US2448540A

US2448540A - Electrophysiotherapeutic apparatus

Info

Publication number: US2448540A
Application number: US492251A
Authority: US
Inventors: Rolland H Maxson
Original assignee: Burdick Corp
Current assignee: Burdick Corp
Priority date: 1943-06-25
Filing date: 1943-06-25
Publication date: 1948-09-07
Anticipated expiration: 1965-09-07

Description

Sept. 7, 1948. R. H.` MAXSON ELEc'rRoPHYsIoTHERAPEuTIc APPARATUS Filed June 25 1943 l/mm Patented Sept. 7, 1948 ELECTROPHYSIOTHERAPEUTIC APPARATUS Rolland H. Maxson, Milton, Wis., assignor to The Burdick Corporation, Milton, Wis., a corporation of Delaware Application June 25, 1943, Serial No. 492,251

(Clt 128-422) 8 Claims. 1

The present invention relates to electrophysiotherapeutic apparatus, and has for its primary object the provision of new and improved apparatus of this type which will not interfere with radio communication and which will operate satisfactorily and be safe under extreme operating conditions. The apparatus of the present invention operates at a specified frequency and within certain tolerance limits, and also in a specified frequency channel.

The electro-physiotherapeutic apparatus., or diathermy equipment as it may be called, of the prior art transmitted electromagnetic waves into space to a, degree depending on a considerable number of complicated conditions difficult to control. As a result, the equipment has at least the potential, if not the actual, ability of transmitting signals which can be heard on radio receivers tuned to the diathermy frequency. The-equipment includes what is essentially a high powered self-excited oscillator generating a frequency which may vary over a range of about 5% to 80%, causing interference in a great number of comm-unication channels.

Most of the frequency shift in the prior art apparatus using oscillators of the self-excited type, is caused by the change in reactance of the plate tank circuit, i. e., the resonant circuit determining the frequency of oscillation. This change in reactance is due to Variations in reactance of a patients circuit coupled to the plate tanlr; circuit, which reactance is reflected back into the tank circuit. A self-excited oscillator automatically adjusts its frequency to the value that causes capacitive and inductive reactance components to be equal. The resulting adjustment or shift in oscillation frequency is great because the reactance of the patients circuit varies greatly with the Wide range of patient applications.

The apparatus of the present invention includes a crystal controlled oscillator, preferably of the harmonic type, the output of which is supplied to an intermediate amplifier driving a final amplifier and isolating the final amplifier from the oscillator. In some respects the apparatus of the present invention may be compared to a crystal controlled radio transmitter, but while such a comparison can be made, there are many problems present in diathermy equipment which are not present in radio transmitters. For instance, in a transmitter the final amplifier feeds energy into a load having a constant resistance and with all reactance substantially tuned out. In contrast, the final amplifier of the present apparatus supplies energy to a load whose resistance as Well as reactance varies over a wide range (because of the different positions of the applicator relative to the patient) and which reactance may or may not be tuned out of the circuit. If a conventional type of radio transmitter final amplifier were used, the reflected reactance in the tank circuit would not change the oscillator frequency, contrary to what takes place in the conventional self-excited oscillator type diathermy equipment, as heretofore set forth. However, even though a conventional radio transmitter final amplifier were used, the change in refiected reactance would cause an increased plate input at a lower plate eiciency and thereby overload the tubes. The reason for this is that the frequency cannot adjust itself to cause equality and cancellation of capacitive and inductive reactances in the plate tank circuit so that the reflected reactance in the tank circuit decreases the tank impedance and increasesl the plate input at a lower plate efficiency, the lowered plate efficiency being caused by a phase shift in the radio frequency plate current resulting from the reflected reactance. Thus the instantaneous radio frequency plate voltage is not out of phase With the instantaneous radio frequency grid voltage. The tube space current thus flows at a higher value of plate voltage and produces an increased plate dissipation which might well be in excess of the tubes rated value, even though no energy whatever is being transmitted to the patient. This condition might arise when the reflected reactance was suiiiciently high, as in the case where the applicator, which may take the form of a treatment drum, supplied by the apparatus is not in contact with the patient and is not tuned to resonance but is closely coupled to the final amplifier.

It is, therefore, one of the primary objects of the present-l invention to minimize changes in plate .tank circuit reactance due to reflected reactance from the load or patients circuit. According to the instant invention, this is accomplished by (l) connecting the patients circuit to the plate tank circuit of the final amplifier 4by means of a loose and variable coupling; (2) utilizing aplate tank circuit having a very high full load effective Q; and (3) preventing thermal deformation of elements of the ltank circuit. The coupling to the plate tank circuit 'has associated with ita pair of series variable condensers Whereby tuning out yof the reactance in the patients circuit can be effected. 'Ihe coupling is variable, and while the coupling to Ithe plate [tank circ-uit is loose, it is sufcient to provide the requisite energy transfer to the patient. There is, however, a limit to the extent to which it'he coupling can be reduced and still have sufcient coupling to provide the requisite energy transfer to the patient. There exists also a probability the operator may-.not always accurately tune the patients circuit `@by 'means -of `the series condensers) so that some reactance will be reflected into the plate tank circuit. Accordingly, the present 'invention provides a further minimization of the effect of the reflected reactance upon the plate tank .circuit by utilizing tank circuit elements having very low capacitive and inductive reactanoes, 1i. re., reac'tances that are relatively small lcompared to the shunt reiiected reactance. The plate tank capacitance is made large and the inductance small in value, with the result that the tank circuit has a very high full load effective Q. For instance, the apparatus of the present invention may have at full load an effective Q preferably of about 85 as compared with value of about 10 to 2G for radiotransmitters.

