|Publication number||US3783287 A|
|Publication date||Jan 1, 1974|
|Filing date||May 18, 1972|
|Priority date||May 18, 1972|
|Also published as||CA980011A, CA980011A1, DE2318493A1|
|Publication number||US 3783287 A, US 3783287A, US-A-3783287, US3783287 A, US3783287A|
|Inventors||Atlee Z, Fulton G, Gager R|
|Original Assignee||Picker Corp|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (2), Referenced by (19), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent [191 Fulton et al. Jan. 1, 1974 ANODE CURRENT STABILIZATION  ABSTRACT CIRCUIT XRAY TUBE HAVING A stabilization circuit for maintaining the anode cur- STABILIZER ELECTRODE rent of an x-ray tube substantially constant is de-  Inventors: George L Fulton, Chicago; Robert scribed including a first stabilization means in which a Gaga. Elmhurst; Zed J. Aflee, stabilization voltage signal proportional to changes in Oak Brook a" CH the anode current is applied to a stabilization electrode in such tube. A stabilization electrode is posi- Assigneer Pick" Corporation, Cklveland, tioned behind the cathode filament with respect to the Ohio anode and collects a portion of the electrons emitted  Filed: 18, 1972 from the filament to reduce the number of electrons striking the anode. The amplitude of the stabilization PP ,646 voltage determines the amount of electrons collected by the stabilizer electrode and thereby maintains the 52 us. Cl. 250/404, 250/409 anode current Constant A mild Stabilization means 51 lm. Cl H05g 1/34 of Slower Operation gradually takes from the first 58 Field of Search 250/95, 97, 99, 103 Stabilization means and varies the cathode filament heating current in order to maintain the electron flow  I References Cited to the anode constant in response to a control signal UNITED STATES PATENTS produced by a differential amplifier comparator means which is proportional to the stabilization volt- 2,767,327 l0/l956 Helterline,Jr. 250/97 age applied to the Stabilizer electrode Lightsignal 3,633,029 l/l972 Duffy, Jr 250/99 Primary Examiner-James W. Lawrence Assistant E.\'aminer-B. C. Anderson Attorney-Stephen W. Blore et al.
l4 [0 f 30 24 l l8 EXPOSURE UGHT CONTROL ELECTRODE PULSER PULSE POWER SUPPLY JJ20 26 J X-RAY TUBE coupling is employed between the x-ray tube anode current monitor and the stabilizer electrode power supply, as well as between such power supply and the differential amplifier for voltage insulation purposes.
10 Claims, 2 Drawing Figures FAST STAB] LIZE LIGHT SIGNAL SIGNAL POWER SUPPLY i s AMPLIFlER LNQ'VATLIGHT SIGNAL 38 46 i I" w CATHODE ANODE i VOLTS vou's I LIGHT 48 5O 52 22 i 5.8G- CATHODE FILAMENT HIGH VOLTAGE mgvtwDlFFERENTl/Rl. v CURRENT HEATER TRANSFORMER i AMPUF'ER STABILIZER TRANSFORMER J i 43 56 HIGH LOW ANODE L REEV corsa r iURRENT CURRENT n w m w 54 LIGHT MONITOR SIG. 35
ANODE CURRENT STABILIZATION CIRCUIT X-RAY THEE HAVING STABILIZER ELECTRODE BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to the stabilization of electron discharge current flowing between the cathode and anode of an x-ray tube or other high voltage electron discharge device. In particular, the invention relates to an electrical circuit for operating an x-ray tube having a stabilizer electrode of the type shown in pending US. Pat. application Ser. No. 229,136 filed Feb. 24, 1972, by Zed J. Atlee and entitled X-RAY TUBE WITH IMPROVED CON- TROL ELECTRODE ARRANGEMENT, in order to maintain the anode current of such tube substantially constant. This is achieved by applying a stabilizing voltage signal to the stabilizer electrode which is proportional to any variation in the anode current, such stabilizing voltage being of a polarity and amplitude which causes a reduction in such anode current variations. The stabilization circuit of the present invention is especially useful in pulsed x-ray apparatus, such as cineradiography, and may be employed for an x-ray tube having two filament cathodes, one of which is for low current fluoroscopy and the other for high current spot film radiography.
