Voltage switching circuit for a color display system

A power supply for a color display device of the type in which the color of light emitted by a phosphor screen is changed by switching an electron accelerating voltage between different levels, the switching being digitally controlled and being connected so that descending voltage changes are recycled to efficiently drive the supply.

BACKGROUND OF THE INVENTION 
As is well known, any instrument which uses a cathode-ray tube, such as the 
oscilloscope, requires direct current operating voltages much higher than 
the direct current potentials usually derived from conventional 
alternating current line voltages. These high voltages cannot be 
efficiently and economically attained for a plurality of reasons such as 
having to incorporate heavily insulated transformer windings, bulky and 
dangerous capacitors and other objectionable features. In addition, when 
the cathode-ray tube is a beam penetration type cathode-ray tube wherein 
the color of light emitted by a phosphor screen is changed by switching an 
electron accelerating voltage between different levels for a color display 
system the devariation of the necessary high voltages is even more 
particular. For example cost, reliability, switching speed and the 
above-mentioned efficiency must be considered. 
Heretofore, voltage switching circuits in a color display system of this 
type in which the color of light emitted by a phosphor screen is changed 
by switching an electron accelerating voltage between different levels 
have included two or more silicon controlled rectifiers (SCR's) which are 
alternately triggered to switch the acclerating voltage between two 
different levels. As described in U.S. Pat. No. 3,512,036 which teaches a 
protective circuit for use with SCR high voltage switching circuits, such 
circuits connect the SCR's so that both SCR's could conduct simultaneously 
due to some malfunction. In such a case, the switching is latched-up in 
which neither of the SCR's can be turned off unless the current is 
interrupted. Therefore, the use of such a switching circuit along with a 
protection circuit as described in the above-mentioned patent becomes 
expensive. 
In other prior art power supplies, especially where voltages are in the 
kilovolt ranges, switching of various voltages is usually difficult in 
that a plurality of relays, or precautions taken to adequately insulate 
switches and wiring must be undertaken, are utilized. With relays and 
switches, switching speeds of up to 200 microseconds are necessary to 
switch the accelerating voltages between different levels in order to 
change the color of the light emitted by the cathode-ray tube. These long 
switching times are not effective in state of the art equipment, another 
disadvantage of the prior art. Also, these prior art power supplies 
usually employ transistor "stacks" to do their high voltage switching 
which magnifies control interface due to the operating voltages of the 
transistor "stacks". Further, these transistor "stacks" are succeptable to 
breakdown thereby decreasing reliability. 
SUMMARY OF THE INVENTION 
The present invention overcomes these mentioned disadvantages of the prior 
art by providing a voltage switching circuit wherein all driver 
transistors operate at low voltages which simplifies the control interface 
which also means less transistors, hence less cost; since the transistors 
are operated at low voltages there will be less breakdown thereof which 
increases relaibility; during descending voltage changes, the energy 
delivered to the cathode-ray tube is recycled back into the supply to 
conserve energy; and the circuit uses simple digital control and 
sequencing to increase switching speeds. 
Basically, the entire voltage switching circuit consists of two series 
connected power supplies; a conventional-floating high voltage supply, and 
a switching supply which includes a high speed driver for loading energy 
into the load, a low impedance driver for maintaining the energy in the 
load and a digitally controlled reset which recycles the energy back into 
the driver supply. 
It is therefore an object of the present invention to provide a voltage 
switching circuit for a color display system which overcomes the 
disadvantages of the prior art. 
It is another object of the present invention to provide an improved power 
supply for a color display system of the type in which the color of the 
light emitted by a phosphor screen is changed by switching an electron 
accelerating voltage between different levels. 
It is still another object of the present invention to provide an improved 
power supply for a color display system whereby switching is digitally 
controlled and being connected so that energy on descending voltage 
changes is recycled to efficiently drive the supply. 
The foregoing and numerous other objects, advantages, and inherent 
functions of the present invention will become apparent as the same is 
more fully understood from the following description and drawings which 
describes the invention in one of its preferred embodiments; it is to be 
understood, however, that the embodiment described is not intending to be 
limiting nor exhausting of the invention but is given for the purpose of 
illustration in order that others skilled in the art may fully understand 
the invention and principles thereof and the manner of applying it in 
practical use so that they may modify it in various forms, each as may be 
best suited to the conditions of the particular use.

