US2319378A

US2319378A - Stabilizer system

Info

Publication number: US2319378A
Application number: US414307A
Authority: US
Inventors: Louis L Weisglass
Original assignee: Westinghouse Electric and Manufacturing Co
Current assignee: CBS Corp
Priority date: 1941-10-09
Filing date: 1941-10-09
Publication date: 1943-05-18
Anticipated expiration: 1960-05-18
Also published as: GB636357A

Description

May 18, 1943 L. L. WEISGLASS STABILIZER SYST'EM Filed Oct. 9, 1941 INIVENTIOR 400/64Wf/561M55 ATTORNEY Patented May 18, 1943 UNITED STATES PATENT OFFICE STABELIZER SYSTEM Fennsylvania Appiication October 9, 1941, Serial No. 414,307

6 Claims.

The present invention relates to X-ray systems and has particular reference to such a system wherein the discharge current flowing through the tube is stabilized, i. e., maintained substantially constant.

In X-ray systems when it is desired to energize the X-ray tube with relatively high milliamperage current, the current utilized to heat the thermionic cathode of the X-ray tube or rectifying valve tubes is comparatively high and must be maintained constant. As the low-tension transformer for heating the cathode and the high tension transformer for supplying the discharge current are usually connected to the same domestic source of supply, there are many factors which aifect the discharge current.

For example, the commercial source of supply is subject to line voltage fluctuations caused by the application of periodic loads thereto in the immediate vicinity. Also, the cathode heating current is directly affected by connection of the load of the high tension transformer to the source due to the resulting voltage drop.

This problem has long been recognized in the art and numerous circuit arrangements have been employed in an efiort to stabilize the operation of the X-ray tube. In most instances these systems result merely in maintaining a constant temperature of the thermionic cathode by causing an inverse variation in the cathode heating current in response to line voltage fluctuations. There are, however, several other factors tending to cause a variation in the discharge current of the X-ray tube; and since this latter together with time, and expressed as milliampere-seconds,

determines the quality of the resulting radiograph, it is more essential that the milliamperage flowing through the discharge be maintained constant rather than the cathode heating current in order to obtain optimum stabilization.

It is accordingly an object of the present invention to provide an X-ray system wherein the milliamperage flowing through the X-ray tube is maintained substantially constant.

Another object of the present invention is the provision of an X-ray system for energizing an X-ray tube wherein the discharge current is maintained constant regardless of the time period of energization of the X-ray tube.

Another object of the present invention is the provision of an X-ray system wherein any tendency for the discharge current to vary automatically causes a variation in the cathode heating current, thereby maintaining the milliamperage of the discharge current substantially constant.

A further object of the present invention is the provision of an X-ray system wherein current flow through an impedance device included in the excitation circuit causes the automatic break down of a gaseous discharge tube, which in turn renders an auxiliary circuit operative to cause a variation in the cathode heating current inversely to the variation in the discharge current, thereby maintaining the latter substantially constant regardless of the factor tending to cause variation of the latter.

Still further objects of the present invention will become obvious to those skilled in the art by reference to the accompanying drawing wherein:

Fig. 1 is a diagrammatic illustration of the stabilizer system in accordance with the present invention, and

Fig. 2 is a diagrammatic illustration of a modification which an X-ray stabilizing system may take in accordance with the present invention.

Fig. 3 is a graphic illustration of the voltage and current wave form through certain of the elements of the stabilizer circuit.

Referring now to the drawing in detail, there is shown in Fig. 1 a source of commercial alternating current potential Ill-L2. A high potential transformer 5 has its primary winding 6 connected by means of conductors I and 8 to the source of supply LIL2. As shown, the transformer 5 is of the split type having two secondary windings 9 and i0 adapted through an excitation circuit to supply unidirectional current to an X-ray tube l2. This excitation circuit extends from either end of the secondary winding 9 (depending upon which end is instantaneously positive), through either rectifying valve tube [3 or 14, again depending upon the instantaneous polarity of the current, through a conductor 1 5 to the X-ray tube l2, thence through a conductor it, one or the other of a second pair of valve tubes H or l8, to either end of the secondary winding i0 and from the center point of this winding through a conductor 19 (a connection terminal D for a purpose hereinafter described more in detail) to an adjustable resistance 2!] and back through a conductor 22 (connection terminal E similar to D), and milliampere meter 23 to the center point of the secondary winding 9, thus completing the excitation circuit which is grounded at 24.