Previous x-ray tube stabilization circuits such as that shown in U.S. Pat. No. 2,617,045 of M. R. Coe and U5. Pat. No. 2,810,838 of C. W. Clapp et al stabilized the cathode to anode discharge current of the x-ray tube by adjusting the heating current flowing through the cathode filament to change the electron emission current of the cathode. This technique is satisfactory for tungsten cathode filaments which are heated to an extremely high temperature during normal operation so that heating and cooling of the filament to adjust the emission current may be accomplished rapidly by changing the heating current. However, this type of stabilization is too slow for some application of x-ray tubes employing thoriated tungsten filaments because they are operated at a much lower temperature than pure tungsten filaments, and therefore cool at a slower rate. Thus, for example, when making rapid exposures less than about a second duration, x-ray tubes employing thoriated tungsten filaments cannot be operated using prior stabilization circuits without some compromise in the degree of stabilization obtained because they do not stabilize the anode current fast enough.
The stabilization circuit of the present invention overcomes these disadvantages by providing a separate stabilization electrode in the x-ray tube behind the cathode filament with respect to the anode and applying a stabilizing voltage signalto such stabilizer electrode which is proportional to variations in the anode current of the x-ray tube. The stabilizer electrode responds immediately and collects a portion of the emitted electrons to reduce the electron discharge current to the anode with the amount of such collected portion changing in accordance with the amplitude of the stabilizing voltage applied thereto, and thereby maintains the anode current substantially constant. This new stabilization means operates substantially instantaneously since it does not depend upon the heating and cooling rate of the cathode filament.
In addition, the stabilization circuit of the present invention may also employ a second stabilization means which is activated by a control signal porportional to 2. the stabilizing voltage output signal of the first stabilization means described above and operates by varying the cathode filament heating current in a similar manner to that of the above-cited references. The second stabilization means gradually takes over stabilization of the anode current from the first stabilization means and reduces the stabilizing voltage signal. This provides a more efficient operation and prevents undue heating of the stabilizer electrode clue to electron discharge current. Light signal coupling is employed between an anode current monitor in the high voltage transformer circuit and the input of the first stabilizer means to produce the stabilizing voltage signal without the need for high voltage insulation. A similar light coupling is employed between the output of the first stabilization means and the input of a differential amplifier in the second stabilization means in order to apply a control signal to the second means which is proportional to the stabilizing voltage signal of the first means.
It is, therefore, one subject of the present invention to provide an improved stabilization circuit of fast re spouse for maintaining the anode current of an x-ray tube or other electron discharge device substantially constant.
Another object of the inventin is to provide such a stabilization circuit, including a first stabilization means, in which a stabilizing voltage signal proportionalto changes in the anode current is applied to a stabilizer electrode within the x-ray tube in order to collect a portion of the emitted electrons and reduce the electron discharge current reaching the anode.
A further object of the invention is to provide such a stabilization circuit of more efficient operation in which a second stabilization means of slower response is employed which gradually takes over stabilization from the first stabilization means by adjusting the cathode filament heating current in order to maintain the anode current substantially constant in accordance with a control signal proportional to the stabilizing voltage of the first stabilization means.
An additional object of the invention is to provide such a stabilization circuit in which light signal coupling is employed in order to provide high voltage isolation and fast response.
An additional object is to provide such a stabilization circuit suitable for pulsed x-ray apparatus using x-ray tubes having filament cathodes made of thoriated tungsten in order to quickly stabilize the cathode to anode discharge current of such tubes.
BRIEF DESCRIPTION OF DRAWINGS DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIG. 1, one embodiment of the stabilization circuit of the present invention includes an x-ray tube ll) or other electron discharge device, having an anode l2 and a thermionic cathode 14 which ordinarily is a filament cathode of thoriated tungsten but could be an indirectly heated cathode having a separate heating filament. A stabilizer electrode 16 is provided within the evacuated tube envelope outside the focusing field of the cathode and preferably in a position behind the cathode 14 with respect to the anode 12. This position of the stabilizer electrode is necessary to prevent the stabilizing signal voltages applied thereto from defocusing the electron beam and changing the focal spot of such beam on the anode 12. Thus, the source of the xrays provided by such focal spot does not vary in size with changes in stabilizing voltage. In addition, a control electrode 18 is provided between the cathode 14 and anode 12 to enable the x-ray tube to be pulsed on and off. The control electrode 18 is shown as a grid, but which is actually a cathode cup type focusing electrode as shown in copending U. S. Pat. application Ser. No. 229,136, filed Feb. 24, 19-72, by Zed J. Atlee entitled: X-RAY TUBE WITH IMPROVED CONTROL ELECTRODE ARRANGEMENT. This control electrode is quiescently biased to cut off the cathode 14 and is momentarily pulsed to a more positive voltage to allow electron discharge current to flow between the cathode l4 and the anode 12. This enables the cathode filament 14 to be heated continuously even while the tube is cut off and thereby provides the tube with a faster emission response when it is pulsed on, which is extremely useful for cineradiography requiring rapid pulses of high current. In addition, a second cathode filament 20 may be provided in the tube to provide a continuous electron discharge of low current during fluoroscopy so that the patient is exposed to low x-ray radiation levels.