DESCRIPTION OF THE INVENTION 
Referring now to the drawings and in particular FIG. 1, there is indicated 
a load 10 such as a cathode-ray tube of a type with which the present 
invention is useful. Positive direct current is supplied to load 10 from a 
high voltage supply stage 12 which, in turn, is under the control of a 
first driving stage 14 and a second driving stage 16. These driving stages 
are urged into operation by first and second driver control means 18 and 
20, respectively. Each of the driver control means includes first inputs 
which, in turn, are connected to each other and to the high voltage supply 
stage 12 for feedback purposes as well as having second inputs which, in 
turn, are connected to each other and to a selector stage 22. Selector 
stage 22 provides selectable reference signals necessary to drive the 
control and driver stages so that voltage stage 12 provides a certain 
magnitude of direct current voltage relative thereto. Selector stage 22 is 
digitally controlled by two signals obtained from a sequence control stage 
24 which receives at inputs 26 and 28 coded information as to the 
necessary value of the reference signal discussed above. Sequence control 
stage 24 additionally provides an enable signal to first driver control 
means 18 as well as a signal to initiate a reset of the supply via a reset 
stage 30 operatively disposed between the output of the high voltage 
supply stage 12 and the first and second driving stages 14 and 16. 
The FIG. 1 embodiment therefore represents a circuit which can best be 
described as consisting of two series connected power supplies. The first 
is high voltage supply stage 12 which is adapted to provide load 10 the 
lowest output level deliverable thereby. The second supply is, of course, 
the switching section supply as previously mentioned in the specification. 
In essence, first driving stage 14 is a circuit for loading energy into 
load 10 and is operated so that the energy is deliverable thereby to the 
load only during a certain time period, second driving stage 16 maintains 
the voltage on the load after being loaded by driver 14 at a frequency 
equal to the resonance of the transformer and finally the section which 
unloads energy off of load 10 during descending voltage changes and 
recycles it back to both the first and second driving stages in accordance 
with timing and common signals needed to satisfy the coded information 
applied at inputs 26 and 28. 
To fully understand the invention, reference should now be directed to the 
drawings shown in FIGS. 2-9. In these drawings, a preferred circuit is 
shown and each figure corresponds to a particular block as shown in FIG. 
1. It should be mentioned that the circuits shown and described 
thereinafter are not intended to be limiting thereto. An exception to this 
is the stage shown in FIG. 2 for conditioning the coded information for 
application to the sequence control stage 24 and which consists of a pair 
of inverting amplifiers comprising NPN transistors 50 and 52 whose 
emitters are grounded and whose collectors are coupled to a low voltage 
source of potential A, say positive 15 volts, via load resistors 66 and 
68. The base of each transistor is biased to a selected level by biasing 
networks comprising resistor pairs 54-56 and 58-60, each pair serially 
connected between ground and another source of low voltage potential B, 
say negative 15 volts. In addition to being connected between the bias 
resistors, the base of each transistor is coupled to the input terminals 
26 and 28 via input resistors 62 and 64 respectively. 
As was previously explained, each input terminal may receive coded 
information. Typical of such coded information are the waveforms 70 and 72 
as shown in FIG. 3. Waveform 70 is a TTL compatible digital signal having 
two stable levels existing, for example, say at +5 volts and at zero volts 
i.e., a logical 1 and a logical 0 by definition; waveform 72 is similar. 
Assuming that the load 10 is a cathode-ray tube and further assuming that 
two phosphors are utilized therein, for example red and green phosphors, 
then waveform 70 and 72 represent coded information which will cause the 
power supply to switch the electron accelerating voltage between different 
levels to change the color of light emitted by the phosphor screen to 
thereby provide red light, green light or combinations thereof such as 
orange and yellow light. Digitally, if waveform 70 is considered as the 
most significant bit (MSB) and waveform 72 as the least significant bit 
(LSB), then as shown in the drawings and by definition red=00, orange=01, 
yellow=10 and green=11. In the circuit of FIG. 2, the MSB signal is 
applied to input 28 and the LSB signal is applied to input 26 whereby they 
are inverted by the amplifiers and are available in inverted and level 
shifted form, waveforms 70 and 72 (pronounced not 70, not 72), to be 
utilized by the sequence control stage 24 and the selector stage 22. It 
should be mentioned that hereinafter the waveforms 70, 72, 70 and 72 will 
be referred to as the MSB and LSB signals or MSB and LSB (inverted) 
signals, respectively. 