It will thus be seen that the X-ray tube is energized by full wave rectified alternating current with one-half of each of the secondary windings 9 and I0 supplying the energy during each half wave of the alternating current cycle, the particular portion of each winding depending upon the polarity of the ends of the windings during each half-wave, as is customary in the art. The thermionic cathode of the X-ra tube i2 receives heating current from a low tension transformer 25, the low voltage secondary Winding 26 of which is connected directly to the cathode. The primary winding 21 of this low tension transformer is connected to the source of commercial potential Li-LZ in series with a pair of parallel-connected oppositely wound winding 28 and 29 disposed on the outer legs of a saturable reactor 39. From the stabilizer system thus far described it will be obvious that th heating current for the thermionic cathode of the X-ray tube 12 passes through the windings 28 and 29 of the saturable reactor 39, and inasmuch as the variable resistance 28 is connected in the grounded center of the high tension transformer and thus in series with the X-ray tube excitation circuit, the discharge current for the tube will flow through this resistance and through the milliampere meter 23 always in one direction (making connection terminal D negative polarity and that of E positive).

When the X-ray tube 12 is energized, and depending upon the milliamperage flowing therethrough, a voltage of a determinable value will be built up across the variable resistance 29 which will have a voltage curve as shown in the upper portion of Fig. 3. A gaseous discharg device 32 having a definite breakdown voltage is connected through a pair of

resistors

33 and 34 in parallel with the variable resistance 29, and the latter is adjusted to this value and once set, need not be further disturbed. In addition, a smoothing condenser 35 may be shunted across the variable resistance 29, if desired. Upon the voltage across the variable resistance 29 rising to the breakdown potential of the gaseous discharge device 32, the latter becomes conducting, thereby causing current flow through the

resistors

33 and 34 in the manner shown by the current curve in the lower half of Fig. 3. A dual grid amplifier tube 39 has its indirectly heated cathode 31 connected to one end of the resistor 34 and the other end to one of the grid electrodes 38, with the connections being such that when the device 32 breaks down, the ends of the resistor 34 assume different potentials; and as the voltage across this resistor 34 increases, the grid 38 becomes increasingly negative with respect to the indirectly heated cathode 37.

This dual grid amplifier tube is arranged to control direct current flow through a winding 39 carried by the center leg of the saturable reactor 39. Although any suitable source of direct current, such as batteries or a generator, will operate satisfactorily, a rectified system is preferable. Accordingly, to generate this D. C. current a transformer 49 is shown, the variable step primary winding 42 of which may be connected to the same commercial source of supply Ll--L2 as that of the low-tension transformer 25 and high tension transformer 5, with a condenser 43 connected in series therewith so asto maintain the primary potential substantially constant. The secondary winding 44 of this transformer 41] has its center point connected to one plate of a condenser 45 as well as to the indirectly heated cathode 3'! of the dual grid amp ifi respective anodes 46 and 4! of a full wave rec- 32, while its endsare connected to the tifier tube 48, having it thermionic cathode 49 heated by an auxiliary low-tension winding 50 forming a part of the transformer 40. This cathode 49 is connected to the remaining plate of the condenser 45 and to one end of the winding 39 of the saturable reactor 30, while the remaining end of this latter winding is connected to the anode or plate 52 of the amplifier tube 36. The cathode 49 of the rectifier tube 48 is also connected through an adjustabl limiting resistor 53 to the screen grid 54 of the amplifier tube 36 so as to apply a predetermined biasing potential thereto.

The transformer 4i! accordingly generates an alternating current which is converted into unidirectional current by the rectifier tube 48 for charging the condenser 4%. Discharge of this condenser 43 through the D. C. winding 39 of the saturable reactor at is directly controlled by the amplifier tube 39.

From the foregoing it will become obvious that if the milliamperage passing through the X-ray tube 52 exceeds a given value, and regardless of the factor to which such rise is attributable, a comparatively high voltage is built up across the resistance 29 of a definite polarity. This rise in voltage causes the parallel-connected gaseous discharge device to break down and become conducting, so that current flows through resistor 34. Current flow through the latter makes the grid 38 of the amplifier tube 36 negative with respect to the indirectly heated cathode 47, as hereinbefore explained, whereas prior to current flow through the resistor 34 the grid 38 had a zero potential. The application of a negative potential to this grid accordingly decreases the current passing through the amplifier tube 35, and consequently the D. C. winding of th saturable reactor 39.

Since the passing of a direct current through a winding of the reactor has a considerable controlling influence on alternating current flowing through the other windings, even when of a considerably higher order of magnitude, this decrease in D. C. current through. winding 39 in turn decreases the primary and consequently the secondary current passing through the filamerit transformer 25, and hence the cathode of the X-ray tube. Reduction in the cathode heating current thus causes the milliamperage passing through the X-ray tube to decrease, thereby maintaining the same substantially constant.

