US2569945A - Device containing electron tubes for automatically stabilizing high-voltage sources with symmetrical potential distribution - Google Patents

Description

06L 1951 o. NETTELAND DEVICE CONTAINING ELECTRON TUBES FOR AUTOMATICALLY STABILIZING HIGH-VOLTAGE SOURCES WITH SYMMETRICAL POTENTIAL DISTRIBUTION Filed Dec. so, 1947 IN VEN TOR.

Fatented Oct. 2, I951 DEVICE CONTAINING ELECTRON TUBES FOR AUTOMATICALLY STABILIZING HIGH-VOLTAGE SOURCES WITH SYM- METRICAL POTENTIAL DISTRIBUTION Olav Netteland, Oslo, Norway Application December 30, 1947, Serial No. 794,572

In Norway November 1, 1945 3 Claims.

The present invention relates to devices for automatically stabilizing high voltage sources by means of electron tubes inserted directly in the high voltage circuit in series with the load and acting to maintain constant the output voltage at a predetermined value and simultaneously to maintain symmetrical the voltage distribution relatively to an earthed center point or a center point with a fixed potential relatively to ground independently of the variations of the input voltage.

The devices are especially intended for stabi- I lizing high voltage sources used for the production of radiation, such as X-rays generators for medical or technical purposes, but the same means may also be used for other purposes in science and technics, without making any principal alterations therein.

The use of electron tubes in voltage stabilization is not new, a variety of different circuits being described in the literature some of which are widely used in practical technics. All different methods, however, are based upon the same fundamental property of grid controlled electron tubes, i. e. that their anode voltage is a function, especially a linear function of their control grid potential, thus the tube can be used as a substitute for an automatically variable ohmic resistance.

This tube operating as a resistance can therefore be inserted in series or in parallel with the output load, the grid potential being varied in a suitable way to maintain constant the output voltage.

Electron tube stabilizers are chiefly used in connection with low or medium voltages, but have also been applied for high voltage in the order of 100 kv. for producing radiations, especially intended for scientific purposes. Those stabilizers are usually provided with a stabilizer tube in the positive branch of the high voltage circuit and are preferably used with the cathode of the electron tube grounded to give a high voltage source with grounded positive pole and negative pole at high tension, an arrangement which is often very advantageous.

In connection with X-ray generators for medical purposes, electron tube stabilizers of the said v type are not so much used, in spite of the existing need for the highest possible constancy and I exact reproduction of the working conditions.

The decisive reason for this fact is that modern X-ray generators usually give a symmetrical high voltage relatively to a grounded center point, an arrangement which is of fundamental importance R3 connected across the input of the sta in connection with the so called shockproof X-ray tubes in combination with high voltage cables with earthed metal sheaths. The insertion, of an electron tube as a stabilizing means in the positive high voltage lead will always give a nonsymmetrical potential distribution which gives unfavourable working conditions for the generator as well as for the X-ray tube and its cables.

This nonsymmetrical potential distribution is especially important when the high voltage source is a charged condenser which is gradually discharged by the X-ray tube. In this case the potential drop across the condenser may be in the order of 30-50 kv. to give a constant output voltage during the whole exposure, the stabilizer tube will have to compensate this total voltage difference and must consequently introduce a nonsymmetry of the same magnitude in the output voltage.

If high voltage generators with electron tube stabilization are to be commonly used in medical X-ray technics, it seems necessary to bring forward stabilizer circuits and devices which can give a constant output voltage with a symmetrical potential distribution relatively to a, center at fixed potential relatively to ground. The object of the present invention is to provide such a stabilizing system which is to be described below with reference to the drawing in which Fig. 1 shows the principal diagram of a stabilizer having a stabilizing device in the positive as well as in the negative lead,

Fig. 2 shows the positive stabilizer, and

Fig. 3 shows the negative stabilizer.

An independent stabilization of the positive and the negative voltage, so that the potential distribution relative to a fixed potential is kept symmetrical, may be obtained according to this invention by means of two independently working stabilizer tubes T1 and T2 as diagrammatically shown in Fig. 1. Said tubes are inserted directly in the high voltage circuit in such a way that T1 with its cathode is connected to the negative high voltage pole of the generator and with its anode to the cathode of the X-ray tube, whereas the anode of T2 is connected to the positive pole of the generator with its cathode connected to the anode of the X-ray tube.