Referring now to FIG. 4 there is shown a schematic diagram of the sequence 
control stage 24. The LSB and MSB signals are respectively applied to D 
inputs of flip-flops 80 and 82 whose reset inputs R are grounded. The set 
inputs (S) of each D flip-flop are coupled together as are the clock 
inputs (C). Flip-flop 80 and 82 are preferably the MC14013B dual type D 
flip-flop device (indicated by solid box 83 surrounding both devices) 
commercially manufactured by MOTOROLA Semiconductor Products Inc., and 
which are fully explained on pages 5-31 to 5-34 of "Semiconductor Data 
Library", Volume 5/Series B, .COPYRGT.MOTOROLA INC., 1976. The purpose of 
flip-flops 80 and 82 is to remember the previous conditions of the LSB and 
MSB signals and applies this information to the B0 and B1 inputs 
respectively, of a 4-bit magnitude comparator 84 (unused data input held 
low) such as the MC 14585B 4-bit Magnitude Comparator also commercially 
manufactured by MOTOROLA Semiconductor Products Inc., and which is fully 
explained on pages 5-492 to 5-495 of the above mentioned reference. Also 
applied to the comparator 84 via inputs A0 and A1 are the present LSB and 
MSB signals respectively. In operation, the inputs to comparator 84, which 
are labeled B0 and B1, are compared against the inputs to comparator 84, 
which are labeled A0 and A1, and if the present values of LSB and MSB are 
less than the previous values obtained from flip-flops 80 and 82, then an 
output A&lt;B (present values of LSB and MSB less than previous values of LSB 
and MSB) is provided on a line 86 or if the current value of LSB and MSB 
are greater than the previous values, an output A&lt;B is obtained on a line 
88. When an output A&lt;B is obtained on line 88, such output is applied to 
reset stage 30 as well as to the set inputs of flip-flops 80 and 82. An 
example to exemplify this action is as follows: assume that the previous 
LSB and MSB are such that B.sub.0 =0, B.sub.1 =1, and that the present LSB 
and MSB are A.sub.0 =0 and A.sub.1 =0. With these assumptions the previous 
inputs infer a 10 or orange condition (see FIG. 3) and the current inputs 
infer a green condition 00. In other words, load 10 has an accelerating 
voltage which produces orange light and the coded information is calling 
for green light to be produced. The comparator 84 senses this condition 
and an output on line 86 is obtained to effect the change. Alternatively, 
if the assumed inputs were reversed, an output on line 88 would be 
provided. 
The signal on line 86 (if A&lt;B) is applied to a one-shot multivibrator 90 
whose clear input is always held high by virtue of being connected to the 
source of low voltage A. Once fired, or clocked, the Q and Q outputs 
thereof are independent of further transitions of the input and are a 
function of external timing components comprising a capacitor 92 and the 
resistors 94 and 97 serially connected to the source of voltage A. So that 
these limitations may be made varied, the resistor 94 has been made 
variable. The Q output of this multivibrator is an enable signal utilized 
by the control means 18 and is the waveform 163 in FIG. 3. The Q output 
thereof is utilized as the clock input of the already mentioned D 
flip-flops 80 and 82. Thus, it can be seen that if the previous LSB and 
MSB inputs are found to be greater than the current LSB and MSB inputs, 
the switching circuit is enabled and the current values of LSB or MSB are 
now loaded into flip-flop 82 and 84 to await new information. As 
previously stated if the current LSB and MSB signals are greater than the 
previous, a signal on line 88 is obtained. In accordance with the 
invention, such case defines that the accelerating voltage is to be 
lowered. Accordingly, the signal on line 88 is utilized to initiate the 
recycling of power during the descending change to improve the efficiency 
of the supply. Additionally, this signal is used to set the flip-flops 80 
and 82 after a time delay. This time delay is provided by the capacitor 98 
and resistors 100 and 102, resistor 100 being variable to select the time. 