While the use of a plate tank circuit having a ^very high full load Q is 'eective in minimizing Atank reactance changes due to reflected load r..- 'actance it 'produces an increase in the effective vradio frequency current circulating in the plate tank circuit. In accordance With 'an-other feature of the present invention, the effect of this increased current to nullify the stabilizing effect produced by the high Q circuit is prevented by maintain-ing the circuit at an even temperature.

VI-t Nmay be seen, therefore, that an object of the present invention to provide a new and improved diathermy equipment producing an adequate energy input to the patient under all conditions without exceeding ithe rated .plate ldissipat-ion of the elec-tronic tubes even under the -worst conditions of out of resonance operation of .ian unloaded -applicator with full coupling, and wherein it is not necessary substantially continu-- Y ously-to adjust the resonance of the patients circuit or the final `amplifier plate tank circuit.

Another object of ,the present .invention is the vprovision ofa new and improved diathermy equipment of the type wherein the transfer of energy to the patient is effected electromagnetically through the Iuse of a low impedance cable or applicator :drum inductively coupled, as 'by a pickup coil, to |the plate :tank circuit of :an amplifier supplied with. high frequency oscillations by a crystal controlled oscillator having the advantages set forth in the preceding paragraph.

Further objects and advantages of 'the present invention will become apparent from the ensuing description, in .the course of which reference is had to the single gure of the accompanying drawing illustrating schematically one embodiment -of 'the invention.

It may be well 'brie-fly to refer to the primary components of one embodiment of the apparatus of the present invention before proceeding with a detailed description thereof. The apparatus comprises, in the main, an oscillation generator and electron -coupled amplier I0, the output of which is supplied to an exciter amplifier I2 supplying -a nal amplifier I4 having a pair of output Orpatients .terminals I6`` 1n .the embodiment fof the invention illustrated, these terminals .are adapted to receive an applicator drum or a cable of a Itype commonly used with high frequency therapeutic apparatus, which has not been illusvtrated. The apparatus inclu-des, in addition, a low voltage power supply unit Vlll, .a high voltage metal.

The oscillation generator and electron coupled -amplifier l0 is piezoelectric crystal controlled and of the harmonic type. It is preferred that the crystal, indicated by the reference character 36, be a quartz crystal and that the output of the oscillator be taken at the second harmonie of the crystal frequency. In the instant embodiment,

the crystal has a frequency of 6.830 megacycles anda very low frequency drift with temperature. The oscillation generator and electron coupled amplier comprises an electron tube 32 (preferably cf the RCA type 6L6, a metal beam tetrode) including an anode 34 a screen grid 36, a control grid 348, a cathode 40, and a cathode heater 42 connected by conductors 44 to a secondary Winding 4G of transformer 22. A high frequency bypass condenser 48 is connected across the terminals of .the heater 42.

Thecathode of tube 32 is connected -to a ground (at ground for radio frequency only) bus wire 5D through a self-biasing resistor 52, shunted by a radio frequency by-pass condenser 58, and a cathode reactance tank 4circuit comprising inductance coil 54 and an adjustable air spaced condenser 56 tuned to a frequency approximately halfway between the -crystal frequency and the second harmonic, i. e. to a frequency of about 10.245 megacycles. This tank circuit provides plate impedance and radio frequency excitation (or feedback) to the control grid 38. The crystal 3e, which is in the grid circuit, acts as a very high Q oscillation circuit and determines the oscillator frequency. Grid bias is furnished to the tube by the cathode resistor 52 and the grid leak resistor 60 connected across the ground conductor 50. The plate and screen grid currents of the tube '32 produce a voltage drop across vthe bias resistor 52 and the rectified radio frequency grid current produces a direct current voltage drop across the resistor 50, with the result that the two voltage drops bias the grid.

The electron coupled output circuit of the oscillator tube includes a, plate tank circuit comprising inductance coll 64 and a variable air spaced condenser 66 tuned to a frequency of about 13,660 megacycles, i. e., the second harmonic of the crystal frequency. It is preferred that this tank circuit be tuned slightly off resonance in order to decrease the voltage applied to the grid of the tubeof the exciter amplifier l2. The tank circuit is connected to the plate 34 by conductor 68 and to the positive terminal of the plate supply voltage (to be described in detail hereinafter) by conductor 10. A radio frequency by-pass condenser 'I'2 is connected between the tank circuit and ground conductor 50. The screen grid is also by-passed to ground for radio frequencies by a by-pass condenser 14 connected across the lscreen voltage supply conductor 16 and a conductor 18 leading to the ground conductor 50. The conductor 18 is connected also to one of the terminals of the heater 42.

The tube 32 should have a very small grid-toplate capacitance (as is the case with the type of tube mentioned) in order that there be negligible energy fed back from the plate to the grid.

`Furthermore, the -plate and screen currents are limited well below the tube ratings by using re1- atively low plate and screen voltages and by applying a suitable grid bias through the use of the cathode biasresistor 52.

The exciter amplifier l2 comprises a tube 82 `(of the G. E. type Gli-814, a. transmitting beam -pled to the oscillation generator through a relatively small coupling condenser 96 connected to the plate end of the plate tank circuit 64, 66 and a grid reactor 98 connected to the grid and to a source of biasing voltage (to be described in greater detail hereinafter) by a conductor |00. The loose coupling afforded by condenser 96 cuts down the excitation of exciter amplifier I2 applied to it from the oscillator, the oscillator being made relatively powerful to render it more stable. The radio frequency input circuit for the amplifier tube is completed by a by-pass condenser |02 connected across conductors and |04, the latter of which is connected to conductor 50 and to the cathode 90 through the by-pass condensersV |06. The screen grid 86 of tube 82 is by-passed to ground through a radio frequency by-pass condenser |08. The ground conductor 50, it should be noted, is connected to the chassis of the exciter amplifier and thus to the metal cabinet through a radio frequency by-pass condenser ||0. This connection should be'made at a location having a zero or minimum radio frequency voltage.

