US2931909A - Electric circuit for supplying an X-ray tube with a predetermined anode current which is equal to the substantially constant saturation current of the tube - Google Patents

Description

April 5, 1960 Filed Aug. 19, 1958 0 L. HOPPERMANN 2,931,909

ELECTRIC CIRCUIT FOR SUPPLYING AN X-RAY TUBE WITH A PREDETERMINED ANODE CURRENT WHICH IS EQUAL TO THE SUBSTAN'I'IALLY CONSTANT SATURATION CURRENT OF THE TUBE 2 Sheets-Sheet l cfiwenZ r 0/6 3432 woppermd m 8 4 (a a (V J 72715 April 5, 1960 R SUPPLYING AN X-RAY TUBE WITH A PREDETERMINED ANODE CURRENT WHICH IS EQUAL TO THE SUBSTANTIALLY CONSTANT SATURATION CURRENT OF THE TUBE Filed Aug. 19, 1958 2 Sheets-Sheet 2 R M IIIIIII 'IIIIIIIIII KT O (I) /V /V [e J28? d l opermenn flM LA /M m o. L. HOPPERMANN 2,931,909 ELECTRIC CIRCUIT F0 United States Patent CC ELECTRIC CIRCUIT FOR SUPPLYING AN X-RA TUBE WITH A PREDETERMINED ANODE CUR- RENT WHICH IS EQUAL TO THE SUBSTAN- .TIALLY CONSTANT SATURATION CURRENT OF THE TUBE Ole Lutz Hoppermann, Charlottenlund, near Copenhagen,

Denmark, assignor to Holger Andreasen, Copenhagen 8., Denmark, a Danish firm Application August 19, 1958, Serial No. 755,998

Claims priority, application Denmark August 19, 1957 6 Claims. ((31. 250-91 a special winding on one of the high voltage transformers whose primary windings across adjustable taps-are regulably connected to an autotransformer which is connected to a nominal line voltage through a regulating device for equalising variations in the said nominal line voltage.

The present invention is'more particularly concerned with electric circuits in X-ray apparatus for examination of coarse structures. In such apparatus the anode voltage of the tube which is decisive for'the hardness of radiation and the consequent power of'penetrationmust be adjustable over a suitable range ofvoltage. Furthermore, the anode curre'nt'in the known apparatus of' the said kind can usually be regulated. In the known circuits of the aforesaid kind is therefore usually provided a voltmeter for controlling the anode voltage of the X-ray tube and a milliammeter for controlling the anode current of the tube; in addition, there is a handle for adjusting the apparatus to a given anode voltage by means of the adjustable taps, and another handle for adjustment of Y The, use of an X-ray a predetermined anode current. installation with such a known electric circuit requires thus manipulation of two handles and reading of two instruments.

It is the object of the present invention to devise an electric circuit in which there is only one instrument to be read, the above mentioned voltmeter being dispensed with, and only one handle for, regulating the anode current, whereas the other handle is substituted by a switch the positions of which are adjusted to a series of predetermined exact anode voltages.

The present invention is basedupon the physicalfact that the emission from an incandescent cathode .in accordance with Dushmanns formula: I i

V V v I.m=AT. where A and b are constants ofmaterial and T the absolute temperature of the incandescent cathode, is solely dependent upon the said temperature and upon the material used. Under constant cooling conditions as in an X-ray apparatus the proportion between the electric energy converted into X-rays and the electric energy supplied to the cathode is with substantial approximation constant. 9 I v i 'At an'anode current predetermined for the type of the tube and which,-for example, may be five milliamps, the voltage across the incandescentcathode and consequently across the primary voltage ofv the filament current transformer will be constant, andthis is utilised by 2,931,909 Pel -re the present invention for measuring the anodevoltage of the X-ray tube and the connected line voltage.

An essential feature of. an electric" circuit according to the invention is that the primary winding or the filament current transformer or the primaryjwinding of high voltage transformers having the special'winding is directly connected to permanent taps provided on the, autotransformer in such manner that the tube. has the predetera mined anode current when the autotransformer is ad justed to the nominal line voltage.