This delay insures a finite time for recycling energy off the load before 
setting the flip-flops. A diode 104 is utilized to speed up, shorten, the 
time that the set signal is applied to the set inputs of the flip-flops. 
It should also be mentioned that the one-shot multivibrator 90 is 
preferrably a MM74C221 Dual Monostable Multivibrator commercially 
available from National Semiconductor Corporation and is fully documented 
in "CMOS DATABOOK" , .COPYRGT.National Semiconductor Corporation, pages 
1-91 to 1-94. 
Referring now to FIG. 5 there is shown the selector stage 22 according to 
the present invention. Selector stage 22 functions as a 4 channel analog 
multiplexer whereby four information units (voltages) are derived from a 
voltage divider network comprising series connected resistors 110, 112, 
114, 116, 118 and 120 connected between the source of low voltage A and 
ground. Variable portions of resistors 114, 116, 118 and 120 are utilized 
to couple a selectable value of voltage obtained from the divider to the 
multiplexer. Variable resistor 112 enables the overall maximum value 
across the divider to be precisely set. The selection of one of the 
voltages derived across the voltage divider is controlled by the LSB and 
MSB signals applied thereto and the selected value is then available to 
the first and second drive control means. In the embodiment shown, the 
multiplexer is a CMOS integrated circuit device 122 such as the MC 14052 
Dual 4-Channel Analog Multiplexer/Demultiplexer commercially available 
from MOTOROLA Semiconductor Products Inc. All technical data concerning 
this device is also published in the mentioned Motorola reference on pages 
5-124 to 5-129. A typical output of multiplexer 122 with inputs LSB and 
MSB as shown in FIG. 3 would be the waveform 124 also shown in FIG. 3. 
Referring now to FIG. 6 there is shown the schematic diagram of the first 
driver control means 18 and the first driving stage 14. The means 18 
comprises the controlled amplifier 150 such as, for example, a National 
LM311 Voltage Comparator commercially available from National 
Semiconductor Corp. and described in "Linear Data Book", pages 5-18 to 
5-23 copyrighted 1976 by National Semiconductor Corp. having its inverting 
input adapted to receive feedback from the supply stage 12 and its 
non-inverting input adapted to receive the multiplexed voltage, waveform 
124, from selector stage 22. The amplifier 150 is referenced from a 
voltage divider comprising the resistors 152 and 154 serially connected 
between ground and the low voltage source B. The output of amplifier 150 
is coupled to the base of an inverting amplifier comprising a NPN 
transistor 156 whose emitter and collector are coupled to ground and the 
low voltage source B via resistors 158 and 160, respectively. As 
previously mentioned, the first driver control stage is enable, such 
enable signal being applied from the output of one-shot multivibrator 90 
(see FIG. 4) via resistors 162 and 164 for controllably enabling the 
output signal of amplifier 150 to pass through the transistor 156. The 
enable signal is the waveform 163 (see FIG. 3) and will occur for, say, 
approximately 25 microseconds. Any signal at the collector of transistor 
156 is then coupled by capacitor 166 to the base of yet another transistor 
amplifier comprising PNP transistor 168. 
Transistor 168 has its emitter coupled to low voltage source A via resistor 
170, the combination of which forms a current source. Bias for transistor 
168 is provided by current via return resistor 174 coupled between the 
base thereof and the voltage source A. A diode 172 protects the 
emitter-base junction of transistor 168. Current from this current source 
is applied to load resistor 176 coupled between the collector of 
transistor 168 and ground. The signal at the collector of transistor 168 
is then applied to the base of a current drive NPN transistor 186 via a 
diode 178. Transistor 186 has its emitter grounded and its collector 
coupled to a source of potential, RECYCLE, via the primary winding 
W.sub.p1 of a transformer 188. Transistor 186 is preferably a 2N6546 which 
may require heat sinking, and such transistor also has its base coupled to 
a voltage divider comprising resistors 182 and 184 via a resistor 180. 
Resistors 182 and 184 are also coupled between ground and the low voltage 
source B, respectively. Disposed between a junction formed by the 
connection of resistors 180, 182 and 184 and ground is a capacitor 183. 
