CA1317687C - Portable x-ray system with ceramic tube - Google Patents

Abstract

PORTABLE X-RAY SYSTEM WITH CERAMIC TUBE
Abstract of the Disclosure A small, hand held X-ray machine is constructed to be readily portable, including a portable power supply, yet is sufficiently powerful to take an X-ray of a human patient. The X-ray machine includes a ceramic X-ray tube which is resistant to shocks and vibrations, structurally strong, as well as light weight, further contributing to the convenient portability of the X-ray machine.

Description

( 1 31 76~7 ~: ~ PORTABLE X-RAY_SY_ EM -~
WITH CERAMIC TUBE

: Backqround of Invention X-ray technolo~y has made many recent :, .
~` : advances, particularly in the more sophisticated : : :
" : systems wherein large units are provided ~or taking `j X-rays of:the ob~ect whether it be a human patient, or an industrlal paFt that must be examined.
. Presently, there is a need for a small handheld X-ray unit with a self-contained power source for use in emergency f~ield oare, such as at the scene of a common accident. 51ncm the accident scene may be ~ ahao~tic, the~need for a ~machlne ~of considerable : ~ :
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1 3 1 76~7 simplicity is required. For such units, exposure control should be simple and automatic, similar to an aim-and-shoot camera. ~ore specifically, for such units it is desirable to turn the unit On, insert the receptor film, verify that the ~Ready~
light is On, and then aim and shoot.

Summarv of the Invention According to the need expressed ~bove, the present invention provides a small, handheld X-ray system machine that is readily portable, that is efficient, that includes a aeramic X-ray tube, that includes a portable power supply, and that is sufficiently powerful to take an X-ray of a human patient and provide various high performance features.

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~1 Brief Descrlptio~n of the Drawinas .i .
Novel features and advantages of the present invention in addition to those mentioned above will ; become apparent to those skilled in the art from a reading of the following detailed description in conjunction with the accompanying drawing wherein:
`~ ~ Figure 1 is a sketch showing a side view of the inventive handheld X-ray unit;
Figure 2 i5 a block diagram of the circuitry :~ :
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', of the handheld X-ray unit of Figure 1, Figure 3 is a circuit diagram of the capaci~or discharge switching regulator of Figure 2;
Figure 4 is a circuit diagram of the filament control circuit of Figure 3;
Figure 5 is a relatively enlarged perspective view of the ceramic tube of Figure 2; and, Figure 6 is a side view, partially in section of the ceramic tube of Figures 2 and 5.

Description of the Invention Figure 1 shows the inventive hand held X-ray unit 11, formed essentially in a C-shaped configuration.
As mentioned above it is s~all, compact, and readily portable. The X-ray unit 11 includes a vertical post 14, an upper arm 16 and a lower arm 18. A grip handle 20 is centrally attached to post 14. A
control panel 25 (see Fig. 2) is also mounted on post 14. A power cable 22 connects via handle 20 to a power pack 17 which comprises, in one embodiment, NiCad rechargeable batteries 19 connected in parallel for charging, and connected in seriee for operation, as will be dlscussed. In : the e~mbodiment of Fig. 1, arms 16 ::