The negative terminal of the plate supply is preferably not grounded. to the chassis upon which the apparatus is mounted and the cabinet, which is conductively connected to it, for safety reasons. There is less dang-er of accidental contact with high voltages by a person working on the apparatus if the negative plate voltage is insulated from the cabinet and chassis.

The negative plate supply circuit and the chassis are, however, maintained at ground with respect to radio frequency by the by-pass condenser ||0 and other condensers hereinafter to be referred to. The condenser H0 is connected across the negative plate supply or ground conductor 50 and the chassis, while the chassis and cabinet are connected to a good ground.

'I'he exciter amplifier output or plate circuit includes'a tank circuit consisting of a variable air spaced condenser ||2 and an inductance coil I|4 tuned to a frequency of 13.660 megacycles. One terminal of the tank circuit is connected to the anode 84 b-y a conductor I IB, and the other is connected to a plate voltage supply through conductor ||`8 and to ground through a, radio frequency by-pass condenser |20. The plate voltage supply conductor ||8 has a radio frequency choke |22 interposed therein isolating the direct current plate voltage from radio frequency voltages. The screen grid 86 of the tube is connected to `a source of screen voltage through a. conductor |24.

The amplifier is prevented from self-oscillating because the screen grid is by-passed to the beam-forming plates of the tube and to ground by the condenser |08, and because the grid circuit is shielded from the plate circuit to prevent any external plate-to-grid coupling. In addition, both terminals of the filament are held at ground potential, in so far as radio frequency is co`n' cerned, by the condensers |06 and ||0.

The fixed grid bias applied to the grid 88 of tube 82 through conductor |00 is sufficient to limit t-he plate current to a value such that the plate and screen dissipation will be well below their rated values when radio frequency grid excitation is removed asv by removal of or damage to the crystal 30.

The tubes of the final amplifier I4, which are operated as class C amplifiers, are preferably of a low impedance type operable at relatively high direct current plate voltages. In the present embodiment, the two tubes and |32 (type G. E. FiF-265) of the final amplifier have a high grid-.plate transconductance (6,000 micro mhos) a high mu (amplification factor, of about '15), operate at a direct current plate voltage of about 1500 volts, and are connected to provide pushpull amplification. The high Inu characteristics of the tubes provide suicient plate current cutoff at zero bias, when no radio frequency Voltage is supplied to the amplifier, so that the plate dissipation rating is not exceeded even when grid excitation is missing, as would happen if the crystal were removed or damaged.

The output of amplifier |2 is supplied to the final amplifier through a radio frequency transmission line |34 of the concentric cable type. The line is of low impedance and grounded and terminates at each end with two turn coils |36 and 33. The former is inductively coupled to coil ||4 of the amplifier plate tank circuit, and the latter is inductively coupled to a center tapped inductance coil |40 forming part of a grid tank circuit including also a dual section variable air spaced condenser |42, the center plate of which is connected to the chassis. The terminals of the tank circuit are connected by conductors |44 and |46 to the grids |48 and |50 of the tubes |30 and |32, respectively. The filamentary cathodes |52 and |54 of the tubes are connected in parallel by the conductors |56 which are connected by conductors |51 to the secondary winding |58 of the filament transformer 22. A radio frequency by-pass condenser |60 is connected across con-ductors |56 and the separate conductors are connected to the chassis by the radio frequency by--pass condensers |62 and |64 symmetrically arranged relative to conductors |56. These condensers provide return circuits for the radio frequency grid and plate currents and keep the filaments at ground for radio frequency cur-rents.

Grid bias is supplied to the grids |48 and |50 of the two amplifier tubes by a grid leak resistor |66 connected to one of the. conductors |56 and to the center tap of inductance coll |40. A rectified radio frequency grid current flows through the resistor |58, thereby setting up a direct current voltage providing bias for the grids.

The final amplifier includes also the plate tank circuit having, in accordance with one of the Afeatures of the present invention, a very high full load effective Q, and comprising a two-part inductance coil |68, a condenser |10 of the fixed air spaced type, and a trimmer condenser |12 also of the air spaced type and preferably constructed to have but a limited range of adjustment. Coil |88 and condenser |10 are preferably constructed and arranged so as to be symmetrically located relative to each other and ground, i. e., the chassis. The plate tank circuit is connected to the anodes |14` and |1i6 of tubes |30 and |32 through conductors |18 and |80. The

trimmer condenser |12 is adjusted with the patient circuit coupling coil |82 set4 for zero coupling, in a manner to be described ingreater detail hereinafter. Plate voltage for` the two tubes is supplied by a conductor |84; connected to the midpoint of inductance coil |68 and having interposed therein a radio frequency. choke coil |86. A by-pass condenser |88 serves, together with the coil |86, as a filter toisolate the direct, plate current voltage from the radio frequency voltage. The condenser |88 also. maintains the plate circuit at ground potential with respect to radio frequency and it is symmetrically located relative to condensers |62 and |64-, The plates of the condensers |18 and |12 are connected to eachother and by conductor |88 to the .plate voltage supply line |84 so that. there is novhigli direct current potential between said plates and so that the only voltage across the capacitors is the radio frequency voltage appearing across the tank circuit. 1 Selfeoscillation of the final amplifier is prevented by neutralizing the feedback voltages due to the internal plate-to-grd capacitance of the tube by. the variable -condensers |98 and |92. The former connects the plate. |18 of tube- |32 to the grid |48 of .tube |38, and the latter connects the -plate |14- of tube |30 to the grid. |50 of tube |32. The self-oscillation is furtherl prevented by shielding and isolation of the external plate and grid circuits to prevent plate-togrid coupling.