As a result, the voltmeter used in the known circuits, of the aforesaid kind is rendered superfluous. When the milliammeter isadjusted to indicate the predeterminedanode current which may be indicated by an individual division on the graduated scale of the milliammeterfjtheconstant of the autotransformer for volts per winding will be determined on the basis of the number of wind ings of the winding to which the nominal voltage is con-. nected and the remaining taps provided on the trans-r former will give off an exact voltage determined by the; number of windings between these taps, for example also the taps'for the high voltage transformers regulably con nected to the autotransformer, and the said taps maythus f give off anode voltages of predetermined values, an'cllth'e' individual positions of the handle for the adjustable 'taps' for the andoe voltage can thus bedenoted by the said exact values. V

v l twill be'appreciated that' if the line voltage during" the operation of the apparatus deviates from the nominal value the constant of the autotransformer'for volts perwinding will deviate from its normal valueand the fila-, ment current transformer .will therefore not be supplied with the primary voltage corresponding to the predeter mined value of the anode current, wherefore the regulating device for equalising variations in the nominal line voltage has to be displaced until the milliammeter shows the predetermined anode current, and when this has taken place the autotransformer has the normal value for volts per winding and the voltages across its different taps are of the exact values determined by the number of wind ings.

This indirect measurement of the anode voltage'of said voltage corresponds to a great increase in the anode; current, which will be evident from Dushmanns'formula quoted above. It should be observed here that experi-i ments have disclosed that a 1% increase in the voltage across the incandescent cathode corresponds to'an increase of about 10% in anode current, that is, the anode current may be measured with an accuracy ten times that obtained when measuring the voltage across the. incandescent cathode. Assuming that the milliammeter in the anode circuit and the voltmeter used in the known apparatus for measuring the anode voltage are of identical precision, it will under the conditions referred as be ir'n possible, even by voltmeters which are of maximum precision and provided with knife blade pointer and mirror scale, to adjust the said voltage as accurately as the anode current may be adjusted by'the milliammeter. i

In addition, however, also the anode voltage is measured with corresponding accuracy owing to the proportionality between the voltage across the incandescent cathode and the voltage across the taps for the high volt-' age transformer, and the said taps may thus be marked with voltages corresponding to the respective numbers of windings, the load on the high voltage transformers being always constant, for exampleequal to 5 milliamps.

- stereos The results obtained by the present invention are thus, compared with that obtained in the known circuit dia-' grams of the aforesaid kind;

I (1 Asubstantiallyv moreaccurateadjustment of fila ment voltage and anode voltage, v t

(2 A cost economy, since' the voltmeter is super fiuous and may be dispensed with, and

(3 That at anexposure the manipulation of an X-ray apparatus with anelectric circuitaccording to the inventionbecomes very simple.

In certain types of X-ray tubes the saturation current of, thetube is practically independent of the anode voltage, and the present invention may, be'used directly in v connection with such X ray tubes. In other typesv of tubes the saturation current increasesslightly and substantially proportionally with the anode voltage. It is, however, possible by simple measures to adjust the lastmentioned type of X-ray tubes to electric circuits according to the present inventionby compensating for thesaid increase in'the saturation current.

Inone embodiment of the electric circuitusing a de vice to compensate for the increase, if any, in the saturation current of the X-ray tube the device according to the invention consists of apermanentresistance inserted into one of, the supply lines from thesystem voltage to the autotransformer and so dimensioned that the' voltage drop in the resistance compensates for the increase in the anode current which increase'is substantially proportional with the anode voltage. V

The result obtained is that by suitable dimensioning of the resistance the voltage drop in same will produce a voltage drop in the primary voltage of the autotrans former and consequently a compensation against the said increase in the anode current sothat the latter remains substantially constant, even though.theautotransformer is adjusted to supply a higher prirnarywoltage to the high voltage. transformers. I

In another embodiment of the electric circuit according to the invention the device consists of a permanent re-.

in the supply line to the said high voltage. transformer; produces a compensation against the increase. in the anode, current.