In operation, amplifier 150 compares the value of the accelerating voltage 
available to the load (via feedback) with the value of the selected 
reference voltage and, in turn, produces a drive pulse to the transistor 
156 only during the occurrence of the enable signal 163. This drive pulse, 
which is not shown in FIG. 3 is similar to signal 163 but which only 
occurs for about 10 micro-seconds, is next converted to a current and 
utilized to drive the primary winding W.sub.p1 of transformer 188 so that 
across the secondary winding, a voltage is developed which will be used to 
provide the necessary value of accelerating voltage. (The "on" time of 
transistor 186 is generally indicated by the dashed lines 186 in FIG. 3.) 
In other words, during a portion of the pulse time of waveform 163, 
transformer 188 is initialized by this circuit. It should be mentioned 
that capacitor 183 is utilized to "speed-up" the turn off of transistor 
186. 
In FIG. 7 there is shown the circuits comprising second driving control 
stage 20 and the second driving stage 16. The waveform 124, (see FIG. 3) 
obtained from selector stage 22 is applied to the non-inverting input of 
an amplifier 200 across an input resistor 202 (or additional resistors) 
whereas the inverting input of this amplifier receives feedback from stage 
12 via a resistor 206. Amplifier 200 may be a commercially available 
National LF356 Operational Amplifier fully described in the mentioned 
reference by National Semiconductor Corp. on pages 3-1 to 3-13 thereof. 
The output of amplifier 200 is coupled to a current mirror comprising 
transistor 204 and a diode 205 via resistor 210 as well as to its 
inverting input via a capacitor 207. The emitter of transistor 204 is 
directly coupled to the low voltage source B and its collector supplies 
current via a resistor 212 to the emitters of an emitter coupled amplifier 
pair comprising NPN transistors 214 and 216. The base of transistor 214 is 
coupled to a voltage divider comprising resistors 218 and 220 and has 
waveform 124 applied to the divider from the selector stage 22. The 
collectors of transistors 214 and 216 are connected to the low voltage 
source A via load resistors 222 and 224 as well as to the non-inverting 
and inverting inputs of an amplifier 226, respectively. Amplifier 226 may 
be a National LM741 Operational Amplifier which is described on pages 
3-191 to 3-193 of the reference data book. The collector of transistor 214 
is also connected to the anode of a protection diode 226 having its 
cathode coupled to the low voltage source A, whereas the collector and 
base of transistor 216 has resistors 228 and 230 connected to ground, 
respectively. 
The output of amplifier 226 is utilized to drive the base of an NPN 
transistor 232 via a parallel network comprising a resistor 234 and a 
capacitor 236. The emitter of transistor 232 is coupled to the low voltage 
source B via a resistor 238 and to ground via a resistor 240 whereas the 
collector is coupled to a source of power (RECYCLE) via a load resistor 
242. Also coupled to the collector of transistor 232 is the base of an NPN 
transistor 244 and the cathode of a diode 246, the anode of which is 
connected to the emitter of transistor 244, the cathode of a diode 248 and 
the base of 2N6546 current transistor 250. The collectors of transistors 
244 and 250 are coupled together and, in turn, coupled to the source of 
power (RECYCLE). The emitter of transistor 250 is coupled to the anode of 
the already mentioned diode 248 and to the non-inverting input of 
amplifier 226 via feedback resistor 252. Additionally, the emitter of 
transistor 250 is coupled to a second primary winding W.sub.p2 of the 
already mentioned transformer 188 and the W.sub.p2 non-inverting input of 
an amplifier 254 via a voltage divider comprising resistors 256 and 258. 
Amplifier 254 which may be a National LM311 Voltage Comparator, also 
includes an inverting input which is coupled to the other side of winding 
W.sub.P2 via the adjustable voltage divider comprising resistors 260 and 
262, the latter being of the adjustable type. The output of amplifier 254 
directly connects to a one-shot multivibrator 264. Multivibrator 264 is 
preferrably the second half of the already mentioned multivibrator 90 and 
which also includes external timing components comprising the capacitor 
266 and resistors 268 and 270 which are referenced to the low voltage 
source A. The output, Q, of multivibrator 264 is coupled to the low 
voltage source B via series connected resistors 272 and 274. 