A high voltage transformer 22 is connected at it positive output of about +50 kilovolts to the anode l2 and has its negative output of about-O kilovolts connected through a control electrode power supply circuit 24 to a common lead 26 of both cathode filaments 14 and 20. Another output of the control electrode power supply 24 is connected through lead 28 to the control electrode 18 to apply positive voltage pulses of about 53.5 kilovolts quiescent value and 50.0 kilovolts peak value to the control electrode in order to switch the x-ray tube momentarily on from its normal off state. These pulses-are produced by an exposure pulser circuit 30 which produces a light pulse 32 that is detected by a photoelectric detector in the power supply 24. Thus, the durationof the light pulse 32 determines the width of the voltage pulses produced on output 28.
The stabilizer electrode 16 is connected through a lead 34 to the output of a stabilizer electrode power supply and amplifier 36 whose input is a light signal 38 produced by an anode current monitor 40. The anode current monitor 40 is connected to the high voltage transformer 26 so it detects changes in the electron discharge current flowing from one of the cathodes l4 and to the anode 12 of the x-ray tube and produces a corresponding light signal output 38 whose light intensity corresponds to the magnitude of the anode current variations. This light signal 38 is transmitted to a photoelectric detector in the stabilizer electrode power supply 36 which produces a corresponding stabilizing voltage at output 34. The stabilizing voltage is proportional to the intensity of the light signal 38 and is of a polarity to oppose the change in anode current detected by monitor circuit 40. Thus, when the current in the anode 12 decreases, a negative going stabilizing voltage signal is applied to the stabilizer electrode 16 so that fewer emitted electrons are collected by the stabilizer electrode and more emitted electrons are attracted to the anode 12, thereby increasing the anode current to its desired value. Conversely when the anode current sensed by monitor 40 increases a positive going stabilizing voltage is produced on lead 34 which causes more of the emitted electrons to be attracted to the stabilizer electrode 16. This reduces the number of emitted electrons transmitted to anode 12 and thereby decreases the anode current to its desired value. Thus, the stabilizer electrode 16, the power supply and amplifier 36, and the anode current monitor 40 provide a first stabilization means which acts in the manner of the negative feedback to maintain the anode current of the x-ray tube substantially constant. This first stabilization means has the advantage that it has a substantially instantaneous response.
In addition to the first stabilization means, a second stabilization means of slower response which gradually takes over from the first means, may also be employed for more efficient operation. The second stabilization means includes a differential amplifier 42 connected as a comparator means having one input 43 connected to a DC. reference voltage on the movable contact of a potentiometer 44 whose end terminals are connected, respectively, to positive and negative D.C. supply voltages. The other input of the differential amplifier is a light signal 46 transmitted from the output of the stabilizer electrode power supply 36 and having a light intensity corresponding to the value of the stabilizer voltage produced on output 34 of such power supply. The light signal 46 is transmitted to a photoelectric detector in the differential amplifier 40 which produces a corresponding input voltage signal that is compared with the reference voltage on input 43 and produces a control signal across the output terminals 48 of the differential amplifier. The output 48 of differential amplifier 42 is applied to the input of a cathode current stabilizer circuit 50 whose output is connected to a filament heater transformer 52 in order tovary the heating current transmitted to the cathodes l4 and 20 of the x-ray tube. Thus, the cathode current stabilizer circuit 50 may be of the type shown in the U. S. Pat. No. 2,617,045 of Coe or in U. S. Pat. No. 2,810,838 of Clapp et al. which changes the impedance in series with the primary winding of the heater transformer in order to vary the heater current in the secondary winding of such transformer transmitted to the cathode filaments 14 and 20.