, ,, ' , ~ 1317687 and 18 are hingeable as at 12 for nesting or folding onto post 14.
Refer now also to Fig. 2. An X-ray tube head 24 is mounted in the upper arm 16. Tube head 24 consist~ of an X-ray tube 2G, the high voltage generation circults 28, and a beam limiting cone 30. A receptor asse~bly 32 i5 mounted in the lower arm 18 and includes a conventional X-ray grid, a film eassette holder, and a sensor unit.
The power pack 17 comprises the Nicad batteries lg, energy storing electrolytic capacitors 21 and a capacitor charging circuit 23.
The embodiment shown uses three O.Olf electrolytic eapaeitors whi~h are charged to 305 volts. Ten 1.2V NiCad eells are arranged in two banks o five cells each. The banks are in series for use and in parallel for charging with the parallel/series connection b~ing made at the cable 22 connection to the power pack 17. ~hen the cable 22 is removed, a charging cabIe (not shown) can be connected to charge the batterles. The charging cable can be fed from a variety of charging circuits rànging fro~ a series resistoricigarette lighter plug such as used on a passenger vehicle, or a small ~ D~ 4 ~ ~ ~ "'`''" :
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~ `" 1 31 76~7 appliancc charger, or a regul~ted fast-charger device.
The battery pack 17 is capable of providing power ~or up to fi~ty exposures and can be recharged from almost any external source.
Charging the energy storage capacitors 21 for the first exposure, requires less than three minutes.
Additional exposureæ can be taken at 30 second intervals.
A battery indicator circuit 74 indicates the status of the battery voltage to the user so as prevent exposures when the batteries 19 voltage is below a s~t threshold.
A capacitor discharge switching circuit 50 controls the selective discharge o the energy . storing capacitors 21. The capacitors 21 provide the energy needed to power the X-ray tube 26 as will be descri~ed.
The principle of operation is analogous to a .
photographic ~lash gun in that energy is accumulated in the electrolytic capacitors 21 and is selectively discharged to power the X-ray exposure. The energy storage capacitor charging circuit 23, of suitable known design, comprises a ;suitching converter wh;ich: supplies 400 volt :
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'' , charging spikes to the storage capacltors 21. ~In Figure 3, the storage capacitors are indicated as a single capacitor 21 for purposes o simplicity in drawing.) In operation, the voltage on the energy storage capacitors 21 diminishes exponentially as energy is withdrawn; however, as will be described~ the capacitor switching regulator discharge circuit 50 (Fig. 2 and 3) i5 a switching regulator which provides a constant voltage output o~ 65 to 160V, depending on the desired X-ray tube voltage.
; The cwitching regulator scheme of Figure 3 is more efficient th2n a linear regulator, but normally has the characteristic o generating radio frequency interference. Attention must thus be given to power supply bypassing, grounding, shielding as will now be described. .In conventional step down converter schemes, a P-channel FET (Field Effec~ Transistor) or a PNP
bipolar transistor is used as the switch between the DC input and the assoclated inductor. P-channel FETs~ are not ~ava~ilable which have sufficient current and voltage ratings for the present applicatlon.~ Accordingly, the invention 6 ~ ~

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' incorporates N-clanlel MOSFET's with the drive circuitry, to permit switching to ground in order to reduce interference and noi-~e.
Refer now to Fig 3 in more detail. Capacitor 21 has one terminal or plate A connected to the charging circuit and its other terminal or plate B
connected to the upper terminal of inductor 56.
The lower terminal of inductor 56 is connected to the upper terminal o a toroid 57. Also the lower terminal B of capacitor 21 is connected through diode 60 to ground. The output terminal of toroid 57 provides a regulated output. As shown in Fig.
3. the DC input source, that ls the storage capacitor(s) 21, are positioned after MOSFET
switches 52 and 53 rather than before the switches. The negative terminal of the storage capacitor~s) 21 are no longer tied to ground. This results in the DC output pulse being negative with respect to ground.
A pulse width modulation integrated circuit chip 54, which is of conventional design, alternately gates the MOSFET;switches 52 and 53 On and Off. Each of the MOSFET switches 52 and 53 switches at lOkHz for an effective regulator ~ switching frequency of 20kHz. By alternately : : :~
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' '' ' ' " '' . ' firing the MOSFETs 52 and 53, each MOSFET carries the load current for h~lf the length of time as compared to single MOSFET operating at 20 }cz; thus reliability is increased. When either MOSFET 52 and 53 switches ON, energy is transferred to the load. When the MOSFET switches are Off, energy stored in the inductor 56 is fed to the load.
The junction of inductor 56 and toroid 57 is connected through resistor SB, potentiometer 59 and resis~or 65 as an input to the pulse modulator chip 54. Thus, re istor 58, potentiometer 59 and resistor 65 close the loop between the variable output load voltage, and the pulse modulator 54.
In the embodiment shown, the on-time of the gating pulses is varied to regulate the output voltage from -65 to -160V. The voltage across the X-ray tube is proportional to the regulator output and is adjusted by the potentiometer 59.
The resistor 66 oonnected across the filter capacitor 67 hol`ds the output near ground when the capacitor charging circuit 23 and capacitor discharge switching regulator 50 are idle. The reslstor 66 and 11ter~apacltor 67 prevent the output from~drifting and will~ assure the charging , : :