The patients circuit, including a cable (not shown, or an applicator drum, also not shown) adapted to be connected to the conductor terminals I6, comprises the previously mentioned two-turn pickup coil |82. The output circuit includes also a pair of variable air spaced 'tuning condensers |911 and IQGin'terpGSed between the terminals I6 and opposite terminals of .the pickup coil and adapted to be adjusted to tune the patients circuit into resonance with the final amplifier. The condensers are mounted on a com-mon shaft and are adjustable, as by means of a control knob |91 suitably connected-to the shaft, as through reduction gearing, to provide an accurate contro-l.

The amount of energy transferred to the patient may .be varied by adjusting the coupling between the pickup coil |82 andy the coil |68 of the final amplifier plate tank circuit. This adjustment is effected by movement of a support ist upon which the pickup coil |82 may be suitably mounted. The support is Ipreferably made of insulating material, and it may be moved in some suitable manner (not shown) by movement of a knob 280 or other device having an indicating pointer and operatively connected to the support |98.

One of the important features of the present invention is the provision of an apparatus which is capable of providing an adequate energy input to the patient under al1 conditions, in which excessive plate dissipation of the tubes is prevented even under the worst condition of out of resonance operation of the unloaded'applicator with full coupling between the final amplifier and the patients circuit, and which does not require substantially continuous adjustment of resonance of the patients circuit and of the final amplifier plate tank circuit. These desirable characteristics are attained by utilizing (l) a loose coupling between the patients circuit and the plate tank circuit of the final amplifier, (2) a final amplifier plate tank circuit having a very 8 high full load. effective Q, and; (3) preventing deformation of the tankcircuit elements asarrefsult of temperature changes.

The patients circuit, as, heretofore brought out, is not coupled to a self-excited' oscillator, but. is coupled to the oscillator throughseveral stages of amplification. The push-pull final amplifier feeds the high frequency energy to the patientfs terminals and thence through acable or other applicator device to a patient. The patient-1 and applicator constitute a load Whose resistance and reactance vary over a wide range,.andwhich. reactance may or may. not be tuned out of the circuit. Thexreflectedreactance inthel tank circuit willnot, however, changethe frequency of. the oscillation generator by reason ofv theinterposition of the amplifiers between the patients terminal and the oscillation generator. Variations in the plate tank reactance due tothe reflected reactance from the patients circuit are mini mized by reducing the coupling betweenthe. plate tank. circuit. and the patients circuit. Thereflected reactance is in effect in shunt to the plate tank reactance and is proportional to the degree of couplingA between the tank circuit and patients circuit.

Since there is a limit to the amount'y the maximum coupling can be reduced' and still have enough coupling to provide sufficient energy transfer to the patient, and because ofthe possibility that an operator may not always properly tune the patients circuit by adjustment of condensers |94 and |96, there exists a possibility that some reactance will be reflected into the plate circuit. Further to minimize the reactance change in plate circuit, the capacitive and inductivere:- actance components of the plate tank circuit are designed with very low reactanceV values, i. e.. reactances having a very small value ascompared with the reflected reactancawhich is in effect in shunt with the tank reactance. In other words, the plate'tankcapacitance is made large `andthe inductance small in value, thereby to provide a tank circuit having a very high full load ef'- fective Q.

In an apparatus utilizing electromagnetic transfer of energy to a patient through a cable or drum, i.A e., apparatus of the character herein illustrating one embodimentv of the present invention, it is preferable to use a low impedance cable or drum so that high current can be obtained at low voltage. The low impedance, While preferable from the foregoing standpoint, creates greater difficulties because the reflected reactance variations have a greater effect on the plate tank circuit, but even these difficulties are obviatedby the plate tank circuit having avery high full load effective Q of the present invention. The present invention may be utilized also in apparatus wherein the transfer of energy to the patient is effected by the use of condenser plates. Such apparatus would preferably be operatedv at a higher frequency and utilize a high impedance patients circuit so as to provide a higher-voltage for more effective energy transfer.

The coupling between the final amplifier plate tank circuit and the patients circuit is preferably electromagnetic and provided by the tank coil |68 and the two-turn pickup loop |82, which is connected to the cable or drum through condensers |94 and 96. A tank coil found satisfactory-consists of two sections of slightly* less than three turns each (a total of abcut'five'y and three-quarters turns) of 20- guage copper tubing having an outside diameter of three-eighths inch. The

mean loop diameter is three inches and the space between tube centers is five-eighth in'ch, and the two sections are spaced two and one-quarter inches apart. A satisfactory pickup loop |82 is made of two turns of one-eighth inch outside diameter copper tubing, having a mean loop diameter of one and three-eighths inches and spaced three-eighths of 'an inch apart. The cable is preferably about twelve feet long and made of flexible braided copper tubing (384 strand, #36, tinned) surrounding a rope center and covered by ilexible rubber insulation. If a drum be used, it may be connected to the terminals by short lengths of the above described cable and it may be made of copper tubing like that used in the construction of the tank coil, the total length of cable and tubing being about twelve feet. The tubing is preferably wound spirally and placed within a Bakelite receptacle. With the cable and drum described, the condensers |94 and |96 preferably have values of about 100 mmf, these values being determined primarily by the inductance of the cable or drum and of the pickup loop, the inductance of which is kept low, as by using but two turns. In the case of a condenser applicator the pickup loop would preferably consist of more turns and additional inductances would be placed in series with the connections between the loop and tenninals; J

The plate tank capacitance of the final amplifier |4 comprises the parallel capacitances of condensers (77 mmf.) and |12 (8.5 mmf), and of condensers |90 (18 mmf.) and |92 (18 mmf.) in series with the plate-to-grid -capacitances of the tubes, the series capacitances of the plates to I the laments to the tubes, and the distributed capacitance of inductance coil |68, totaling, with the values given, about A103 mmf. The condenser values are illustrative only, and for Vthose given the inductance of the tank circuit is equal to about 1.315 micro-henries at the resonant frequency. Y