Upon a decrease in the milliamperage current through the X-ray tube below a given value, the stabilizer system operates in the opposite manner.v That is to say, when this current tends to decrease, the voltage across the resistance 20 decreases below the breakdown value of the gaseous discharge device 32. Current flow through resistor 34 is accordingly interrupted, resulting in zero potential being applied to the grid 38 of the amplifier tube 36, causing an attendant increase in the D. C. current flow therethrough and through the winding 39 of the saturable reactor. This increase'in D. C. current through the center leg of the saturable reactor 39 decreases the impedance of the alternating current circuit which includes the windings 28 and 29, thus increasingthe'currentsupplied to the primary winding 21 of the low-tension transformer 25,'and consequentlyincreasing the current supplied by the'secondarywinding Z6 to'the cathode of the'X-ray tube I2. Thus upon a tendency of the milliar'npera'ge current in the X-ray tube excitation circuit 'to increase or decrease, regardless oi the factor causing it, the cathode heating current is inversely varied so that the effect is to maintain the milliamperage substantially constant during the entire energization of the X-ray tube.

It should perhaps also be noted that although a gradual variation of the D. C. voltage through the Winding of a saturable reactor has very little effect on the A. C. component, in that a change in the magnetic fiux does not occur very rapidly, this does not hold true when there is an abrupt change in the D. C. current flow. Accordingly, since the gaseous discharge tube controlling the D. C. current flow breaks down cyclically, its operation is exceedingly abrupt, thus causing periodic impulses of D. 0. current flow through the saturable reactor and a very marked and sudden change to the A. C. component, resulting in an almost instantaneous variation in the flow of alternating current to the cathode of the X-ray tube, thereby maintaining the discharge current substantially constant, as above mentioned.

The modification oi the system as shown in Fig. 2, in so far as the stabilizer system is concerned, diifers in no way from that shown in Fig. l. The difference resides merely in the fact that the X-ray tube excitation circuit is energized from a high tension transformer b which employs a single secondary winding 62, rather than its being of the split type shown in Fig. 1. Since the stabilizer circuit portion is identical to that shown in Fig. 1, it is believed unnecessary that it be again illustrated. However, for the sake of clarity, a series of connections previously herein referred to, namely, A, B, C, D, and E, are shown in both figures and the connection of the stabilizer circuit portion of the system, as shown in Fig. 1 below the broken line, will connect to these same connections in the case of the X-ray tube excitation circuit illustrated in Fig. 2.

There is, however, one additional difference in the excitation circuit as shown in Fig. 2 in that the grounded center tap of the high tension secondary winding 52 is connected to a full wave rectifier connection, shown generally at 53, which is similar to that shown for the X-ray tube [2, except that since the rectifier tubes 5-4 are connected in the grounded leg of the transformer, they do not have to be insulated for as high a voltage as the valve tubes l3, l6 and i7, 3. The variable resistance 26 as well as the milliampere meter 23 are connected to the unidirectional output side of the rectifier arrangement 63 so that substantially D. C. current flows through the meter 23 and a positive and negative potential is applied across the variable resistance 28 in the same manner and for the same purpose as previously described with respect to Fig. 1.

From the foregoing it should be obvious to one skilled in the art that by proper selection of the circuit constants the gaseous discharge device 32 will break down once for every half cycle, and the average milliamperage passing through the X-ray tube 12 will be kept constant with a high degree of precision, regardless of the factors tending to cause a variation of the discharge current. Moreover, this high degree of precision is obtained by causing an inverse variation in the cathode heating current to that of the discharge current which, occurring cyclically during each half wave, necessarily maintains the discharge current substantially constant during the entire period of energization of the X-ray tube.

It should also be understood that while one modification of the stabilizer system of the present invention has been herein shown and described, still further embodiments thereof may be made without departing from the spirit and scope of the appended claims.

I claim:

1. The combination of an electron discharge device provided with a cathode to be heated, an excitation circuit for said device including a high tension transformer having its secondary Winding connected through a rectifying arrangement to said electron discharge device for supplying unidirectional current thereto, a source of alternating current electrical energy for heating the cathode of said electron discharge device, inductive means associated with said source of heatin energy and operable upon the imposition of a unidirectional current component thereupon differing from that of the heating source to control the flow of heating current to the cathode of said electron discharge device, means connected to said excitation circuit and subject to variations in potential thereacross in response to variations in discharge current through said electron discharge device, and circuit means electrically associated with both said aforementioned means and operable upon a variation in the potential across said means connected to said excitation circuit to cause the imposition of a current component upon said inductive means differing from that of said heating source accompanied by a variation in the heating current supplied to the cathode of said electron discharge device inversely to variations in the discharge current flowing through the latter.