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~, - " - 1 31 7687 circuit 23 is turned off when the storage capacitor 21 voltage level has reached the desired full charge level. The inductor 56 stores energy between switching cycles of the FET's 52 and 53.
The inductor 57 is a current limiting device for the saturated power inverter which feeds the high voltage supply. Because of the distributed capacity of the secondary winding of the high voltage transformer, a portion of the inverter cycle is spent charging/discharging that stored charge. An alternative configuration of the circuit combines the functions of inductors 56 and 57 by obviating capacitor 67 and decoupling potentiometer 59.
The outputs of the pulse modulator chip 54 are low until the input pin (pin 10 in the embodiment shown) is pulled low from its normally high state by a di~charge control signal. When the input pin is pulled low by the discharge control signal an exposure is taken for the duration that the input pin 10 is held low. Ihe capacitor 61 connected to pin 8 controls the fall time of the negative output pulse and, to some degree, the leading edge overshoot. The resistor 62 connected between input pins 1 and 9 of pulse modulator chip 54 determines the gain provided by chip 54. The RC networks 63 and 64 determine the switching frequency of the chip 54.
In operation, the capacitor(s) 21 are selectively discharged to produce a short, powerful exposure. The voltage from capacitor(s) 21 is applied to a saturated high frequency ~25 kHz) power inverter 70 which feeds the high voltage transormer 71. A voltage multiplier 28 converts the high frequency energy from transformer 71 to a ~: : 9 ~:

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, , 13176~7 constant potential to operate t.he X-ray tube 26.
The tube 26 filament transformer 72 is controlled by filament control circuit 80 also operates at a high frequency to permit it to be made very small.
As mentioned above, capacitor discharge energy systems normally have the problem of a diminishing voltage on the capacitor during discharge.
Importantly, s~itching regulator circuit 50 provides a voltage to the X-r~y tube 26 which is constant during discharge, such that up to 70% of the energy stored in capacitors 21 can be used ~or the production of X-rays. The X-ray power level of the inventive system is adequate for a motion-stopping image of the chest ard capable of penetrating an average abdomen.
The negative DC voltage pulse. from tha discharge regulator is fed to the main output inverter 70. As mentioned above, the inverter 70 converts the DC voltage to an AC voltage so that the voltage Gan be stepped up by the high voltage transformer 71 and voItage multiplier 2~.
The X-ray tube anode potential lS set by control of the output voltage of the dischar~e : :regulator 50. The combination of the switching : ~ ~ : 1 0 , :

, . , 1 31 76~7 regulator 50 and the saturated inverter 70 keeps the circuit simple and efficient since no variable resistance elements are used.
The high voltage multiplier circuit 28 receives the output of the high voltage transformer 71 and steps it up to 40-9OkV across the X-ray tube.
An ion chamber 76 sense circuit provides an ion sense input signal to the ion chamber sense circuit 77 which in turn provides a signal to an exposure control/timing circuit 78. During an exposure the ion sense signal proportionally reflects the number of ions created in the ion chamber 76. When enough ~ions for the proper exposure have been collected a signal is provided to terminate the exposure. The Automatic Exposure Control (AEC)/timing circuit 78 receives a signal from the ion chamber. The AEC
circuit determines the timing mode, and times the X-ray exposure in the selected timing mode. In the AEC mode, the length of the exposure is determined by the ion chamber 76 and ion chamber sense circuit 77.
The floating power supply circuit 79 provides the floating supply operating voltages that are required such as by the inverter circuit 70, and :: : :
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~ ' to bias the ion chamber 76.
The filament control circuit 80, shown in Figure 4 consists of a pulse width modulator 81, a pulse width modulator 82, a switching regulator 83, and two switching FETQ 84. The circuit 80 provides a tightly regulated current needed for the filament of the X-ray tube 26. The filament control circuit 80 in essence comprises as it.q major subcomponents, a voltage regulator including pulse width modulator 81 and switching regulator 83, and a saturated inverter comprising modulator 82 and FETs 84.
The voltage regulator portion comprising units 81 and 83 of the circuit 80 provides an adjustable and extre~ely stable voltaye to the saturated inverter comprising units 82 and 84. The saturated inverter (82,84) converts She DC regulated voltage to a 20kHz square wave which iæ then fed to the filament transformer 72. The transformer 72 provides isolation between the control circuitry and the filament as well as lowering the average DC
voltage level supplied to the ilament of the X-ray tube 26.
: ~ Modulator 81 provides a pulse width modulated ~ signal~ whi~ch ~ l S used ;to; qate the switching :~ . : :
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t317687( regulator 83 which comprises an integrated package concistin~ of a PNP power t~ansistor and a clamping diode. When the regulator 83 is gated ON, energy is transferred to the load. When the regulator 83 is switched OFF, energy stored in the inductor 90 is fed to ~he load. The DC regulator vol~age is monitored and adjusted via the feedback potentiometer 85 connected to the inverting input o~ the modulator 81. The DC voltage is proportional to filament current, therefore, the potentiometer 85 is used to set the filament current.
The internal output transistors of the modulator (IC) 81 are connected in parallel for single-ended output operation. With the outputs parallel, an effective du y cycle of 0-90% i~
attained and the frequency of the internal oscillator is the frequency of the output. The resistor 86 and capacitor 87 provide for an internal oscillator requency of 40kHz. A warmup siqnal is connected to the shutdown input (pin 10) of the modulator 81 so that the circuit can be switched on just prior to an exposure.
As mentioned, the saturated inverter consists of the modulator 82, two FETs 84, and the filament ' ~ ' ', , ~ '` ~