Disposed between a junction formed by the connection of resistors 272 and 
274 is the base of an NPN transistor follower 276 whose collector is 
directly coupled to the source of low voltage A and whose emitter is 
returned to ground via a resistor 278. Also coupled between the emitter of 
transistor 276 and the mentioned junction is a resistor 280. The emitter 
of transistor 276 also connects to the base of an NPN transistor 282 whose 
emitter is grounded and whose collector is coupled to the winding W.sub.p2 
of transformer 188 via the cathode-anode of a diode 284. The circuit is 
completed by a diode 286 having its cathode coupled to the collector of 
transistor 282 and its anode connected to the base of transistor 276. 
Basically, amplifier 200 and transistors 232, 244 and 250 form a 
conventional series pass regulator circuit which provides a low impedance 
DC voltage to the transformer 188. This low impedance DC voltage is then 
converted to a low impedance AC voltage by aiding the resonant condition 
of the transformer (already initialized by transistor 186 current as 
previously mentioned) by a resonate switch comprising the active elements 
254, 264 and transistors 276 and 282. The difference pair or multiplier 
comprising transistors 214 and 216 adjusts the gain of the series pass 
regulator and the integrator amplifier 200 adjusts the gain (via the 
multiplier) of the series pass regulator such that the accelerating 
voltage conforms (ratio wise) with the reference input. Since the 
integrator is utilized, very precise adjustment is provided thereby 
minimizing error. Action of this described control and driver stage and 
the previously described control and drive stage can best be seen in FIG. 
3 as the waveform 189. Waveform 189 is, of course, across the secondary 
winding W.sub.s1. 
Referring now to FIG. 8, there is shown a portion of the circuit diagram of 
a conventional high voltage power supply which may be utilized as a 
portion of stage 12. As is generally the case, feedback from the output 
stage is applied to an amplifier portion generally indicated by the broken 
arrow 290. The amplifier 290 controls an oscillator portion indicated by 
the broken arrow 292 which drives an output stage which is generally a 
current source such as a transistor 294 providing current to the primary 
winding W.sub.p1 ' of a transformer 296 having one terminal thereof 
connected to a higher voltage source C, say of 100 volts. The output is 
across secondary winding W.sub.s2 '. Of course, the oscillator portion is 
substained by second primary winding W.sub.p2 '. Since this circuit is 
conventional, no further discussion thereof is believed necessary. Further 
information however, is hereby incorporated by reference to "Power Supply 
Circuits", Circuit Concepts, copyrighted by Tektronix, Inc. 
Referring now to FIG. 9 there is shown another portion of the high voltage 
power supply, the load 10 such as a cathode-ray tube, and the reset stage. 
As can be discerned, the secondary winding W.sub.s2 ' of transformer 296 
provides to a tripler 300 the necessary power such that tripler 300 
produces the electron accelerating voltage to the load 10 after filtering 
by capacitor 302. Load 10 can, of course, be considered a capacitive load 
as indicated by the capacitor 304 as is well known. As previously stated, 
this portion of the supply provides the lowest output level deliverable 
thereby. 
In accordance with the present invention however, windings W.sub.p1 and 
W.sub.p2 of transformer 188 are also driven so that across winding 
W.sub.s2 the power necessary to change the accelerating potential on load 
10 is provided. One end of winding W.sub.s2 is connected to voltage 
doubler, comprising in part a parallel diode 306 and capacitor 308. The 
cathode of diode 306 is also coupled to the anode of a diode 310 and the 
cathode of a diode 312, the former diode having its cathode coupled to the 
low or negative side of the tripler 300 and the latter diode having its 
anode grounded. This secondary circuit and the high voltage stage are 
therefore "stacked". Serially disposed between the high or positive side 
of tripler 300 and ground is a first voltage divider network comprising 
resistors 320, 322, and 324 and serially disposed between the low side of 
tripler 300 and ground is a second voltage divider network comprising 
resistors 326, 328 and 330. It is across these two divider networks that 
feedback is obtained, and as indicated in the drawing could be across 
resistors 328 and 322 as indicated by terminals 332, 334, 336 and 338. 