It is known that for a tank circuit of the character of that here under consideration,

e I t-rnuEl-rmu where In the instant case, the full load eiective Q is about 85, a value considerably higher than the effective values of Q in the tank circuits of radio transmitters, which range from 10 to 20. From the calculations given above, it may be noted that with xed voltage and power, an increase in Q causes an increase in the circulating tank current. The increased current would, if the high Q plate tank circuit contemplated by the present invention is used, reach a value causing excessive' heating (if not prevented, as explained hereinafter) of the tankcoil |58 and condenser |10 that would actually deform these enough to change the reactance of the tank circuit, thereby tending to nullify the stabilizing effect produced by the high Q of the circuit. This excessive heating is, however, prevented from occurring by means of an air blower indicated generally by reference character ZID, which is of the exhaust type and creates a flow of air past the tank circuit elements which are, like the blower, mounted within a suitable cabinet. The blower is arranged to.be operative whenever a main switch is closed, as will be described hereinafter.

By thus minimizing changes in the plate circuit reactance as a result of variations in the reected reactance, the space current and plate load of the amplifier tubes are maintained more constant and maintained within the rated dissipation values of the tubes. Furthermore, the instantaneous radio frequency plate voltage is prevented from materially shifting from its proper position out of phase with the instantaneous radio frequency grid voltage) so that the tube space current flows at a lower value of plate supply voltage.

The low voltage power supply unit I8 includes a transformer 2|2 having a primary winding 2I4 and a center tapped secondary winding 2|6 connected in conventional manner to a full wave rectiiier tube 218 (RCA type 5Y3), the filament of which is connected to the plate voltage supply conductor 'l0 through a, low frequency choke coil 220 and to the secondary winding 22| of the filament transformer 22 by conductors 22I-A. The center tap of transformer 2|6 is connected directly to the grid bias supply conductor |00. A voltage divider resistor 222 is connected across conductors 'lll and IDB and intermediate points thereof are connected to conductors 50 and 16, so that conductor |00 is at a more negative potential than ground conductor 50 and conductor 16 is at a potential intermediate ground and that of conductor 1U. The choke coil 220 has associated with it the two conventional by-pass condensers 224 constituting, with the choke coil, a lter. An additional filter condenser 226 is connected between conductor 50 and conductor IDS. Transformer 2 |2 includes also a second secondary winding 228 supplying power through conductors 229 to a pilot light 230 (which may be red) which therefore glows whenever the transformer is energized.

As heretofore indicated, the power supply unit |8 delivers a low voltage. In actual practice the maximum voltage appearing across the voltage divider 222 is about 285 volts. y

The high voltage supply unit 20 includes a transformer 232 having a primary winding 234 and a center tapped secondary winding 236 connected in conventional manner to the pair of rectiiier tubes 238 (of the RCA type 868A), the filaments of which are supplied with current from the secondary winding 24D of the filament transformer 22 through conductors 24|. The output of the rectifier' is connected directly to a voltage divider resistor 242, across which there thus appears a pulsating direct current voltage. The negative terminal of the voltage divider is also `connected to the ground conductor 50, while the positive terminal is connected to the plate supply conductor |84. Conductors |18 and |24 are connected to intermediate portions of the voltage divider.

The high voltage power supply unit is constructed and arranged to deliver a maximum voltage of about 1500 volts across the terminals of the Voltage divider resistor 242.

'The transformers 2|2 and 232 are supplied with energy from the combined voltage adjusting auto and filament transformer 22, their primary windings 42|4 and 234 being adapted to be connected in parallel to a portion of the primary winding 244 of transformer 22 through the

conductors

246 and 248 and upon closure of a switch 25|), -hereinafter to be called the oscillator switch. The conductor 246 is connected t0 a tap on the winding 244, whereas the conductor 2-48 is adapted to be connected to the lower terminal of the winding upon closure of switch 250, the closure of which, however, will not complete the energizing circuit unless a main switch 252 has been closed for a certain length of time.

Power is supplied to the transformer 22 from a suitable alternating current source, which may be connected to the terminals 253 of a plug rtype connector. The supply of power to the primary winding 244 is under the control of the lmain switch 252, illustrated diagrammatically as la simple knife switch (but which is preferably a circuit breaker type of switch capable vof `opening vunder predetermined overload conditions). One terminal of the primary winding 244 l(the lower one) is connected to one ofthe terminals 253 through a radio frequency choke coil 254 lcon stituting, with condenser 256, a power line filter. The other terminal of Winding 244 is shown connected by conductor 256 and a rotatable voltage adjuster arm 258 to the other terminal 25| through the switch 252, conductor 259 and a radio frequency choke coil 268 constituting, with condenser 262, a second power line filter. Adjacent terminals of

condensers

256 and 262 are connected to a third terminal 263 by conductor 264. Terminal 263 is also connected to the cabinet and chassis and is adapted to be connected to a good ground, such as a water pipe, through a suitable conductor which may be the third wire of a three-wire cable.