The stabilizer tube T1 is controlled by means of an electronic amplifier F1 of linear character istic', the input of which is connected to variable contact of a non-inductive voltage divid system. The second stabilizer tube T2 is com trolled by means of another electronic amplifier 3 F2 the input of which is connected to the variable contact of a non-inductive voltage divider Rt, Rs connected across the output of the stabilizing sys tem.

This arrangement acts in such a way that the potential of the respective control grids is varied in a proper way by means of the independent D. C. amplifiers F1 and F2 with the consequence that the potential drop acros the stabilizer tubes is varied automatically in such a way that the output Voltage (the voltagedrop' across the X- ray tube) is maintained constant, and simultaneously a symmetrical potential distribution relative to a grounded center, or a center with a fixed potential to ground is obtained. According to this principal arrangement both stabilizer tubes including their necessary amplifiers and other equipments are as a Whole kept at a high negative or positive potential respectively rela-' tively to the symmetry'center of the system and are consequently surrounded by electrostatically shielding metal boxes which are maintained at a high potential and preferably connected with the cathodes of the stabilizer tubes T1 and T2 respectively.

Let the unstabilized but symmetrical generator voltage be U11 and the stabilized output voltage be E as in Figure l, the condition for symmetrical potential distribution in the output voltage is that the potential drop across the tubes T1 and T2 in every moment is equal to When Uh varies the potential difference will have to be distributed in equal amounts on the two tubes, but this demand is not absolute. For all practical purposes a symmetrydegree of 35% will sufiice but the total variations in the output Voltage E must be much's'maller than this, frequently in the order of 0.1% or even less than 0.01% for special scientific purposes.

According to the above described principal manner of operationthe control gridsof the tubes are to be supplied with a varying voltage causing the anode voltage to vary in a predetermined manner to fulfill the demands as to stability and symmetry. For both stabilizer units this control is obtained by means of high 'volttge potentiometers R2, R3 and R4, Rs connected across the unstabilized input and the stabilized output respectively. The variable voltages from these potentiometers are partly compensated by means of constant potential sources and amplified by the two independently Working D. C. amplifiers F1 and F2, the resulting voltages preferably being applied to the tubes T1 and T2 as negative grid potentials.

According to the invention the stabilizer units are acting independently of each other and in a principally different manner. The negative stabilizer unit will have to be a balanced system dependent on the potentiometer ratio R2, R3, the gain f1 of the amplifier and the amplification factor 1 of the tube T1. The positive stabilizer unit is controlled by negative feed back and is accordingly auto stable and does not need any critical tuning. The setting of the potentiometer ratio R4, R5 has only a small influence upon the degree of stability, but is a convenient method in regulating the stabilized output voltage.

In this paper the two stabilizer units have been named the negative and the positive stabilizer respectively and will be separately described in detail with reference to theFigures 2 and 3. i

a. The negative stabilizer In Fig. 2 a complete circuit is shown for a negative stabilizing system according to the general principles as described above. T1 is the electronic stabilizer tube the cathode of which is connected in series with an ohmic resistance R1 placed in the negative high voltage lead. Ta is an electronic amplifier tube the cathode of which is connected with the cathode of stabilizer tube T1. Re is a load resistance connected between cathodes of tubes T1 and T3, and the anode voltage source B2, in the'embodiment shown a voltage regulated power supply in which the voltage drop across the resistance Rs may be partly compensated by the voltage drop across one or more glow discharge lamps in the power supply unit. R2, R3 is the control voltage divider and B1 a battery or other constant voltage source shunted by a potentiometer P1 to give a suitable grid bias of the amplifier tube T3. By varying the potentiometer P the grid bias of the stabilizer tube Ti may also be adjusted to a decided value.