This second stabilization means 42, 50 and 52 is slower operating than the first means, and also acts to maintain the anode current of the ,x-ray tube substantially constant and gradually takes over stabilization from such first means. Thus, when the anode current increases greater than the desired value, a positive going stabilizing voltage is produced at the output 34 of the stabilizer electrode power supply 36 and a corresponding light signal 46 is transmitted to the differential amplifier 42 which compares it with the reference voltage 43 and produces a control signal at the output 48 of such amplifier. This control signal decreases the heater current flowing in filament 14 to lower the temperature of the filament and reduces the electron emission of such filament. As a result, the anode current decreases and the light signal 38 of monitor 40 reduces in intensity which in turn lowers the stabilizing voltage 34 and lowers the intensity of light signal 46. This continues until, the anode current of the x-ray tube is stabilized at the desired value at which time the control signal output 48 of the differential amplifier 40 reduces to zero.
It should be noted that a selector switch 54 may be connected in series with an AC. line voltage source 56 for selectively. applying such line voltage to the primary winding of the heater transformer of either the high current filament 14 of the low current filament 20 for selective energization thereof. In addition, it is possible to use the switch 54 to selectively energize the filament merely by shorting a high series impedance to increase the heating current therethrough to an emission level, while maintaining the heating currents in both filaments at a lower standby current level in order to increase the speed of response when switching between the filaments.
The stabilizer apparatus of FIG. 1 may employ the electrical circuit of FIG. 2 which is hereafter described, using the same reference numerals to indicate the dashed line boxes in FIG. 2 which corresponds to the blocks of FIG. 1 for easier understanding of the circuit. Thus, the high voltage transformer 22 is a three phase transformer including three secondary coils 58 which are connected through diodes 60 to a common anode voltage lead 62 for applying a positive voltage of about +50 kilovolts to the anode 12 of the x-ray tube 10. A current limiter diode tube 64 is connected in series between the output terminal 62 of the high voltage trans-' former and the anode 12 of the x-ray tube and a filter including a resistor 66 in parallel with a capacitor 68 is connected from the anode of such limiter tube to ground. Three other secondary coils 70 on the high voltage transformer are connected through other rectifier diodes 72 to a common cathode voltage lead 74 to supply a negative voltage of about 50 kilovolts to the common lead 26 at cathodes 14 and 20 of the x-ray tube through the controlelectrode power supply 24. Another current limiter diode tube 76 is connected in series with the output 74 of the high voltage transformer. A filter including a resistor 75 and a parallel capacitor 77 are connected between the cathode of diode 76 and the ground. The anode of diode 76 is connected to a common point 78 in the control electrode power supply to apply a negative voltage of about 50 kilovolts thereto. This 50 kilovolts serves a reference voltage for the control electrode 18, the cathodes 14 and and the stabilizer electrode 16, and is transmitted through a zenner diode 80 to the secondary windings of a control electrode power supply transformer 82.
The control electrode power supply 24 includes a first secondary winding 84 on transofrmer 82 which is connected across a full wave rectifier bridge 86 to produce a positive D.C. voltage of about +l,750 volts on the right side of the bridge rectifier which is applied to the upper terminal of a resistor 88 and produces a negative D.C. voltage of about 1 ,750 volts on the left side of the bridge which is applied to the lower terminal of resistor 88. A capacitor 90 is connected in parallel with resistor 88 to provide a ripple filter and the upper terminal of such filter is connected through a current limiter resistor 92 to the common terminal 26 of the cathode filaments l4 and 20 of the x-ray tube. The control electrode 18 is connected by lead 28 through a pair of series resistors 94 and 96 to the common connection 78 so that a negative voltage of about -50 kilovolts is applied to such control electrode. A quiescent voltage of about 47 kilovolts is applied to the cathode 14 which is the sum of the +3 kilovolts produced across bridge 86 and the 50 kilovolts applied to such bridge through a lead 98. Thus, the x-ray tube is quiescently reversed biased approximately 3,000 volts to cut off such tube.
A pair of vacuum tubes 100 and 102 which may be of the pentode type are connected in parallel with their cathodes connected in common to the lead 98 and their anodes connected in common to the current limiter resistor 92. The control grids of tubes 100 and 102 are connected through coupling resistors 104 and 106, respectively, to a source of positive voltage switching pulses which momentarily render such tubes conducting from their quiescent nonconducting state. This effectively short circuits the control electrode power supply so that the entire 3,000 volts is dissipated in current limiter resistor 92. As a result, the cathode voltage on lead 26 momentarily reduces to 50kilovolts so that it equals the control electrode voltage on lead 28, thereby removing the reverse bias on the x-ray tube and rendering such tube conducting. As a result, electrons emitted from the cathode 14 are caused to bombard the anode 12 to produce an x-ray pulse, which may be employed in cineradiography. to record an x-ray image on film. Thus, at this time, the control electrode 18 and the cathode 14 of the x-ray tube are maintained at the same potential by a reference resistor 108 and diode 110 connected in series between leads 26 and 28.
