Constant current bias color switch for a beam penetration CRT

A high voltage color switch for a beam penetration CRT includes a constant current driver which provides a bias current to the high voltage transformer in the first color write period when the baseline color is being displayed. One or more secondary colors can be displayed during a second color write period during which a switch connects the transformer to a conventional voltage driver that provides the needed voltage swing. At the end of a secondary color write period the constant current driver is again connected to reset the flux level in the transformer to one end of its dynamic range. During a core reset interval only a small voltage offset appears on the anode of the CRT so that color information written in the baseline color appears without any perceivable change to a viewer. At the end of the core reset period the constant current source continues to supply a constant bias current to the primary winding of the transformer to maintain the flux level at one end of its dynamic range.

CROSS REFERENCE TO RELATED APPLICATIONS 
The subject matter of this patent application is related to that disclosed 
in U.S. pat. application Ser. No. 259,342 filed May 1, 1981 by M. H. 
Kalmanash for RANDOM COLOR SWITCH FOR BEAM PENETRATION CRT now U.S. Pat. 
No. 4,356,435, issued Oct. 26, 1982; to U.S. patent application Ser. No. 
259,343 filed May 1, 1981 by M. H. Kalmanash for DUAL MODE COLOR SWITCH 
FOR BEAM PENETRATION CRT now U.S. Pat. No. 4,337,420, issued June 29, 
1982; to U.S. patent application Ser. No. 259,344 filed May 1, 1981 for 
IMPROVED SEQUENTIAL COLOR SWITCH FOR BEAM PENETRATION CRT now U.S. Pat. 
No. 4,337,421, issued June 29, 1982; to U.S. patent application Ser. No. 
259,381 filed May 1, 1981 by M. H. Kalmanash et al for DIFFERENTIAL RANDOM 
COLOR SWITCH FOR BEAM PENETRATION CRT; to U.S. patent application Ser. No. 
259,383 filed May 1, 1981 by M. H. Kalmanash for STROKE DURING RETRACE 
COLOR SWITCH; and to U.S. patent application Ser. No. 284,831 filed July 
20, 1981 by M. H. Kalmanash for MODULAR HIGH SPEED COLOR SWITCH, now 
abandoned, all of which are assigned to the same assignee as the present 
case. 
DESCRIPTION 1. Technical Field 
This invention relates to a high voltage color switch for a beam 
penetration CRT, and more particularly, to a color switch which uses a 
constant current source which biases the primary winding of the high 
voltage transformer during the primary color write period. 2. Background 
Art 
A beam penetration-type color CRT (cathode-ray tube) is generally known and 
is a display device having a faceplate on which an image or alphanumeric 
characters can be written. One or more phosphor layers on the inner 
surface of the faceplate can be selected to emit almost any desired 
wavelength of visible light. If two layers of phosphor are deposited on 
the faceplate, it is possible to display more than two distinct colors by 
changing the depth of penetration of the electron beam into the phosphor 
layers. Because the electron beam emitted by the cathode in the neck of 
the CRT strikes the phosphor layers at a velocity influenced primarily by 
the voltage level on the accelerating anode, a change in the voltage level 
applied to the accelerating anode will correspondingly change the 
proportion of light emitted by the two phosphor layers. In other words, in 
a penetration CRT with two layers of different light emitting phosphor up 
to about four colors can be displayed to a viewer by changing the DC 
voltage level applied to the accelerating anode positioned near the front 
of the CRT. 
A significant limitation encountered in the use of penetration-type CRT's 
is related to the length of the reset period between write periods. 
Because the DC voltage level on the accelerating anode must be changed 
during the reset period, the length of the reset period is primarily 
defined by the electrical capacitance associated with the anode. The anode 
has a relatively large physical size and, as such, inherently has a large 
capacitance resulting in a significant amount of electrical charge being 
stored thereon during a write period. Of course, any additional 
capacitors, particularly large capacitors which are often used in high 
voltage power supplies, also increase the capacitance in the high voltage 
circuit and add to the reset period. Because this electrical charge is 
increased, or decreased, to change the voltage level on the anode, the 
reset period separating two write periods is related to the 
charge/discharge rate inherently associated with the total capacitance 
seen by the high voltage power supply. 
Another limitation found in prior art color switches used with beam 
penetration CRT's is related to the sequencing of the colors to be 
displayed on the CRT faceplate. Although it is possible to display between 
three or four distinguishable colors on a two-layer penetration CRT, some 
high voltage color switches must operate in a particular sequence. In 
other words, the high voltage color switch provides one preselected 
voltage level to the anode in successive write periods, that is, the anode 
voltage is changed from 10 KV to 14 KV, from 14 KV to 18 KV and finally 
from 18 KV back to 10 KV. During each of these sequential write periods, 
images or alphanumerics written by the electron beam are displayed only in 
that color corresponding to the voltage level impressed on the anode. If 
images or alphanumerics are to be displayed in one color, such as red, 
during a particular write period, then at the completion of that write 
period no additional red information can be displayed until the high 
voltage color switch sequences through its preselected voltage levels to 
the next write period at which red information can be displayed. 
