation 
unit 21. In the initial heating unit 22, an output terminal of the instant 
heating signal generation unit 21 is connected to the base electrode of a 
first switching transistor Q21 through a resistor R22, and the collector 
electrode of the transistor Q21 is connected to the base electrode of the 
second switching transistor Q22 through a resistor R23. Further, an output 
terminal of the high voltage generation unit 20 is connected to the 
emitter electrode of the transistor Q22, the emitter electrode of the 
transistor Q22 is connected to the base electrode of the transistor Q22 
and the resistor R23 through a resistor R24, and the collector electrode 
of the transistor Q22 is connected to the cathode heater 24 of the cathode 
ray tube 25. 
In the circuit in FIG. 3, as previously stated, the driving pulse signal is 
output from the instant heating signal unit 21 and is applied to the 
initial heating unit 22. More specifically, the driving pulse signal is 
applied to the base electrode of the transistor Q21 through the resistor 
R22 of the initial heating unit 22 so that the transistor Q21 is turned 
on. When the transistor Q21 is turned on, a DC voltage which is output 
from the high voltage generation unit 20 is applied to the transistor Q21 
through the resistors R23 and R24 and a low voltage is applied to the base 
electrode of the transistor Q22. At this time, with the transistor Q22 
turned on, since the DC voltage output from the high voltage generation 
unit 20 is applied to the cathode heater 24 of the cathode ray tube 25 
through the transistor Q22, the cathode heater 24 is energized and rapidly 
warmed by a high voltage of about 8 volts which is output by the high 
voltage generation unit 20. As a result, a cathode is rapidly heated by 
the cathode heater 24. It is preferable that a time period during which 
the instant heating signal generates the driving pulse signal of high 
voltage shall be set to be a time period required to heat the cathode up 
to a normal operating temperature due to the heating performed by the 
cathode heater 24 caused by the output voltage of the high voltage 
generation unit 20. Then, at the moment when the cathode is heated up to 
the normal operating temperature, and the predetermined time period has 
elapsed, the instant heating signal generation unit 21 shall output a low 
voltage. Said low voltage shall be applied to the base electrode of the 
transistor Q21. At this time, the transistor Q21 is turned off and the 
transistor Q22 is turned off. With the two transistors turned off, the 
high voltage output from the high voltage generation unit 20 is dropped to 
about 6.3 volts through the resistor R21, which is the rated voltage of 
the cathode heater 24. The rated voltage of about 6.3 volts is applied to 
the cathode heater 24 to keep the cathode at a normal operating 
temperature. 
In FIG. 3, the voltage drop unit 23 having a resistor R21 is used for 
dropping the operational to voltage of the high voltage generation unit 20 
to the rated voltage of the cathode heater 24 of the cathode ray tube 25 
and for applying the rated voltage to the cathode heater 24. 
As mentioned above, the present invention rapidly heats the cathode in a 
cathode ray tube by applying a high voltage, above the rated voltage of 
the cathode heater, to the cathode heater at the initiation of the 
operating mode. Subsequently, after a cathode is heated up to a normal 
operating temperature, the present invention drops the voltage down to the 
rated voltage of the cathode heater and applies that rated voltage to the 
cathode heater, so that a cathode ray tube can present a stable image 
within a short time period.