An overvoltage protection resistor 112 in series with a zener diode 114 are connected across the vacuum tubes 100 and 102 to prevent damage to such tubes when they are in a nonconducting state. Thus, when overvoltage occurs, the zener diode 114 conducts and transmits current through resistor 112 to ground through another resistor 116 connected thereto. An A.C. coupling capacitor 118 is connected between the common lead 98 and the junction of resistors 94 and 96 to handle transient voltages. The heater voltage applied to the filaments of the vacuum tubes and 102 and diode 76 is supplied by another secondary winding 120 on the transformer 82 which is referenced to the common lead 98 and is connected to such filaments by lead 121. In addition, the middle grids of tubes 100 and 102 are connected through resistors 1122 and 124. to the common point 78, with the upper grids connected to the cathodes of such tubes.
The exposure pulser circuit 30 includes a first NPN type switching transistor 126 whose base is connected through a coupling resistor 128 to an input terminal 130 to which is applied a positive rectangular pulse having a width corresponding to the desired x-ray exposure time. This pulse switches transistor 126 conducting from a quiescent nonconducting state to produce a negative pulse on its collector. The collector of transistor 126 is connected through a load resistor 132 to a source of D.C. supply voltage of about +20 volts, while the emitter of such transistor is connected directly to ground and its base is connected to ground through a bias resistor 134. The negative going output pulse on the collector of transistor 126 is applied to the base of a second NPN switching transistor to cause it to switch to a nonconducting state from its quiescent conducting state. The collector of transistor 136 is connected through a load resistor 138 to the +20 volts D.C. supply voltage and has its emitter grounded. When transistor 136 switches off it produces a positive going pulse on its collector which is applied to the base of a third NPN switching transistor 140 to switch it to a conducting state from its normally nonconducting state. The emitter of transistor 140 is grounded, while its collector is connected through a light emitting diode 142 and a load resistor 144 to the volts D.C. supply. When transistor 140 is rendered conducting a current pulse is transmitted through the light emitting diode 142 causing such diode to emit a light pulse 32 ofa duration corresponding to the width of the x-ray exposure pulse ap plied to the input terminal 130 of the pulser 30.
This light pulse 32 is transmitted to a light sensitive transistor 146 connected as a diode with its base shorted to its emitter, in the control electrode power supply 24. The light pulse renders the transistor 146 conducting so that it effectively short circuits a voltage divider resistor 148 connected in parallel between the emitter and collector of such transistor. This causes more current to flow through a second voltage divider resistor 150 connected between the base and emitter of a normally nonconducting NPN transistor 152 to switch such transistor to a'conducting state. The common terminal of the emitter of transistor 152 and the voltage divider resistor 150 is connected to the common lead 98 so that a potential of 50 kilovolts is applied thereto. For this reason, the light coupling 32 is employed to electrically insulate the low-voltage pulser 38 from the high voltage of the control electrode power supply 24. The collector of transistor 152 is connected through a load resistor 154 to a +50 volt D.C. supply voltage having an actual value of about -49,95O volts produced at the output of a rectifier bridge 156 having its input connected across a third secondary winding 158 on transformer 82. Thus, the other output of the bridge is reference to -50 kilovolts on the common lead 98 which is added to the +50 volts produced across the bridge. When transistor 152 is switched conducting, it produces a negative going pulse which is transmitted through a coupling resistor 160 to the base of an NPN output transistor 162 in order to switch such output transistor to a nonconducting state from its normally quiescent conducting state. The output transistor 162 has its emitter connected to lead 98 and has it collector connected through a load resistor 164 to the +50 volts supply voltage provided at the output of bridge 156. It. should be noted that a resistor 166 in parallel with the capacitor 168 are connected across the output of the bridge 156 to provide a ripple filter which smooths the 50 volts D.C. supply voltage The positive going exposure control pulses produced on the collector of the output transistor 162 are transmitted through a protection diode 170 to the grids of the tubes 100 and 102 in order to switch such tubes into a momentarily conducting state. In this manner, the x-ray tube is provided with a pulsed operation since the reverse bias between the control electrode 18 and the cathode 14 is momentarily removed to enable electrons to flow from such cathode to the anode 12, and thereby produce an x-ray pulse.