Of particular interest is U.S. Pat. No. 3,906,333 issued Sept. 16, 1975 to 
M. Kalmanash for LOW COST SWITCHING HIGH VOLTAGE SUPPLY, assigned to the 
same assignee as the present case, which describes a switching high 
voltage power supply for use with a beam penetration-type cathode-ray 
tube. This power supply has the secondary of a high voltage step-up 
transformer in series with the accelerating anode of the cathode-ray tube. 
The primary of the transformer is connected to ground through a capacitor 
for developing a DC voltage level. This voltage across the capacitor is 
fed to the regulating input of the baseline DC high voltage power supply. 
The color switching power supply of the present invention is an 
improvement over that described in this patent. 
Another patent of interest is U.S. Pat. No. 4,092,556 issued May 30, 1978 
to D. Chambers et al for SWITCHED HIGH VOLTAGE POWER SUPPLY SYSTEM. This 
patent describes a high voltage power supply for the rapid switching of 
high voltage applied to the anode of a beam penetration color cathode-ray 
tube. The energy for making the rapid transition between voltage levels is 
stored in two inductors, one for upward transitions and the other for 
downward transitions. When it is desired to change the voltage applied to 
the cathode-ray tube, the appropriate one of the storage inductors is 
coupled through a control switch to the anode causing the voltage applied 
to the anode to change at a rapid rate. The voltage rises until the 
desired voltage level corresponding to a desired upward color is reached 
at which time the switch is turned off and the storage inductor recharged. 
A tracking high voltage supply maintains the anode at the predetermined 
voltage level once that level has been reached. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide a color switch for a 
beam penetration CRT which is capable of random operation in a write 
period from a non-baseline color without the need for a core reset period 
during which the flux level in the high voltage transformer is reset to 
prevent saturation. 
A particular feature of the present invention is to provide a color switch 
for a beam penetration CRT in which color write periods in a secondary 
color can be randomly selected. 
Yet another feature of the present invention is to provide a color switch 
for a beam penetration CRT in which a constant current driver provides a 
bias current in the primary winding of a high voltage transformer during 
the write period for the primary color thereby eliminating the need for a 
core reset period at the end of a secondary color write period to reset 
the transformer flux level. 
According to the present invention a constant current color switch for a 
beam penetration CRT includes a high voltage power supply having an 
out-put voltage which is supplied to the anode of a CRT through the 
secondary of a high voltage transformer. The voltage level of the high 
voltage power supply is set to the voltage level required to generate the 
color in which information is displayed most of the time or the primary 
color, e.g., green. In the preferred embodiment, this corresponds to an 
anode voltage of 18 KV. This green is typically chosen as the primary 
color since it has the best brightness and resolution since it is written 
at the highest available anode voltage. A high voltage transformer has its 
secondary winding in the high voltage circuit to the anode so that the 
voltage level presented to the anode can be changed throughout a range, 
e.g., 18 KV to 10 KV by changing the transformer voltage. One side of the 
primary winding is connected to a low voltage power supply, e.g., 5 volts. 
A constant current driver as well is powered by this low voltage power 
supply. The other side of the primary winding is connected to a switch so 
that a signal from either the constant current driver or a color bit 
processor can be presented to the primary winding. When the CRT is to 
display the primary color, which is most of the time, the switch connects 
the current driver to the primary winding of the transformer so that a 
constant bias current is provided to the primary winding of the 
transformer which sets a magnetic field in the transformer core just short 
of core saturation. To generate a write period in a secondary color, the 
switch is transitioned connecting the voltage driver to the primary 
winding of the transformer. This allows a sufficient voltage to be 
impressed on the primary winding to change the voltage level on the 
secondary side which combines with the high voltage power supply output to 
set the appropriate level for the secondary color write period. The 
secondary colors all involve a unidirectional drive on the transformer, 
with respect to the primary color. Thus, the constant current drive which 
sets the magnetic field and thus the core flux density to one extreme 
during the primary color interval permits utilization of the full dynamic 
range of flux during random operation, which minimizes the size of the 
transformer. It also resets the transformer core during the primary color 
interval. 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent from the following description of a 
preferred embodiment and accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring first to FIG. 1, there is seen one embodiment of a constant 
current color switch for a beam penetration CRT according to the present 
invention. In preferred form, a high voltage power supply 10 of a known 
type is provided and its DC output level is selected to provide a 
predetermined baseline color (described in greater detail hereinafter) in 
a beam penetration CRT (cathode-ray tube) 12. The CRT 12 includes a 
faceplate 14 on which images are written by a beam of electrons. The inner 
surface of the faceplate 14 typically has at least two layers of phosphor 
deposited thereon, each layer emitting a distinct wavelength, or color, of 
light in response to being excited by the electron beam. For the purpose 
of appreciating the present invention, it will be assumed that there is 
one layer of green phosphor (closest to the faceplate) and one layer of 
red phosphor located on the inner surface of the faceplate 14. Of course, 
more than two layers of phosphor could be deposited on the faceplate 14. 