The cathode of the stabilizer tube T1 is principally connected with the negative high voltage pole of the generator'and i suitably controlled in such a manner that in a fixed ratio, thevariations of the unstabilized input voltage Us are applied to the controlgrid of the tube in phase opposition. As mentioned above'this is attained by means of a preferably non inductive potentiometer R2, R3; the potential drop across the resistance R2 is partly compensated by the battery B1 or 'a similar constant potential source and is applied to the D. C. amplifier F1.' The amplified and phase inversed output voltage of this amplifier is suitably compensated and applied to the control grid of the tube T1 as a' preferably negative grid potential, which will have to vary in'the decided way. i

In order to obtain the stabilizing efiect aimed, it is necessary that the amplification factor of the tube T1 can be considered as constant within the range of regulation, and assumed the anode current is maintained coristantfthe potential drop U1 across the tubewillbe an approximately linear function of its grid potential Ugl. If F1 has a linear working characteristic and a constant gain f1, the grid voltage U of the stabilizer tube consequently is a linear function of the potential drop across theresistance R2, which is equal to:

R2 rt s and by critically adjusting R2 according to the equation:

(where the sum resistance R2+Ra is approximately constant), the negative stabilizer will compensate 1/11 of the input voltage'variatlons. If the input voltages is exactly symmetrical, n is equal to 2, and mostly it will diflef'only'slightly from this value. But it is principally possible to obtain a symmetrical output voltage even if the input'voltage is considerably non-symmetrical Above it has been presupposed that the load current I be constant but this is the case seldom only. To a certain degree the effect of small current variations'rnay be reduced by usinga stabilizer tube with a'high mutual'co'nducta'nce S1 and a low amplification factor. However, generally a special automatic compensation is necessary to obtain a voltage drop across the tube,

which is sufiiciently independent of the load current. According to the present invention, for

. this purpose, a variable ohmic resistance R1 has been inserted between the negative terminal of the resistance R2 and the cathode of the tube Ti, and across this resistance the load current I will produce a potential drop I.R1 acting to alter the effective grid potential of the amplifier tube Ts.

By adjusting the resistance R1 according to the equation:

m-sl (2) and when working on a part of the operating characteristic of T1, within which its mutual conductance S1 may be considered as constant, it may be obtained that its grid voltage will vary linearly as a function of the tube current I, in such a manner that the anode voltage (U1) of the tube will be approximately independent of the anode current. In the special case, shown in Figure 2, the total potential drop I.R1 is applied to the grid of T3, and m is equal to one.

Mostly it is difiicult to obtain exact balancing of the system according to the Equations 1 and 2, or the balance will be exact within rather narrow limits only. However, as mentioned above, the voltage symmetry in practice mostly is not very critical, differences of a few percent being allowed. Such a degree of stability easily can be obtained, and in general the adjustments of the resistors R1 and R2 can be done once for ever. When the variations in load current I are very great, S1 will not be sufficiently constant, and it is preferred to vary R1 in a few coarse steps in connection with the regulation of the X-ray tube current to cover the total operating range.

The potential drop (U1) across the tube T1 may be varied at will to obtain proper working conditions by regulating the compensating voltage (B1) defining the grid potential of the amplifier tube T3 and consequently the anode voltage of the stabilizer tube T1.

By this means it is possible to obtain a symmetrical output voltage at different values of E without disturbing the fixed adjustment of the potentiometer ratio R2, R3.

b. The positive" stabilizer The positive stabilizer will now be described with reference to Fig. 3.

As mentioned above, the positive" stabilizer is controlled in the ordinary way by negative feed back by means of the potentiometer R4, R5

which is connected across the terminals of the stabilized output volta e.- The principal operation of this type of electron tube stabilizer is well known and need not be described here.

Difierent types of amplifiers may be used even the symmetrical stabilizer, but on account of .totalgain of the amplifier F2 be Ex, ,u: and f2 6 respectively, the stabilizating efiiciency of the system is approximately:

To obtain a good stabilization it is therefore preferable to use a relatively high compensation voltage as well as a high gain amplifier and a tube T2 with a high amplification factor; the special amplifier needed should have the following specifications: D. C. amplifier without phase inversion, high amplification, high input compensation potential, elimination of special input and output compensation voltage sources, and if possible all the necessary operating potentials delivered from a single, stable voltage source which is independent of load variations.

An efficient amplifier which fulfil the above mentioned requirements, is shown in Figure 3 in its principal form.

The several stages of the amplifier are all driven by a single, stablized voltage source with a constant potential Ek, its positive pole being con nected to the cathode of the stabilizer tube T2. The cathode of the input tube T4 is connected to the negative terminal of the anode voltage source and its control grid is connected directly to the control potentiometer R4, R5, across the resistance R5 of which is produced a potential drop of nearly the value Ek which gives a suitbale input tube grid potential. In this manner a special input compensation voltage source is avoided, and El: may easily be given a value which is sufficiently high to produce an eflicient stabilization.