As stated previously with regard to FIG. 1, in order to maintain the cathode to anode discharge current of the x-ray tube substantially constant during rapid exposure times, a stabilizing voltage is transmitted from the output of the stabilizing electrode power supply and amplifier 36 through lead 34 to the stabilizer electrode 16. This causes the stabilizer electrode to collect a portion of the electrons emitted by the cathode 14 and thereby control the amount of discharge current reach ing the anode 14. The stabilizer electrode power supply 36 includes a secondary winding 172 which may be provided on the transformer 82 and a bridge rectifier 174 connected across such winding to produce a rectified D.C. voltage of about 50 volts at the output terminals of such bridge. A ripple filter, including a capacitor 176 in parallel with resistor 178, is connected across the bridge. The lower terminal of the rectifier bridge 174 is connected to the common lead 98 so that it is also referenced to 50 kilovolts. The light signal 38 produced by the monitor 40 is received by a photosensitive transistor 180 of NPN type connected as a diode in the stabilizer electrode power supply 36. The emitter and collector of the phototransistor are connected in parallel with a voltage divider resistor 182. The voltage divider resistor 182 is connected through a current limiting resistor 184 to the upper terminal of bridge 94 and is connected in series with another voltage divider resistor 186 which also acts as the base bias resistor of an NPN switching transistor 188. Thus, when a light signal 38 is received, the emitter to collector impedance of transistor 180 reduces so that the current through the bias resistor 186 increased and produces a positive going signal on the base of transistor 188 which acts as a first inverter amplifier. The collector of transistor 188 is connected through a load resistor 190 to the positive D.C. supply voltage, while its emitter is connected to the 50 kilovolts D.C. reference voltage on lead 98. Thus, transistor 188 inverts the positive signal and transmits a negative signal from its collector whose voltage amplitude depends on the intensity of the light signal 38.
The negative going signal produced on the collector of transistor 188 is transmitted through a peaking circuit including a coupling resistor 192 in parallel with a capacitor 194, to the base of another NPN transistor 196 connected as a second inverter amplifier. The base of transistor 196 is connected to the negative D.C. supply voltage through a base bias resistor 198 while its emitter is directly connected thereto and its collector is connected to the positive D.C. supply voltage through a load resistor 200. The negative going signal is amplified and inverted to produce a positive going signal on the collector of transistor 196 which is then transmitted through another peaking circuit including a coupling resistor 202 in parallel with a capacitor 204. These peaking circuits increase the slope of the leading edge of the signal to provide it with a faster rise time. The positive going signal is transmitted to the base of a third inverter amplifier transistor 205 which produces a negative signal on its collector. The base of transistor 205 is connected to the negative D.C. power supply through a bias resistor 206 while its emitter is directly connected thereto, and its collector is connected to the positive D.C. power supply through a load resistor 207. A fourth inverter amplifier transistor 208 having its base connected to the collector of transistor 205 inverts the negative signal on its base to produce a positive output signal on its collector. This transistor 208 is also provided with a base bias resistor 209 and a collector load resistor 210.