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transformer 72. The modulator 82 is synchronized to the oscillator 81 and is configured for push-pull operation. In this configuration the two outputs are alternately switched with 45X duty cycles and an overall frequency of one-half the oscillator requency; that is, 20kHz. The filament transformer 72 steps the regulator voltage down to the required 24V. The snubber across the leads o the transformer comprising resistor 88 and capacitor ~9 are needed to suppress the switching spikes on the outputs of the FETs 84.
An ion chamber 36 o~ known design is sandwiched between an X-ray grid 34 and an X-ray receptor, film cassette or Polaroid cassette 38.
Ion chamber 36 is used by the AEC circuitry in ~ . .
order to automatically time an exposure. While a var1ety of iLage receptors may be used a co~mercially available positive print type film has been found to be quite suitable. X-rays thus go through the patient and the grid, then pas.s through the ion chamber, and finally strike the receptor.
` ~ Conventional ready circuits provide a signal which is used to en~b1e an X-ray exposure and to :
~ indicate tha~t~an~e~xposure can be taken. A
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': ~ ' " '~' " : ~. -- -, ~ ' metering circuit 75 provides a signal to deter~ine whether or not enough energy is available to make an exposure.
An important part of the invention, namely, the ceramic tube 26 will now be described with reference to Figs. 5 and 6~ Prior art X-ray tubes are usually made of glass and are used within oil-filled enclosures. The prior art enclosures are lined with lead so that the X-ray beam can exit only frsm a window of the enclosure. Prior art X-ray tubes which use glass envelopes cannot withstand mechanical shock or vibration.
The ceramic X-ray tube 26 of the invention comprises a tungsten oxide based ceramic cylinder 101 as the body of the tube. Importantly, the ceramic material serves as the structural body of the tube, and also as an inherent X-ray shield. The cylinder 101 is placed in a housing enclosure 112 filled with a silicone gel 111 which bonds to the outer surface of cylinder 101~ Sllicone gels 111 can bond to the ceramic material but do not bond to glass without the addition of special primers or adhesion compounds. Metallic end caps 102 and 103 ~are brazed at the ends of the cylinder 101. The X-ray tube 26 effects a reduction of size and weight :, - ' ' ~
:, ' ` , -- 13176~7( of conventional X-ray tubes, and provides an improvement of the shielding characteristics.
The anode 104 of the X-ray tube includes a tungsten taxget 105 plated on a solid bronze or copper body, which, in turn, is mounted on end cap 103 of tube 26. The cathode 107 of tube 26 is mounted by rigid leads 108 to end cap 102.
Tungsten oxide or other exotic metal oxides which are stable at high firing temperatures, may be added to the ceramic material of tube 26. The resulting ceramic compound i5 vacuum-tight and of excellent insulating properties. A portion of the cylinder, the e~it window, is made of a compatible ceramic, but without the tungsten oxide. The use of a tungsten o~ide based cylinder means that the tube is self-shielded and obviates or reduces the amount of lead or litharge X-ray beam shielding.
X-ray tubes of smaller size and reduced weight are therefore possible. The wall thickness indicated at 114 of the cylinder 101 can be selected to provide the desired X-ray shielding to the object.
Alternatively, and as shown in Fig. 6, the ceramic cylinder 101 can be constructed to have a desired extra thickness to provide the necessary shielding.