The output stage of the amplifier is a D. C. differential stage with two preferably identical tubes connected in parallel and with a common cathode resistance R8, the negative terminal of which is connected to the negative pole of the anode voltage source. In the following the tubes will be named output tube T5 and control tube T6 respectively. In Figure 3 the control grid of the output tube is connected to the anode resistance R1 of the input tube T4 with its grid controlled in an ordinary manner. The grid of the control tube has a fixed potential relative to the negative pole of the voltage source and is connected to the voltage divider R11, R12. To obtain maximum sensibility the anode voltage of the control tube preferabl should be maintained constant by directly connecting the anode to the positive pole of the voltage source.

The value of the anode resistance R9 belonging or the output tube is chosen so that the mean anode current delivers directly the negative grid bias for the stabilizer tube T2.

The operation of the amplifier described above is as follows: A voltage variatio applied to the input tube T4, and the voltage variation from the first stage, now amplified and phase inversed, is applied to the control grid of the output tube to cause an anode current variation in this tube and an inversed current variaion of about the same amount in the control tube T6, whereby the common cathode is maintained at an approximately constant potential. The same resulting effect can also be obtained by the use of a single tube in the output stage, connecting it in series with one or more neon tubes inserted between its cathode and the negative terminal of the voltage source, providing the output tube with a suitable constant cathode potential.

c. Output voltage regulation As pointed out above, the regulation of the '7 output voltage of the total stabilizer device may be performed by varying the potentiometer ratio R4, R in the negative feed back control system of the positive stabilizer, maintaining the compensation voltage E1; constant.

According to this method the system will attain a new stable condition at a new output potential E, the total voltage variation being absorbed by the positive stabilizer tube T2, which will be given a new mean anode voltage. In this variation, the negative stabilizer tube T1 will not take part, as the two units are working independently. To obtain a symmetrical output voltage it is necessary to have a simultaneous regulation of the potential drop across the negative tube T1. As mentioned above, this may be done by varying the input compensation voltage of the amplifier F1, without disturbing the balance of the system.

The principle of simultaneous voltage regulation is illustrated in Figure 7a, in which each position of the sliding contact K1 of the positive voltage regulator corresponds to a posi.ion of the sliding contact K2 on the negative potentiomete P1; and when moving the two contacts synchronously, a symmetrical ouput potential may always be attained. The two regulators may have a resistance characteristic allowing continuous voltage regulation, but in practice it is generally more convenient to have a regulation in steps, corresponding steps being adjusted to fulfil the symmetry conditions.

In Figure 7b is shown an arrangement allowing continuous regulation by means of a step switch in combination with a fine regulation between the steps.

As shown in the figure each coarse regulator may have two parallely mounted identical contact fingers K1 with the resistances inserted between corresponding contact points, the regulators having synchronously moving sliding contacts between which the fine regulationpotentioineter P3 is connected, the contact handle of which is indicated b F1.

At low output potentials, this method of voitage regulation may result in rather high voltage drops across the stabilizer tubes. To avoid unfavourable working conditions of the stabilizer tubes the above mentioned coarse regulators K1 may be combined wtih a synchronously moving regulator in the primary of the high voltage transformer, allowing a simultaneous regulation in steps of the unstabilized high voltage U11.

11. Special arrangements For many purposes it is necessary to vary the the potential drops of the stabilizer tubes within very wide limits, up to 56 av, and consequently the variation range of the control grid potentials being in the order of 1000 volts. In combination Wiih apparatus for X-ray diagnostics at high X-ray tube currents, the grid voltage variation may be from negative cut off to positive potential of some hundred volts, resulting in considerable grid currents. Ordinary output stages of control amplifiers will not fulfill these extreme requirements, the above described amplifiers, however, may be modified into a suitable form with rather small alterations.