The positive going output signal of transistor 208 provides the stabilizing voltage produced at the output of the stabilizer electrode power supply 36, and is transmitted through a light emitting diode 212 and an overvoltage protection diode to the stabilizer electrode 16. This positive stabilizing voltage causes a portion of the electrons emitted by the cathode 14 to be collected by the stabilizer electode which reduces the discharge current flowing in the anode 12 of the x-ray tube, and thereby tends to maintain such anode current substantially constant. The stabilizing voltage produced on the lead 34 varies between zero and +50 volts depending upon the brightness of the light signal 38 emitted by a light emitting diode 216 in the monitor 40. The light emitting diode 216 has its cathode grounded and its anode connected through a selector switch 218 in series with one of a plurality of resistors 220 of different value which are selectively connected in parallel with a limiter resistor 222. The common terminal of the resistors 220 and resistor 222 is connected at a common terminal 224 to both the cathode and anode windings 70 and 58 of the high voltage transformer 22. Thus, the sum of the anode and cathode current of the x-ray tube is transmitted to ground through the light emitting diode 216 and a shunt resistor 226 connected in parallel therewith so that the diode 216 emits light only when the total current exceeds a selected value of, for example, about 50 milliamperes. Thus, the light signal 38 emitted by diode 216 has a brightness which is proportional to the anode current flowing in the x-ray tube 10. 1
The differential amplifier 42 of the second stabilizer means includes a phtosensitive transistor 228 of NPN type which is connected as a diode at one input of such differential amplifier. The phototransistor 228 is positioned to receive the light signal 46 transmitted from the light emitting diode 212 of the stabilizer electrode power supply and amplifier 36. The other input of the differential amplifier is a DC. reference voltage on the movable contact 43 of potentiometer 44 which is connected to the base of another NPN transistor 230 connected as an inverter amplifier on one side of the differential amplifier. A second inverter amplifier transistor 232, similar to transistor 230, is provided on the other side of the differential amplifier and has its base connected to the emitter of phototransistor 228. A bias resistor 234 is connected between the emitter of transistor 232 and its base. When light signal 46 is produced, the emitter to collector impedance of phototransistor 222 reduces, so that the current through bias resistor 234 increases, thereby producing a positive voltage on the base of transistor 232. This produces a differential output signal on output conductors 48 of the differential amplifier when such positive voltage exceeds the reference voltage applied to the base of transistor 230 in the manner of a conventional differential amplifier.
Transistors 230 and 232 are connected through emitter biase resistors 236 and 238 in common to a source of negative D. C. supply voltage provided at the lower terminal of a diode bridge rectifier 240 whose input terminals are connected across a secondary winding 242 on transformer 82. The positive output terminal of the bridge 240 is connected through load resistors 244 and 246, respectively, to the collectors of transistors 230 and 232 via additional load resistors 248 and 252. Load resistor 248 may be variable in order to balance the output voltages of the differential amplifier so that it produces no output signal unless a light signal is received by phototransistor 228. The collectors of transistors 230 and 232, respectively, are connected to the bases of second inverter amplifier transistors 252 and 254. Transistors 252 and 254 have their emitters connected through zener diodes 256 and 258 to the positive D.C. supply and have the collectors connected through load resistors 260 and 262 to the negative D.C. supply voltage since these are PNP transistors. A pair of third inverter amplifier transistors 264 and 266 are provided with their bases connected to the collectors of transistors 252 and 254, respectively, and their collectors connected to the output terminals 48 of the differential amplifier at load resistors 244 and 246.
The output signal of the differential amplifier 42 produced across leads 48 upon receipt of the light signal 46 is applied to the cathode current stabilizer 50 which varies an impedance in series with the common lead of a pair of primary windings 268 and 270 of the filament heater transformer 52 in a similar manner to that shown in U. S. Pat. No. 2,810,838 of Clapp et al., or in U. S. Pat. No. 2,617,045 of Coe. The secondary windings 272 and 274 of the transformer 52 are connected, respectively, to the cathode filaments l4 and 20 of the x-ray tube at terminals 276 and 278, respectively. Thus, in the shown position of switch 54, the stabilizer 50 changes the heating current offilament 14 in order to vary the electron emission of such filament and thereby maintain the electron discharge current in anode 12 substantially constant. As stated above, this second stabilizer means 42, 50 and 52 is slower than the first stabilizer means 16, 36, and 40 and gradually takes over stabilization from such first means. This provides a more efficient operation since the second stabilizer means reduces the total electron current emitted from the cathode 14 while the first stabilizer means does not, and thereby reduces power dissipation and heating of the stabilizer electrode 16.
It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiment of the present invention. For example, the two filaments 14 and 20 of the x-ray tube can both be operated without any pulsed control electrode or a second control electrode may be provided for the filament 20' in order to selectively bombard the anode with one of such filaments. In addition, for dual filament stabilization, a second stabilizer electrode can be employed behind filament 20 which is connected like stabilizer electrode 16. Also, it should be noted that since circuits 22, 24', 36 and 52 operate at a high voltage between about 50 kilovolts and ISO kilovolts, they may be provided in an oil filled tank for insulation purposes. Therefore, the scope of the present invention should only be determined by the following claims.