The beam of electrons is emitted by a cathode (not shown) located near the 
end of the neck portion of the CRT 12; the individual electrons are 
accelerated toward the faceplate 14 under the influence of a high voltage 
applied to the anode 16. As is known, the anode 16 is formed from a 
material of high conductivity and is positioned circumferentially around 
the front portion of the tube to influence the beam of electrons. Because 
of its large size, the anode 16 has a relatively high capacitance and, for 
the purposes of simplicity, is illustrated in the preferred embodiment of 
FIG. 1 as a capacitor. 
In accordance with the present invention, the output of the high voltage 
power supply is presented along a line 18 to one end of a secondary 
winding 20 of a transformer 22. The other end of the secondary winding 20 
is connected by a line 24 to a damping resistor 26, the other end of which 
is connected by a line 28 to the anode 16. As will be appreciated, the 
just described circuit is essentially a high voltage circuit operating in 
the range of, for example, 10 KV to 18 KV to provide voltage levels 
suitable for accelerating the beam of electrons toward the faceplate 14. 
In the present invention, that color in which information is displayed on 
the faceplate most of the time is known as the "baseline color" and is 
obtained by impressing the highest voltage level, i.e., 18 KV, on the 
anode 16 from the high voltage power supply 10. The color which 
corresponds to this highest voltage level, green, also has the greatest 
contrast and resolution. A secondary color is a nonbaseline color 
displayed on the faceplate 14 for write periods of a relatively short 
duration, e.g., a total of 2 milliseconds out of a total period of 16 
milliseconds (display refresh period), and typically information written 
during these secondary color write period are to contrast with that 
written in the baseline color. For example, information may be written in 
a red color which requires more immediate attention by the viewer than 
information written in the baseline color, green. In this embodiment, the 
secondary color write period may be a single interval or may be smaller 
intervals randomly placed in the display refresh period. The secondary 
colors are obtained during a secondary color write period by changing the 
voltage level impressed on the anode 16 within the selected voltage range. 
For example, in the present embodiment secondary color write periods can 
be obtained by impressing from 10 KV to 18 KV on the anode which is 
accomplished by subtracting a voltage of 8 KV or less from the high 
voltage power supply via the transformer 22. 
The transformer 22 also has a primary winding and the turns ratio is 
selected to provide the needed voltage swing from the baseline color in 
the known manner. For example, if a 1 to 320 turns ratio were employed in 
the transformer 22, a 25-volt change across the primary winding 30 would 
cause an 8 KV change across the secondary winding 20. Because this voltage 
swing is tied to a baseline voltage level of approximately 18 KV, the 
resulting swing on the line 24 would be within the range of 18 KV to 10 
KV. Still referring to FIG. 1, one end of the primary winding 30 is 
connected by a line 32 to a terminal 34, to which a low voltage portion of 
a low voltage power supply (not shown) is connected. This essentially 
supplies the voltage potential for the biasing current when the baseline 
color is being displayed on the CRT. The other end of the primary winding 
30 is connected by a lead 36 to a switch 38. The switch 38 has two 
positions, one of which is for the baseline color mode (position B) and 
the other of which is for the secondary color mode (position A). A current 
driver 40 is provided and functions as a constant current source with the 
switch 38 in the "B" position to keep a constant current on the line 39. 
In the baseline color mode the current driver 40 is connected to the 
transformer 22 so that the constant bias current is continually passed 
through the primary winding 30 in one direction. In the secondary color 
mode input color information is initially received by a color bit 
processor 44. The color processor 44 generates pulse waveforms at its 
output on a line 45 which is amplified by a voltage driver 46. The switch 
38, in its "A" position, is connected via line 47 to the output of the 
voltage driver 46 so that the amplified waveform corresponding to the 
selected secondary color can be presented to the primary winding 30 to 
change the color displayed by the CRT 12. 