2. The combination of an electron discharge device provided with a cathode to be heated, an excitation circuit for said device including a high tension transformer having its secondary winding connected through a rectifying arrangement to said electron discharge device for supplying unidirectional current thereto, a source of alternating current electrical energy for heating the cathode of said electron discharge device, inductive means electrically connected with said source of heating energy and operable upon the imposition of a unidirectional current component thereupon to control the how of alternating heating current to the cathode of said electron discharge device, means connected to said excitation circuit cyclically subject to variations in potential thereacross during each half-wave of the alternating current cycle in response to variations in the unidirectional discharge current through sald electron discharge device, and circuit means electrically associated with both said aforementioned means and operable upon the cyclic variation in potential across said means connected to said excitation circuit to cause the imposition of a unidirectional current component upon said inductive means accompanied by a Variation in the heating current supplied to the cathode of said electron discharge device inversely to variations in the discharge current flowing therethrough.

3. The combination of an electron discharge device provided with a cathode to be heated, an excitation circuit for said device including a high tension transformer having its secondary winding connected through a rectifier arrangement to said electron discharge device for supplying unidirectional current thereto and an impedance device in said circuit and subject to a variation in potential thereacross upon a variation in the discharge current through said discharge device, means connected across said impedance and pe riodically operable upon a rise in potential across said impedance above a predetermined value to cause current flow through said means, a source of electrical energy for heating the cathode of said electron discharge device including a saturable reactor, means adapted to impress a unidirectional current upon said saturable reactor to control the flow of alternating current to the cathode of said electron discharge device, and control means electrically associated with said last-mentioned means and with said first-mentioned means and operable in response to current flow through the latter to cause operation of said last-mentioned means with an attendant inverse variation in the heating current supplied to the cathode of said electron discharge device in response to variations in discharge current flowing through the latter.

4. The combination of an electron discharge device provided with a cathode to be heated, an excitation circuit for said device including a high tension transformer having its secondary winding connected through a rectifier arrangement to said electron discharge device for supplying unidirectional current thereto and an impedance device in said circuit and subject to a variation in potential thereacross upon a variation in the discharge current through said discharge device, a gaseous discharge tube and a resistor in shunt relation across said irnpedance and operable upon the potential across said impedance rising to a predetermined value to cause breakdown of said gaseous discharge tube with attendant current 2' flow through said resistor, a sourcecf electrical energy for heating the cathode of said electron discharge device including a saturable reactor, means associated with said saturable reactor for impressing a unidirectional current upon the lat said impedance rising to a predetermined value to cause breakdown of said gaseous discharge .tube with attendant current flow through said resistor, a source of electrical energy for heating the oathode of said electron discharge device including a saturable reactor, a source of unidirectional current adapted to be supplied to said saturable reactor to control the flow of alternating current therethrough and to the cathode of said electron discharge device, and a control tube associated with said resistor and with said source of unidirectional current and operable upon the flow of current through said resistor to cause unidirectional current to flow from said source to said saturable reactor with an attendant inverse variation in the heating current supplied to the cathode of said electron discharge device in response to cyclic variations in the discharge current flowing through the latter.

6. The combination of an electron discharge device provided with a cathode to be heated, an excitation circuit for said device including a high tension transformer having its secondary winding connected through a rectifier arrangement to said electron discharge device for supplying unidirectional current thereto and an impedance operable upon the flow of current through said resistor to cause current flow from said means to said reactor with an attendant inverse variation in the heating current supplied to the cathode of said electron discharge device in response to variations in discharge current flowing through the latter.

5. The combination of an electron discharge device provided with a cathode to be heated, an excitation circuit for said device including a high tension transformer having its secondary winding connected through a rectifier arrangement device in series with said circuit and subject to a cyclic variation in potential thereacross upon a variation in the discharge current through said electron discharge device, a gaseous discharge tube and a resistor in shunt relation across said impedance and operable upon the potential across said impedance rising to a predetermined value to cause breakdown of said gaseous discharge tube with attendant current flow through said resistor, a source of alternating current electrical energy for heating the cathode of said electron discharge device including a saturable reactor having at least one of its windings in series relation with said heating source, a source of substantially constant potential unidirectional current adapted to be supplied to a second winding of said saturable reactor to control the flow of alternating current therethrough and to the cathode of said electron discharge device, and a control tube having its anode and cathode connected in series with said source of unidirectional current and the second winding of said saturable reactor to control the flow of current through the latter, and the grid of said control tube being connected to said gaseous discharge tube and said resistor and operable to assume a negative potential with respect to the cathode of said control tube upon the flow of current through said resistor, to cause attendant fiow of current through the second winding of said saturable reactor and an inverse variation in the alternating current flowing through the other winding thereof for heating the cathode of said electron discharge device in response to cyclic variations in the discharge current flowing through the latter and said series-connected impedance.

LOUIS L. WEISGLASS.