The voltage adjusting arm 258 is provided so that the apparatus may be operated at designed Voltage irrespective of the line voltage. To enable the operator to ascertain that .the proper adjustment has been made, there is provided a volt meter 266 supplied with energy from the filament secondary winding |51 through conduc tors |51. Twelve voltage adjustments are obtainable, each constituted by a connection from a fixed contact associated with the movable contact arm 258 and connected to a suitable tap on the transformer winding 244 (the taps being spaced at suitable intervals such as 21/2 volts)`. Only three of these connections are shown-the first, middle, and last-the others being omitted Adelay is provided by a thermal time delay relay including a coll 210, a resistance heater 212 energzable upon closure of main switch 252, and three movable switch blades 214, 218 and 218, of which the last two mentioned are operable by the coil. The switch blade 214 is a bimetallic switch blade adapted, when heated by resistance heater 212, to move to the right to close an energlzing circuit through coil 210, preferably after a delay of about 15 seconds. Resistance heaterl 12 212 -is energized immediately-uponl closure of main switch 252 through a Acircuit including a terminal 263, conductor258, the 4switch 252, conductor 288 fconnected Vto the heater, conductor 282, icontact284, the movableiswitchblade 2:16 in engagement with contactl284, and .conductor 286 leading to the .fother `terminal 253. The energizing circuit :for coil 210 closed by the bimetallic switch blade y214 is :the lsame as that for the .heater-circult to conductor 280, which is also connected to coil 218,1the Vcircuit-from coil -218 to the-other power `terminal being completed Vthrough switch blade 2.14 and conductor 286. When coil "210 is energized it completes a holding circuit for itself through conductor 1288 and switch .blade `216 in its operated position. The heater 212 is deenergized -by movement vof switch blade 21-6 to its operated-position.

When :oscillator switch 250 is closedafter-operation -of relay 210, iconductor -248 is connected to the lower terminal `of transformer winding 244, with the result lthat the .primary `windings 2|4 and -234a1eenergized because the fother terminals of the .,two windings are connected to winding 244 through conductor 24.6. Conductor 24.8 is connected -to the lower lterminal of winding 244 through la vcircuit including oscillator switch .250, conductor 280, switch 4blade 282 v.of an overload relay mechanism including a movable switch blade 284, 4conductor `296, switch :blade V298 of `the time ydelay relay, movable switch blade 218 in contact with it, and conductor 286. It is apparent, therefore, .that -closure of the oscillator switch .250 will .not vrender the power supply uni-ts operati-ve .unless the main power switch has been closed lto energize transformer 422 for a `short period of time.

In `order to obtain vsulllcient energy transfer when high impedance 4patient loads are `connected to the l.terminals -|.6, it is necessary to have a closer coupling between the pickup coil |82 and the plate tank coil |68 of the final amplifier than is the case when low impedance patient loads are connected to the terminals. Thus, when the apparatus is constructed to provide the requisite close coupling under these conditions, then when a very low impedance load is connected to the terminal |6, there is a possibility that overcoupling will occur and result in .the plate current of the final amplier tubes exceeding the ratings of the tubes.

The apparatus includes an overload relay mechanism to cut off the plate voltage when the plate current exceeds a predetermined maximum value, such as the rated maximum value, as disclosed and claimed in my copending divisional application Serial No. 741,769, filed April 16, 1947. According to this arrangement the plate voltage is cut off until the relay mechanism is reset by adjusting the coupling between the pickup coil |82 and the tank coil |68 to a minimum value, such as the zero point of the power control -20|) or preferably slightly below this point. Accordingly, the plate voltage cannot be vrestored until the overcoupled condition causing the overload no longer exists.

The overload mechanism includes an overload relay coil 302 connected in the plate-cathode circuit of the final amplifier tubes |30 and |32. More specifically, the relay coil is in circuit between the ground conductor 50 and the filament transformer |58 connected to the cathodes |52 and I|54 of the tubes. The circuit from the coil to the transformer winding |58 includes conductor 383 and the circuit from the transformer winding |58 to the cathodes includes conductors |51. The coil has associated with it a low frequency filter constituted by a low frequency choke coil 304 in series and a by-pass condenser 306 connected in shunt with it. The filter smooths the pulsating direct current flowing in the plate circuit of the final amplifier, thereby preventing chattering of the relay. The value of current at which the relay operates is adjustable by a resistor 308 also connected in shunt with coil 302.

The plate current of the final amplifier is measured by a direct current milliammeter 3H), also in the plate-cathode circuit of the two final amplifier tubes. After the final amplifier has been operating for a few minutes to allow the final amplifier tank circuit elements to reach their normal operating temperatures, and with the reactance in the patients circuit tuned out by capacitors |94 and |96, the plate current is proportional to the power output of the amplifier. Under these conditions the milliammeter can be used as an output meter to enable the operator to judge relative dosage.

The overload relay operates the previously mentioned switch 294 so that when the switch 294 is operated upon an overload the energizing circuit for the rectifier transformers 2|2 and 232 is broken. When the switch 294 is operated it is latched in its operated position by a spring biased latch 3|2.

In order to reset switch 294 it is necessary to adjust the coupling to slightly below the zero point. When this is done the latch 3|2 is retracted by a solenoid 3|4, the circuit for which can be closed through a pair of contacts 3|6 associated with the pickup coil support |98. These contacts are so arranged that they can be closed only when the pickup coil is manually held slightly beyond the zero setting. At this time solenoid 3I4 is energized from the winding 244 of transformer 22 through a circuit including conductor 246, the contacts 3|6, solenoid 3|4, switch 294 in engagement with fixed contact 3|8,

conductor 296, fixed Contact 298, movable switch 218 in engagement therewith, and conductor 286 When the solenoid 3|4 is energized, the latch 3| 2 is retracted to permit the return of switch blade 294 to its indicated position (to which it may be spring biased) to engage contact 292. When this happens the primary windings of transformers 2|2 and 232 are again energized.

In order to prevent intermittent on-and-off action of the overload relay under conditions in which the overload was not caused by overcoupling, as when the plate tank circuit may be out of resonance because of an improper adjustment of the tank circuit trimmer condenser 12, the pickup coil'support |98 and the contacts 3|B are so constructed and arranged that the contacts are not closed except when; the support is adjusted to move the pickup coil slightly beyond a zero coupling position. Immediately the support |98 is released, as by releasing the power control knob 200, a spring 320 returns the pickup coil support and coil to their zero positions.