'Ina third embodiment of the .electric circuitaccording to. the. invention the deviceconsists of a second switch mechanically connected to an anode voltage switch, the

' firstrnentioned switch being connected to the autotransformer in. such manner that the filament current decreases in predeterminedmanner at increasing anode voltage.

I The result obtained is that by, suitable. placement of.

the connecting pointsof. the switches on the-autotransformer the increased anode current originating from the increased primary voltage for the high. voltage trans-. former is counteracted by a reduction of the filament current, so that the anode current remains substantially constant and the anode voltage only becomes dependent on the position of the mechanically connected switches.

In a fourth embodiment of the circuit according. to the invention the device consists of a compensationtransfprrner whose primarywinding is connected in parallel with the primary winding ofthe anode transformer, whereas its secondary winding is connected in series with and counter-connected with. the primary windingtof the highvoltage transformer havingthe special filament current winding.

The result obtained is that by suitable dimensioning of the ratio of the-compensation transformer the filament current may be reducedby an amount corresponding to the change of the anode voltage caused by a turning of 4 the second switch, so that the anode current despite the said change is kept constant.

In a fifth embodiment of the circuit according to the invention the device consists of a special transformer whose primary winding is. connected in parallel with the primary windings of the high voltage transformer, whereas the secondary winding is in series with. and counter-connected with the primary winding of the filament current transformer.

The result obtained is that at an increase in the primary voltage of the high voltage transformers the increased anode current in the'tube due to the said increase may by suitable dimensioning of the ratio of the special transformer be compensated by a reduction of the filament current ofthe tube.

The invention will now be further described with reference to the drawing, in which Figures 1-7 show various embodiments of an electric circuit for supplying an X-ray tube according to the invention with current.

In the embodiments of the electric circuit according t0 the invention as shown in Figures 1 and 2 the saturation currentof the tube is assumed to be constant and of a predetermined value and that the anode current is substantially independent ofthe anode voltage.

Figure l shows a first. embodiment of the electric circuit according to the invention, comprising two high voltage transformers B and C referredto as anode transformer and cathode transformer, respectively, and an independent filament current transformer D. A regulator transformer A feeds the high voltage transformers B and C with primary voltage the value of which may be adjusted by means of a regulating device E which may consist either of; a switch E sliding across a number of taps provided on the regulating transformer A or of a contact brush L which is adjustable over a stripped part of the surface of the windings of the regulating transformer A, or of a combination of these. In the case mentioned in the middle the regulating transformer A will usually bealong the stripped part of. the surface of the windings ofthe regulating transformer in case of dismantling of the;

apparatus during its adjustment in factory or workshop In the regulating transformer A are connected two poles N of apower line, the said'connection being estab fished-through a permanent tap O and by means of the contact brush K whichis displaceable along the stripped part of the surface of the windings of the regulating transformer and so dimensioned that the regulation covers the expected deviations of the line voltage from the nominal voltage which the apparatus is intended to work at. The secondary windings of the high voltage transformers Band C are interconnected in series with a milliammeter m. Since the electric circuit is assumed to be adapted for use in connection with an X ray tube working at a definitemilliamp value the milliammeter need only have one scale division corresponding to the said definite value. The division may, however, have an indication in volts corresponding to the nominal line voltage. V e

The ratio of the filament current transformer D and the voltage fed to the primary winding of the transformer, which voltage is given at the permanent connecting points of the primary winding on the regulating transformer A, are so adapted that when the regulating transformer A- has the number ofvolts per winding for which it is intended the filament will have an emission corresponding to'the fixed anode current of the apparatus.