1. An electron discharge current stabilization apparatus comprising;
an electron discharge tube having an anode, at least one thermionic cathode and a stabilizer electrode; power supply means for applying high voltage between said anode and cathode, and for applying a heating current to said cathode to cause electrons to be emitted from said cathode and to bombard said anode as an electron discharge current; and stabilization means including a first meats separate from said stabilizer electrode for detecting the amount of the electron discharge current flowing in said anode, said first means applying a stabilizing signal voltage to said stabilzer electrode to cause a portion of said electrons to be diverted from said, anode to said stabilizer electrode depending upon the value of said stabilizing voltage, and said first means varying said stabilizing voltage in accordance with any changes in the anode current in a manner which maintains said anode current substantially constant. 2. A stabilization apparatus in accordance with claim 1 in which the stabilization means also includes a second means for detecting the amount of electron current flowing in said stabilizer electrode and producing a control signal corresponding thereto, and control means for varying the cathode heating current to change the electron emission of said cathode in accordance with said control signal in a manner which maintains said anode current substantially constant.
3. A stabilization apparatus in accordance with claim 2 in which said second means is coupled to said first means by light coupling means.
4. A stabilization apparatus in accordance with claim 3 in which said light coupling means includes an electrical light source in said first means which emits a light signal corresponding to said control signal, and a photoelectric detector in said second means for converting said light signal into said control signal.
5. A stabilization apparatus in accordance with claim 4 in which the second means includes a comparator means having one input connected to said photoelectric detector and another input connected to a DC. reference voltage, and having its output connected to said control means.
6. A stabilization apparatus in accordance with claim 5 in which the tube is an x-ray tube, the power supply includes a filament transformer having its secondary winding connected to a filament cathode in said'x-ray tube and having its primary winding connected to said control means.
7. An x-ray apparatus comprising: an x-ray tube having an anode, at least one thermionic cathode and a stabilizer electrode positioned behind the electron emitter of said cathode with respect to said anode; power supply means for applying high voltage between said anode and cathode, and for applying a heating current to said cathode to cause electrons to be emitted from said cathode and to bombard said anode as an electron discharge current; and
stabilization means including a first means separate from said stabilizer electrode for detecting the amount of the electron discharge current flowing in said anode, said first means applying a stabilizing signal voltage to said stabilizer electrode to cause a portion of said electrons to be diverted from said anode to said stabilizer electrode depending upon the value of said stabilizing voltage and said first means varying said stabilizing voltage in accordance with any changes in the anode current in a manner which maintains said anode current substantially constant.
8. An x-ray apparatus in accordance with claim 7 in which the x-ray tube includes a focusing cup type control electrode separate from said stabilizer electrode and connected to a pulser means for applying pulses to said control electrode of sufficient voltage to change the bias on the x-ray tube to a conducting state from a quiescent nonconducting state.
9. An x-ray apparatus in accordance with claim 7 in which the first means is coupled to a high voltage transformer of the power supply means by a light coupling means for transmitting a light signal corresponding to said stabilizing signal from said power supply means to said first means of said stabilization means.
10. An x-ray apparatus in accordance with claim 7 in which the stabilization means also includes a second means for detecting the amount of current flowing in the stabilizer electrode and for varying the cathode heating current of the x-ray tube to maintain its anode current substantially constant, said first means operating faster than said second -means, and said second means being connected so that it gradually takes over stabilization of the anode current and reduces the stabilizing signal voltage so as to prevent electron heating of the stabilizer electrode.
Patent No. $7 3, Dated January 1, 1974 Inventor) George L. Fulton; Robert M. Gager; and Zed J. Atlee It is certified Lhat error appears in the above-identified patent add that said Letters Patent are hereby corrected as shown below:
Page 1, the title should be corrected to read as follows:
ANODE CURRENT STABILIZATION CIRCUIT FOR X-RAY TUBE HAVING STABILI ZER ELECTRODE Column 1, the title should be corrected to read as follows:
ANODE CURRENT STABILIZATION CIRCUIT FOR X-RAY TUBE HAVING STABILIZER ELECTRODE I Column 5, line S2,"transofrmer" shouldbec --tra n sformer--;
Column 8, line 22, "increased" should be "increases- Column 10, claim 1', line 60, "meats" should be -rmeans--.
oigned and sealed this 16th day of April 19m.
EDWARD 1=I.FLETCHER,JR. I I I c, 'I IARSEMLL DANN I Attesting; Officer I Commissioner of Patents r I I m J "ORM Inc-1050410039)
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|U.S. Classification||378/112, 378/138|
|International Classification||H05G1/00, H02M7/06, H05G1/34|