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! ( 1317687 Also a beryillium window llOA can be for~ed for minimal filteration of the X-rays.
Also, the use of brazed ceramic tube configuration makes it possible to build tubes of exceptional resistance to shock and vibration.
The handheld X-ray machine of Fig. 1 performs as well as more powerful single phase conventional X-ray machines. There are two reasons for this:
the reduced focus to film distance requires half the energy for penetration, and, the constant potential on the tube 26 increases the actual output in MR/mAs (milliroentgens/milliampere second). The tube current of 35 mA for the handheld machine is equivalent to about 150 mA of a single phase generator. The constant potential circuit means that energy is used more efficiently by the X-ray tube.
Referring still to Fig~ 1 for operator and patient safety, a conventional X-ray tube must be shielded with a litharge (lead oxide3 or equivalent cylindrical shield, and the radiation from the open ends of the cylinder must be blocked by means of lead sheet within the housing or equivalent structure close to the tube. The use of the novel X-ray tube fabricated wlth a ceramic containing : : :

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: ~ ' ' :-oxide~ of high atomic number elements eliminates the need for the cylindrical shield and permits the design of smaller tube heads.
The ~tainless steel collimating cone 30 restricts the X-ray field to an area no larger than the receptor assembly 32 which is backed with lead shielding. The exposure switch on the conrol panel 25 is located in a position that reduces the operator exposure to a low value of backscattered radiation. A remote control exposure switch not shown, may be provided for even greater protection for the operator.
~ While the invention has been partic~larly shown and described with reference to a preferred embodiment thereof, it w1ll be understood by those skilled in the art that various changes in ~orm and .
details may be made therein wlthout departing fro.n the spirit and scope of the inv~ention.

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Claims (22)

1. X-ray apparatus comprising:

a) an energy source for providing a DC voltage;

(b) a storage capacitor;

(c) means for recharging the capacitor from the energy source;

(d) power supply means, connected to the capacitor, for providing a regulated output voltage, including an inductor connected between the storage capacitor and the output of the power supply means, a controllable switch connected between the storage capacitor and ground on the other side of the capacitor from the inductor, a pulse-width modulator providing output pulses to control the controllable switch with the width of the pulses from the modulator modulated in accordance with an input signal to provide negative output pulses from the capacitor to the inductor, and means for providing an input to the pulse-width modulator to maintain the output voltage of the power supply means substantially constant as the capacitor voltage changes during an X-ray exposure;

(e) an X-ray tube having at least an anode and a cathode;

(f) means, receiving the output of the power supply means, for selectively providing a high voltage between the anode and cathode of the X-ray tube to drive the same.

2. The X-ray apparatus of Claim 1 wherein the means for providing an input to the pulse-width modulator includes feedback means for feeding back the output of the power supply means to the input of the pulse width modulator.

3. The X-ray apparatus-of Claim 1 wherein the controllable switch comprises two FETs connected in parallel between the capacitor and ground and wherein the pulse-width modulator is connected to the gates of the two FETs to drive them alternatively on every other pulse of the pulse-width modulation so as to split the duty cycle for the pulse width modulation between the two FETS.

4. The X-ray apparatus of Claim 1 including a capacitor and resistor connected in parallel between the output of the inductor and ground to filter the output voltage of the power supply means.

5. The X-ray apparatus of Claim 1 including means for selecting the DC output voltage from the power supply means.

6. An apparatus as in Claim 1 wherein the energy source to charge said capacitor is a storage battery.

7. An apparatus as in Claim 1 wherein the energy source to charge said capacitor is a power line of limited capacity.

8. The X-ray apparatus of Claim 1 wherein the means for providing an input to the pulse width modulator includes a potentiometer connected to the output of the power supply means and connected to the input of the pulse-width-modulator to provide feedback of the output voltage to the pulse width modulator with the desired DC
output voltage level being set by the potentiometer.

9. The X-ray apparatus of Claim 1 wherein the means receiving the output of the power supply means includes a saturated high frequency power inverter which provides its output to a high voltage transformer.

10. The X-ray apparatus of Claim 1 including a diode connected on one side to the connection between the storage capacitor and the inductor and on the other side to ground to allow current charging the capacitor to flow to ground through the diode.