According to the present invention the fixed load resistancein the'anode' circuit of the'output tube may be substituted by a continuousvariable resistance, the momentaneous value of which being a function of the anode current. This is reaiizedby inserting a suitable electron-tube, acting as a load resistance (here named resistance tube T11, Figure 8) in the anode circuit of the output tube. In the cathode circuit of the resistance tube is inserted an ohmic resistance across which the common anode current ofthe two in series connected tubes produce a voltage drop being applied to the control grid giving same a potential which is a linear function of the anode current. The cathode of the resistance tube is connected with the control grid of the stabilizer tub (T2, Figure 8) and its anode is connected to a sufiiciently high positive potential relative to the cathode of the stabilizer tube. If the resistance tube is a screen grid tube, a special floating screen grid voltage source is needed.

The above arrangement acts as follows: At a sufficiently high anode current in the output tube, the potential drop across the resistance tube attains a high value to the high negative grid potential, the anode voltage being gradually reduced with the anode current. When the grid potential of the stabilizer tube be positive, the grid current passes the resistance tube without disturbing the normal grid contro1 of the tube.

By using suitable resistance tubes with preferably high amplification factors, the arrangement is equivalent to a very high resistance in the anode circuit of the amplifier tube, giving the amplifier a practical voltage gain very near to the amplification factorof the amplifier tube. In Figure 10 is shown the corresponding arrangement of the amplifier F1 in the negative stabilizer.

By means of the above described arrangement the stabilizer can convenientl be used not only for ordinary symmetrical stabilization but as well to stop the load current effectively by giving the stabilizer tubes sufficiently high negative grid potentials. Such an arrangement is shown in Figure 10, in which a magnetically operated switch S1 is inserted in the lead between the anode of the input tube T4 and the control grid of the output tube T5. With switch S1 open the grid of the tube T5 automatically attains, a grid potential giving a very high potential drop across the resistance tube with the switch closed, the output tube attains a grid potential according to the potentiometer ratio Eli/R5 giving normal working conditions at constant output voltage E. The magnetic switch is suitably operated by means of thyratrons (R61, R62), which are fired by means of positive pulses applied to the grids.

I claim:

1. A system for stabilizing high tension direct .rcurrent electrical systems, comprising in combination a high voltage source to be stabilized, two operatively independent grid controlled'stabilizing valves connected in series in the negative and positive high voltage lead respectively connecting the load with said source, which two stabilizing valves form the stabilizing circuit, a potentiometer connected across the input of said stabilizing circuit, the variable contact of which potentiometer is connected tothe input of an electronic amplifier the output of which is connected with the control grid of one of said stabilizing valves producing a voltage which is a linear and phase-inverted function of the unstabilized input voltage, a further potentiometer connected across the output of said stabilizing circuit, the variable contact of said second p0- tentiometer being connected to the input of a second'electron'ic amplifier the output of which is connectedwith the control grid of the other I ofsaid-stabiliz'i-ng valves for'producing a grldpotential being a direct linear function of the stabilized output voltage which is applied to the load.

2. A system as claimed in claim 1 and in which the potentiometer connected across the input of said stabilizing circuit is permanently adjusted (or balanced) according to the equation:

:R2+R3 2 1'l 1 R2 being the part resistance of the potentiometer connected to the negative terminal of the voltage source, R3 the other part resistance connected to the positive terminal, n a factor generally nearly equal to 2, 11 the grain of the amplifier controlling the stabilizing valve in the negative lead, and U1 the amplification factor of said stabilizing tube, the potential drop of said negative part resistance being partly compensated by means of a, constant voltage source to give a proper input grid bias for said amplifier and by regulating said compensating voltage equal to the mean potential drop across said stabilizing tube at will Without disturbing the balance of said negative stabilizing system.

3. A system according to claim 1 and in which an ohmic resistance is connected in the cathode circuit of the negatively connected stabilizing tube, whereby a potential drop occurs across said 10 resistance which is proportional to the output load current being produced, said resistance having the value wherein fl is the gain in the amplifier, S1 the mutual conductance of the negatively connected valve, l/m of the potential drop Rr-l being applied to the input of the amplifier (F1), so that the grid potential of the negative stabilizer tube varies approximately linearly with the anode current in said tube in such a way that within wide limits the potential drop across the tube is nearly independent of the output load current.

OLAV N ETTELAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,835,121 Rentschyer Dec. 8, 1931 2,210,393 Braden Aug. 6, 1940 2,394,891 Bowie Feb. 12, 1946 2,434,069 Goldberg Jan. 6, 1948

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