When the contact brush K is adjusted until the-milliammeter m indicates thefixed anode current'oftheas mentioned above.

apparatus, the regulating transformer will receive the:

correct number of volts per winding, wherefore the regulating transformer A will be capable of feeding the high voltage transformers B and C with predetermined The adjusted in kilovolts and connected to a series of taps on the regulating transformer A, or by means of a contact brush L which is displaceable along the stripped winding surface of the regulating transformer A, in which case the regulating transformer usually will be ring wound. Finally, a combination as disclosed in Figure 1 may be used. In the latter case'adjustment of the anode voltage may be continuous, and the scale of the regulating knob of the ring wound transformer may in that case be adjusted in kilovolts.

Figure 2 shows another embodiment of the electric circuit'according to the'invention, comprising two high voltage transformers, an anode transformer B and a cathode transformer F, respectively, the latter being, for example, a combined high voltage and filament current transformer.

The regulating transformer A feeds the anode transformer B and the combined high voltage and filament current transformer F with primary voltage. The primary voltage of the anode transformer B may be adjusted by means of the regulating device E which may be designed An adaptation resistance M may be inserted in the filament current circuit if the filament voltage of the tube R does not correspond to an integral number of filament current windings of the combined high voltage and filament current transformer F. In the transformer F the ratio of the filament current winding and the primary voltage of the transformer, which is determined by the permanent connection points of its primary winding on the regulating transformer A, adapted in such manner that when the last mentioned transformer has the number of volts per winding for which it is calculated the emission of the filament will correspond to the fixed anode current of the apparatus. When the said anode current is obtained, the high voltage winding of the transformer F will furthermore produce a predetermined voltage so that the total high voltage across the anode circuit of the tube has a fixed value. By adjustment of the contact brush K which is displaceable along a stripped part of the surface of the windings of the regulating transformer A until the milliammeter m indicates the fixed anode current of the apparatus the regulating transformer A will be fed with the correct number of volts per winding, wherefore the regulating transformer will be capable of providing the anode transformer B with a predetermined primary voltage without the said voltage having to be measured by a voltmeter. The primary voltages can be adjusted either by means of the regulating device E which may be adjusted in kilovolts or by means of the contact brush L as mentioned above.

If the X-ray tube used in the electric circuit is such one in which the anode current increases at increasing anode voltage and if the electric circuits disclosed above were used directly in connection with such a tube the interval containing the permissible line voltages will be displaced upwards in case a low anode voltage is used, whereas the said interval will be displaced downwards in case a high anode voltage is used. Furthermore, with the use of such tubes it would be necessary to re-adjust the position of the contact brush K after each new adjustment of the anode voltage.

In the embodiments of electric circuits in accordance with the invention as described in the following a compensation for the increase in the anode current is obtained by an increase in the anode voltage.

Figure 3 shows a third embodiment of the electric circuit according to the invention in which a resistance M of; the order of a few ohms is insertedin the supply line from the one polelof the line voltage to the regulating transformers The strength of the current in the resistance M depends on the consumption of the X-ray tube and' is thus, since the apparatus involved is for adjustment of v a definite value of milliamp, solely dependent on thev On adjustment of an increased anode,

anode voltage. voltage the primary'winding of the combined filament current transformer and cathode transformer would owing to the increased voltage drop across the resistance receive a lower voltage, wherefore the temperature and 1 the emission of the filament will decrease so that the in- I crease in the anode current that would have occurred owing to the increased anode voltage is equalised.

In the fourth embodiment of the electric circuit ac cording to the invention as shown in Figure 4 a resistance M of the order of a few ohms is inserted in series with a permanent tap S from the regulating transformer A which is connected with the primary windings of both the high voltage transformers and the filament current.

transformer. In the same way as described above the increased consumptionby adjustment of a higher anode Figure 5 shows a fifth embodiment of the electric. circuit according to the invention, in which the anode voltage is adjusted by means of a double switch 02, so that simultaneous with the increase in the anode voltage the voltage across the combined cathode transformer and filament current transformer F is reduced, wherefore the temperature and emission of the filament will decrease so that the increase in the anode current that would have occurred owing to the increased anode voltage is equalised.