11. X-ray apparatus comprising:
(a) a battery providing a DC voltage;
(b) a storage capacitor;
(c) means for recharging the capacitor from the battery;
(d) power supply means, connected to the capacitor, for providing a regulated output voltage, including a controllable switch, an inductor connected between the capacitor and the output of the power supply means, the controllable switch connected to control the discharge of the capacitor through the inductor, a pulse-width modulator providing output pulses to control the controllable switch with the width of the pulses from the modulator modulated in accordance with an input signal, and feedback means for feeding back the output voltage from the output of the power supply means to the input of the pulse-width modulator to maintain the output voltage substantially constant as the capacitor voltage changes during an X-ray exposure;
(e) an X-ray tube having at least an anode and a cathode; and (f) means, receiving the output of the power supply means, for selectively providing a high voltage between the anode and cathode of the X-ray, tube to drive the same.

12. The X-ray apparatus of Claim 11 wherein the controllable switch is connected between the capacitor and ground, the other side of the capacitor connected to the inductor through which the output from the power supply means is provided, wherein the pulse-width modulator is connected to the controllable switch to drive the same in a pulse-width modulated fashion to provide negative output pulses from the capacitor to the inductor to regulate the output of the power supply means.

13. The X-ray apparatus of Claim 12 wherein the controllable switch comprises two FETs connected in parallel between the capacitor and ground, and wherein the pulse-width modulator is connected to the gates of the two FETs to drive them alternatively on every other pulse of the pulse-width modulation so as to split the duty cycle for the pulse width modulation between the two FETS.

14. The X-ray apparatus of Claim 11 including a capacitor and resistor connected in parallel between the output of the inductor and ground to filter the output voltage of the power supply means.

15. The X-ray apparatus of Claim 11 including means for selecting the DC output voltage from the power supply means.

16. The X-ray apparatus of Claim 11 wherein the feedback means includes a potentiometer connected to the output of the power supply means and connected to the input of the pulse-width modulator to provide feedback of the output voltage to the pulse width modulator with the desired DC output voltage level being set by the potentiometer.

17. The X-ray apparatus of Claim 11 wherein the means receiving the output of the power supply means includes a saturated high frequency power inverter which provides its output to a high voltage transformer.

18. The X-ray apparatus of Claim 12 including a diode connected on one side to the connection between the storage capacitor and the inductor and on the other side to ground to allow current charging the capacitor to flow to ground through the diode.

19. An X-ray tube filament control circuit comprising:
a) switching regulator means, receiving all unregulated source voltage, for providing a regulated DC voltage at its output, the switching regulator means including a controllable switch and an inductor connected between the unregulated source voltage and the output of the switching regulator means, a pulse width modulator providing output pulses to control the controllable switch with the width of the pulses from the modulator modulated in accordance with an input signal, and feedback means for feeding back the DC output voltage to the input of the pulse width modulator to maintain the output voltage substantially constant as the source voltage changes including means for selecting the DC output voltage level;
b) a filament transformer having a primary and a secondary connected to the filament of an X-ray tube;
c) saturated inverter means, receiving the regulated DC voltage from the switching regulator means, for providing a high frequency signal to the primary of the filament transformer, the saturated inverter means including switching devices which are switched in saturation to provide the output signal from the saturated inverter means, wherein the filament current provided from the secondary is proportional to the regulated DC voltage from the switching regulator means.

20. The X-ray tube filament control circuit of Claim 19 wherein the feedback means for the switching regulator means includes a potentiometer connected between the DC output voltage and ground and having its potentiometer output connected to the input of the pulse width modulator, whereby the DC voltage is provided to the saturated inverter means and thereby the filament current can be selected by adjusting the potentiometer.

21. The X-ray tube filament control circuit of Claim 19 wherein the saturated inverter means provides a square wave signal at a frequency of about 20 KHz.

22. An X-ray tube filament control circuit comprising:
a) switching regulator means for receiving an un-regulated source voltage and providing a regulated DC output-voltage, including a potentiometer connected between the DC output voltage and ground and having the output of the potentiometer fed back as the control signal for controlling the output voltage of the switching regulator means;
b) a filament transformer having a primary and a secondary connected to the filament of an X-ray tube;
c) saturated inverter means, receiving the regulated DC
voltage from the switching regulator, for providing a high frequency signal to the primary of the filament transformer, whereby the DC
voltage provided to the saturated inverter can be adjusted by adjusting the potentiometer and thereby adjusting the filament current from the secondary of the filament transformer.