Referring next to FIG. 3, there is seen a typical magnetization curve for 
the transformer 22. As mentioned herebefore, one feature of the constant 
current color switch of the present invention involves maintaining a 
constant current in the primary winding 30 when the baseline color is 
being displayed on the CRT 12. This current essentially drives the flux 
level to its maximum reset position to obtain as much dynamic range in the 
secondary color write periods as possible. As is well known, if the flux 
level is driven beyond H.sub.1 (to the right of H.sub.1) or beyond H.sub.2 
(to the left of H.sub.2) the transformer is driven into saturation and the 
secondary output voltage sags. The flux density, B, is a function of the 
transformer core material and size as well as the number of turns in the 
windings and is related to the amplitude and duration of a voltage pulse 
applied to the primary winding 30. In other words, to increase the flux 
density capability of the transformer 22 it would be necessary to increase 
the physical size of the transformer or the number of turns. It is highly 
desirable to use the entire dynamic range in order to minimize the 
transformer size, since increasing the number of turns slows the dynamic 
response of the transformer, i.e., increases the transition time to 
secondary colors. In the present invention, the constant current applied 
to the primary winding 30 drives the flux level to just below its 
saturation point, i.e., 50. 
A particular feature of the present invention is that it also provides 
dynamic tracking color focus voltage for the CRT 12. In preferred form 
this includes a first potentiometer 60 which is coupled to the output of 
the high voltage power supply 10. This is a separate output which has a 
lower DC level than the output used to supply the anode voltage. The first 
potentiometer 60 is connected through a winding 62 which is an additional 
winding on the transformer 22 to a second potentiometer 64. The 
potentiometer 64 is used to provide dynamic adjustment to the voltage 
applied to the focus electrode 66 which is located near the front portion 
of the electron gun (not shown) of CRT 12. Typically, the focus voltage 
level applied to the focus electrode is a fixed percentage level of the 
voltage applied to the anode 16. A particular feature of the present 
invention is that this focus system requires a minimum of additional 
parts, only a couple of potentiometers and the additional winding on the 
transformer 22. The first potentiometer 60 adjusts the DC baseline voltage 
level while the second potentiometer 64 adjusts the dynamic output to the 
level applied to the focus electrode 66. 
The operation of the constant current color switch according to the present 
invention will now be described with reference to FIG. 3 together with the 
aforementioned figures. Prior to time t.sub.0, the switch 38 is in the "B" 
position and the high voltage power supply 10 provides an 18 KV voltage 
level to the anode 16 of the CRT 12 so that information is written on the 
CRT 12 in the primary or baseline color, i.e., green. At time t.sub.0, the 
color bit processor 44, acting in response to incoming color information, 
is to transition the voltage level on the anode 16 to a suitable level to 
display a secondary color, i.e., red, so that information can be written 
on the faceplate 14 during this secondary color write period in a 
contrasting color. A control pulse presented by the color processor 44 
along the line 43 transitions the switch 38 from its "B" position to its 
"A" position. At the same time, a pulse is provided along line 45 to the 
input of the driver 46. This pulse is amplified by the driver 46 to form 
an output waveform which creates an 8 KV voltage swing in the transformer 
22 that is of opposite polarity to that of the high voltage power supply 
10. The voltage level on the anode 16 is thus driven to the 10 KV level 
during this period t.sub.0 to t.sub.1 so that information written on the 
face-plate 14 during this interval is displayed in the secondary color, 
red. 
At the end of the secondary color write interval, time t.sub.1, the switch 
38 is transitioned back to its "A" position and the color switch of the 
present invention is now ready to display information in the baseline 
color, green. However, it will be noted that the magnetizing current in 
the transformer 22 was driven in the direction toward the other end of its 
dynamic range during the secondary color write period. The constant 
current driver 40 now restores the magnetizing current back toward the 
constant level prior to time t.sub.0, or the end of its dynamic range. 
Because this magnetic reset current is driven by a lower voltage source, 
e.g., 5 volts versus 28 volts, during this reset period, time t.sub.1 to 
time t.sub.2 there is a small change in the associated voltage level on 
the anode 16. As seen in FIG. 2, this small increase in the voltage level 
applied to the anode 16 is about 1 KV, compared to the baseline voltage 
level on the anode 16 during the primary color period of about 18 KV so 
that it essentially is not perceived by a viewer. Finally, by time t.sub.2 
the magnetizing current in the primary winding has been fully restored to 
one end of its dynamic range and the current driver 40 then supplies a 
constant bias current to the primary winding 30. As would be expected, 
because there is no longer any change in current through the primary 
winding 30, there is no voltage change across the secondary winding 20 and 
the voltage impressed on the anode 16 returns to that of the output of the 
high voltage power supply 10, or 18 KV. 
Although this invention has been shown and described with respect to a 
preferred embodiment, it will be understood by those skilled in this art 
that various changes in form and detail thereof may be made without 
departing from the spirit and scope of the claimed invention.