*Figure 6 shows a sixth embodiment of the electric circuit according to the invention, incorporating an anode transformer and a combined cathode/filament current transformer F. A compensation transformer H dimensioned for a few volt-amps has its primary winding parallel with the primary winding of the anode transformer B, whereas its secondary winding is in series with the primary winding of the cathode and filament current transformer F, but counter-connected to the same. It will be evident from this that on an increase in the voltage of the anode transformer H a voltage will be subtracted from the cathode and filament current transformer F, whereby the temperature and emission of the filament will decrease so that the increase in the anode current that would have occurred owing to the increased anode voltage is equalised.

Figure 7 shows a seventh embodiment of the electric circuit according to the invention in which the primary winding of the auxiliary transformer H is connected in parallel with the primary windings of the high voltage transformers B and C, whereas its secondary winding is in series with the primary winding of the filament current transformer D, but counter-connected to same. It will be evident from this that by an increase in the voltage of the anode transformer B a voltage will be subtracted from the filament current transformer, whereby the temperature and emission of the filament will decrease so that the increase in the anode current that would have occurred owing to the increased anode voltage is equalised.

The embodiments of the electric circuit according to the invention as disclosed in Figures 3-7 are based on the same principles as the embodiments of the invention disclosed in Figures 1 and 2, the internal voltage of the apparatus being controlled by means of the milliammeter m and adjusted by means of the contact brush K for adjustment of the input voltage of the apparatus.

a-selected value of the anode current'and of-the kind comprising at least one high voltage transformer, between whose secondary windings is inserted a milliammeter and a filament current transformer which may be formed as a special winding carried on one of the high voltage transformers, whose primary windings across adjustable taps are regulably connected to a supply transformer for instance an autotransformer, which is connected to a nominal supply voltage through a regulating device for equalizing variations in the said nominal supply voltage, characterized in that'the primary winding of the filament current transformer or the primary winding of the high volt-agetransformer carrying said special winding is directly connected to fixed taps so arranged on said autotransformer that when the autotransformer is adjusted to the nominal supply voltage the anode current is equal to the substantially constant anode saturation current of the tube.

2'; An electric circuit as'claimedin claim 1, including a; device to compensatefor the increase, if any, in the saturation current of the X-raytube at increasing anode voltage, characterized in that the said device consists of a fixed resistor inserted in one of the supply lines from the supply voltage to the autotransforrner and so dimensioned that the voltage drop in the resistor compensates for a small increase in the anode saturation current, which increase is substantially proportional with the anode voltage. I

3. An electric circuit as claimed in claim 2, characterized in that the device consists of a fixed resistor inserted in one of the supply lines to the primary winding of the filament current transformer or of the high voltage transformer carrying said special winding and so dimensioned" that the*v'o'lt'a'ge drop in the said. resistors: compensates for a small increase in the anode=saturationcurrent whichincrease is substantially proportionalwitlr';

the anode voltage.

manner that the filament current decreases by moving said switches in predetermined manner at increasing anode voltage. I

5. An electric circuit as claimed in claim 1, characterized in that a compensation transformer whose primary winding is connected in parallel with the primary winding of the anode transformer, whereas its secondary winding is counter-connected in series with the primary winding of the high voltage transformer having the special filament current winding.

' 6. An electric circuit as claimed in claim 5, characterized in said'compensation transformer having its primarywinding connected in parallel with the primary windings of'the high voltage transformers, whereas its secondary winding is counter-connected in series with the primary winding of the filament current transformer;

References Cited in the file of this patent UNITED STATES PATENTS 1,631,625 Coolidge June 7, 1927'" 2,167,802 Goldfield Aug. 1, 1939 2,172,581 Horsley Sept. 12, 1939' 2,492,281 Hall Dec. 27, 1949 2,626,360 Wright Jan. 20, 1953 2,745,020 Graves et al. May 8, 1956' 2,870,340 Fransen Jan. 20, 1959'

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