The blower 2|0 is` adapted to be energized simultaneously with the energization of transformer 22 by connection thereof across

conductors

246 and 286. While the main function of the blower is to maintain constant the temperature of the final amplifier tank circuit elements to prevent change in tank reactance as a result of temperature variations, it also serves the purpose of cooling other parts of the apparatus, such as the transformers.

After the unit has been assemble-d and all the tubes have been installed and the ground connection 264 made, it is desirable to check the apparatus prior to operation. This check includes the following steps:

(l) Make certain that neither a cable nor an applicator drum are connected to terminals I6, and that the resonance control |91 and power control 20|) are at their zero positions.

(2) Turn on the main switch 252 and adjust the voltage adjusted arms 258 so that the volt meter 266 registers as close as possible to the intended operatingvoltage, which may be suitably indicated on the volt meter dial.

(3) Turn` on the oscillator switch 258 and observe the plate` current milliammeter Sill. This meter should indicate a certain current flo'w (approximately milliamperes in an apparatus of the character described, or possibly higher). This current should, however, immediately start to decrease and reach a minimum value (of approximately 90 milliamperes or less) in about four minutes. The trimmer condenser |12 should be adjusted if any of the following results are observed:

(a) The plate current exceeds a certain maximum value (about milliamperes) (b) The plate current does not reach the minimum value even after operating ten minutes.

(c) The plate current increases instead of decreasing.

(4) Should adjustment of the trimmer condenser |12 be necessary, this is done as follows: With both the power and resonance controls at zero anld with nothing plugged in the terminals IS. the unit is operated with the oscillator switch 259 in its closed position for at least ten minutes, after which the trimmer condenser |12 is adjusted to produce minimum plate current. This adjustment is rather critical and should be done carefully. In the event the plate current should be quite high. it is best then to make a preliminary adjustment and then a finlal adjustment after an interval of about ten minutes.

(5) If it is not .possible to obtain the minimum plate current reading, or if the overload relay 302 operates whenever the oscillator switch is turned on, it will be necessary to check the tubes and neutralization.

(6) If at any later time the no load plate current does not decrease to its minimum value or less in four minutes after turning on the 0scillator switch, then the trimmer condenser should be adjusted as indicated above.

After the apparatus has been adjusted as above indicated, it is ready for operation. The rst step in the operation. is to connect either a cable or drum or other type applicator to the terminals I6 and properly place it relative to the patient, and this is done in accordance with conventional modes of application of these devices. The main switch 252 is then closed, with the result that the transformer 22 is energized, as is the exhaust blower 2|0. Energization of the transformer is indicated by the pilot light (white) 268. The transformer immediately supplies current to the filaments of all the tubes, including the rectifier tubes, to condition these for operation. At the same time the volt meter 266 indicates the voltage appearing across the transformer winding |58. In the event of line voltage changes, the volt adjuster 258 may have to be adjusted. This adjustment is also usually necessary, when treatment is started, because of line voltage drop. Closure ofthe main power switch also effects energization of the resistance heater 272 and energlzation of coil 210 after a time interval, through bimetallic switch blade 214, so that transformers 2I2and 232 of .the two power supply units may be thereafter energized upon closure of oscillator switch 256. Energization of the transformers results in the application of voltages to the oscillation generator I0, the exciter amplifier I2 and the final amplifier i4, to render these operative. At the same time, pilot light 239 is energized to indicate the placing of the apparatus into full operation. Full operation of the device is also indicated by the reading of the milliammeter 3I0, which, it may be remembered, indicates the plate current of the final amplifier stage. At this time it is preferable to operate the apparatus for a period of several minutes inorder to allowr the final amplifier to reach an operating temperture at which the plate current decreases to its minimum value.

With the apparatus in! full operation, the next step is the adjustment of the resonance control 'I'97 to tune the patients circuit into resonance with the plate tank circuit of the final amplifier. This is done with the power control 200 advanced to one of its minimum coupling positions. The resonance control should al- -ways'be adjusted to give maximum plate current, the indication of resonance. Adjustment of control knob |91, it may be remembered, adjusts the'condensers |94 and '|86 to effect this tuning. The position of the adjustment depends on the particular application given to the patient. The resonance control does not act the same as the power control because, after resonance has been obtained, further change inthe `resonance control causes a reduction in plate current, i. e., such adjustment causes detuning of the circuit.

The energy transferred to the patient is controlled by the power control 200, which adjusts the coupling between the pickup coil I82 and i the inductance coil |68 of the final amplifier plate tank circuit. The energy transfer is indicated by the plate current milliammeter THU. Should the control be turned too high, i. e., to 'couple too closely the coils, thenI under certain conditions the overload relay coil 302 may be energized sufficiently to operate switch 294 from its indicated position into contact with fixed contact 3I8. This, as heretofore described in detail, results in the termination of the supply of voltage to the oscillation generator, the exciter amplifier and final amplifier tubes, because transformers 2 I2 and 232 are deenergizedby movement of switch blade 294 away from contact 292. The

`apparatus can be restored to operative condition only by adjusting the power control 20D to a minimum coupling position, in which it has to be held and from which it is returned to zero coupling position by the spring 320 acting upon the pickup coil support |98. Such turning of the power control results in the energization of the rest solenoid'SId, which withdraws latch 3I2 Vto enable contact arm 284 to move into its indicated position.

The milliammeter 3I0 indicates the directplate current of the final amplifier, which is directly proportional to the radio frequency power transferred from the oscillator to the patients circuit, provided that the unit has been in operations with the power and resonance controls at zero for a time long enough to enable the plate current to reach a minimum value before starting treatment, and `provided'th'at thefresonance control is tuned for maximum plate current during treatment. In other words, the reading of this meter is an arbitrary guide to the amount of energy generated.