Although not shown in the drawings, terminals 336 and 338 are preferably 
the inner conductors of a coaxial cable with terminal 336 connected such 
that feedback is applied to the input 291 (see FIG. 8) from the divider 
and input 293 receives feedback from terminal 332. Additionally, input 
terminal 291 is coupled to the resistor 206 (see FIG. 7). Similarly, the 
terminal 293 is coupled to the inverting input of amplifier 150 (see FIG. 
6). Simultaneously, the terminals 334 and 338 are grounded at the above 
mentioned inputs whereas the shield of the coaxial cables carrying the 
feedback are only grounded at the divider terminals. This feedback scheme 
allows (a) resistors 324 and 330 to act as isolation means in the event of 
a short across the high voltage section, and (b) since the shield is only 
grounded at the divider end, less noise generated therein is fed-back. 
Also shown in FIG. 9 is the reset section 30 which includes, say, a means 
350 such as a "spark gap" serially coupled between the negative, or low, 
output side of tripler 300 and ground via the primary winding W.sub.p1 " 
of a transformer 352. A secondary winding W.sub.s2 " of transformer 352 
has disposed in parallel therewith a series connected diode 354 and a 
capacitor 356 for providing a rectified voltage, RECYCLE, for use by the 
first and second driver stages 14 and 16. By recycling this energy, the 
supply is efficient. Controlling the rectified voltage (RECYCLE) available 
to the drivers is a control portion including an amplifier comprising an 
NPN transistor 358 whose emitter is grounded via a resistor 360 and whose 
collector is directed coupled to winding W.sub.s2 " and diode 354. The 
second portion of the amplifier is the NPN transistor 362 whose collector 
is directly connected to the low voltage source A and whose emitter is 
directly coupled to the base of transistor 358. Control of the stage is 
provided by applying the output of multivibrator 90 (see FIG. 4) which is 
the waveform 93 of FIG. 3 to the base of transistor 362 via the RC network 
comprising resistors 364 and capacitor 366. Input to the base of 
transistor 362 is limited by a diode 368 coupled between the base thereof 
and ground, and such base is returned to ground via a resistor 370. 
Before continuing, it should be mentioned that all the transformers i.e., 
188, 296 and 352 have "polarity" as indicated by the "dots" adjacent each 
primary and secondary winding. These dots indicate which one of the two 
secondary winding terminals is positive at those times when a particular 
one of the primary winding terminals is positive, and vice versa. It has 
been shown and described the value of secondary voltage, waveform 189, 
increases or decreases in accordance with coded information applied at 
input terminals 26 and 28. On increasing voltage changes, the control and 
driver stages 18 and 14 causes the secondary of transformer 188 to be 
initialized. Once initialized, the secondary is maintained by control and 
driver stages 20 and 16. On descending voltage changes reset stage 30 is 
triggered which causes energy on load 10 to be utilized by drivers 14 and 
16 to increase efficiency of the circuit. Thus, the present invention 
provides a voltage switching circuit for a color display system which 
overcomes the disadvantages of the prior art. 
While there has been shown and described a preferred embodiment of the 
present invention, it will be apparent to those skilled in the art that 
many changes and modifications may be made without departing therefrom in 
its broader aspects. For example, there is shown in FIG. 10 an alternative 
circuit which may be utilized in place of the means 350 ("spark gap"). 
This alternative comprises a plurality of series connected networks 400 . 
. . 400R, the number of networks dependent upon the necessary electron 
accelerating voltages required, and each section generally capable of 
about 500 volts. Each network includes a series connected string of zenor 
diodes 402, 404 and 406 the latter of which has its anode coupled to the 
control electrode of an SCR 408 and a resistor 410. The anode of SCR 408 
is coupled to the cathode of diode 402 whereas the cathode is coupled to 
the other end of resistor 410. Disposed in parallel with the above 
mentioned elements is a resistor 412, the purpose of which is to equalize 
the voltage drops across the diode 402-406. It should be mentioned that 
the gate of SCR 408 may be associated with isolated windings of a 
transformer under the necessary control from the driving stages. 
Therefore, the appended claims are intended to cover all such changes and 
modifications as fall within the true spirit and scope of this invention.