The operation of overload mechanism is indicated by the pilot light 230. This pilot light is supplied with energy from the transformer winding 228 so that it is lighted whenever the oscillator switch 250 is closed and the apparatus is in operation. Should the overload-mechanism operate with the switch 259 closed, then the transformer winding 22B is deenergized and the light 230 extinguished. Thus, the extinguishment of pilot light 235 with the oscillator switch 250 closed indicates operation of overload mechanism.

While there has been described but a single preferred embodiment of my invention, many modifications may be made without departing from the spirit of the invention, as indicated herein and otherwise, and I do not wish to be limited to the precise details of construction set forth, but desire to avail myself of all changes within the scope of the `appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States, is:

l. An electro-therapeutic apparatus' of the type comprising an oscillation generator and a patients circuit, including an amplifier coupled to said generator and patients circuit, the coupling between the amplifier and patients circuit coinprising an amplifier platc'tank circuit having a very high full load effective Q.

2. An electro-therapeutic apparatus of the type comprising van, oscillation generator anda patients circuit, including a class C amplifier between the generator and circuit and having a plate tank circuit coupled to the patients circuit, said tank circuit having a very high full load effective Q.

3. An electro-therapeutic apparatus of the type comprising an oscillation generator and a patients circuit, including a coupling between the oscillation generator and patients circuit comprising a generator output plate tank circuit having a very high full load effective Q, and temperature changing means for preventing thermal deformation of elements of said tank circuit.

4. An electro-therapeutic apparatus of the type comprising an.' oscillation generator and a patients circuit, including an amplifier having a plate tank circuit coupled to the patients circuit, said tank circuit having a very high full load effective Q, and means for cooling the elements of said tank circuit to prevent thermal deformation of elements of said tank circuit.

5. An electro-therapeutic apparatus of the type comprising a crystal controlled oscillation generator and a patients circuit, including a class C amplifier coupled to the generator and patients circuit, the coupling between the amplifier and patients circuit comprising an amplifier plate tank circuit having a very high full load effective Q and comprising an inductance coil, and a coupling coil in said patients circuit loosely coupled to said inductance coil.

6. An electro-therapeutic apparatus of the type comprising a crystal controlled oscillation generator and a patients circuit, including a class C amplifier coupled to the generator and patients circuit, the coupling between the amplifier and patients circuit comprising an amplifier plate tank circuit having a Very high full load effective Q and comprising an inductance coil, a coupling coil in said patients circuit loosely coupled to said inductance coil, said patients circuit having a low impedance, and temperature changing means for preventing thermal deformation of elements of said resonant circuit.

7. An electro-therapeutic apparatus of the type comprising an oscillation generator and a patients circuit, including a coupling between the oscillation generator and patients circuit comprising a. resonant circuit adapted when the generator is operative to be heated to an extent such as to cause thermal deformation of its elements, temperature changing means for preventing thermal deformation of elements of said resonant circuit, and a switch operable to condition said generator for operation and for placing said temperature changing means in operation.

8. An electro-therapeutic apparatus of the type comprising an oscillation generator and a patients circuit, including in combination, an amplifier coupled to the generator and having a 25 Number plate tank circuit coupled to the patients cir- 18 cuit, said tank circuit having a very high full load eiective Q, and means loosely coupling said patients circuit to said tank circuit, said patients circuit having a low impedance.

ROLLAND H. MAXSON.

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

UNITED STATES PATENTS Number Name Date 1,083,085 Gowing et al. Dec. 30, 1913 1,338,812 Carlson et al. May 4, 1920 1,927,926 Dow et al Sept. 26, 1933 1,943,619 Mudge et al Jan. 16, 1934 1,962,796 Wappler June 12, 1934 2,017,858 Halstead Oct. 22, 1935 2,097,868 Beard Nov. 12, 1937 2,190,282 Browner Feb. 13, 1940 2,198,073 Bayless et al. Apr. 23, 1940 2,333,760 Babo et al Nov. 9, 1943 FOREIGN PATENTS Country Date 681,396 France May 14, 1930 Certificate of Correction Patent No. 2,448,540. September 7, 1948. ROLLAND H. MAXSON It is hereby certified that errors appear in the printed specication of the above numbered patent requiring correction as follows:

Column 4, line 54, for about 13,660 read about 13.660; column 11, line 47, for Winding 157 read winding 158; column 14, line 13, for the Word adjusted read adjuster; column 18, line 19, list of references cited, for the date Nov. 12, 1937 read Nov. 2, 1937; and that the Said Letters Patent should be read With these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 7th day of December, A. D. 1948.

THOMAS F. MURPHY,

Assistant 'ommzssz'oner of Patents.

Certificate of Correction Patent No. 2,448,540. September 7, 1948. ROLLAND H. MAXSON It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

. Column 4, line 54, for about 13,660 read about 13.660; column 11, line 47, for Wihding 157 read winding 158; column 14, line 13, for the Word adjusted read adjuster; column 18, line 19, list of references cited, for the date Nov. 12, 1937 readv Nov. 2, 1.937;

and-ftbat the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofce.

Signed and sealed this 7th day of December, A. D. 1948.

THOMAS F. MURPHY,

Assistant Uommz'ssz'oner of Patents.

Column 4, line 54, for about 13,660 read about 13.660; column 11, line 47, for winding 157" read 'winding 158; column 14, line 13, for the Word adjusted read adjuster; column 18, line 19, list of references cited, for the date Nov. 12, 1937 read Nov. 2, 1.937;

'and-' that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofce.

Signed and sealed this 7th day of December, A. D. 1948.

THOMAS F. MURPHY,

Assistant Govmnz'ssione'1l of Patents.