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Claims  |
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What is claimed is:
1. An electrosurgical unit comprising:
vacuum tube oscillator means for providing an ungrounded output of a given
frequency;
power supply means including a transformer for supplying said oscillator
means with a high voltage;
coagulation output means for coupling to said oscillator means, and
providing a coagulation signal for use in coagulation procedures;
cutting output means for coupling to said oscillator means, and providing a
cutting signal for use in cutting procedures;
switching circuit means for selectively connecting said coagulation output
means or said cutting output means to said oscillator means;
intensity control means for coupling to said power supply means, and
varying the voltage supplied to said oscillator means, said intensity
control means including a variable coil in parallel with said transformer
for controlling the output voltage across said transformer;
modulation means for coupling to said oscillator means, and providing
intermittent operation of said oscillator means, said modulation means
including a multivibrator means, and coagulation level control means for
varying the duty cycle of said multivibrator means;
and operation control means in said switching circuit means for
interconnecting said intensity control means to said power supply means
when said cutting output means is connected to said oscillator means, for
interconnecting said modulation means to said oscillator means when said
coagulation output means is connected to said oscillator means, and for
disconnecting from said oscillator means one of said modulation means or
said intensity control means when the other is connected thereto.
2. An electrosurgical unit as in claim 1, and wherein said cutting output
means comprises a handpiece unit for applying said cutting signal as a
high current density RF signal, and a return plate for returning the
signal to said oscillator means, and wherein both said handpiece unit and
said return plate are isolated from ground.
3. An electrosurgical unit as in claim 1, and wherein said oscillator means
comprises first and second vacuum tubes coupled in push-pull arrangement,
and a balanced three coil transformer unit, including a first coil having
its ends respectively connected to the grids of said vacuum tubes and its
mid point coupled to ground, a second coil having its ends respectively
connected to the plates of said vacuum tube and its mid point coupled to
said power supply means, and a third output coil having its ends coupled
through said switching circuit means to said coagulation output means and
said cutting output means.
4. An electrosurgical unit as in claim 3 and wherein said transformer unit
comprises three cylindrical tubes of equal length and of sequentially
increasing diameter size, all containing aligned mounting holes at the
ends thereof, said three tubes being mounted together through the holes,
said three coils being wound respectively on said three tubes, the center
portion of each tube being utilized as the winding area to completely
balance the windings on each side of the center, thereby removing any
capacitive effects between the tubes.
5. An electrosurgical unit as in claim 4 and further comprising mounting
screws, and wherein said tubes are coupled together by said mounting
screws cooperating with said mounting holes maintaining said cylindrical
tubes in a tangentially interconnected arrangement.
6. An electrosurgical unit as in claim 3, and wherein the number of turns
in said third coil is substantially less than the number of turns in said
second coil and positioned centrally thereof, thereby providing a high
voltage across the output, a low source impedance, and reducing capacitive
coupling between said second and third coils.
7. An electrosurgical unit as in claim 1, and wherein said switching
circuit means including said operation control means comprises a first
switch interconnecting said oscillator means to a first and second
position, said cutting output means being connected to said first position
and said coagulation output means being connected to said second position,
a second switch having a first position interconnecting said intensity
control means to said power supply means and a second position
disconnecting said intensity control means from said power supply means, a
third switch having a first position disconnecting said modulation means
from said oscillator means and a second position connecting said
modulation means to said oscillator means, said first, second and third
switches normally being in their respective first positions, a first relay
coil means for switching said first and third switches into their
respective second positions upon energization thereof, control switch
means energizing said first relay coil means when in a closed position,
and a second relay coil means energized by the closing of said third
switch for switching said second switch into its respective second
position.
8. An electrosurgical unit as in claim 7, and wherein said coagulation
output means includes forceps means connected across said osciallator
means output, said control switch means being coupled to said forceps
means.
9. An elctrosurgical unit as in claim 7, and wherein said cutting output
means includes handpiece means and an operating switch thereon, said
operating switch having a cutting position and a coagulation position,
said operating switch connected in parallel with said third switch,
whereby said cutting position corresponds to the first position of said
third switch and said coagulation position corresponds to the second
position of said third switch, and wherein said cutting oiutput means can
be utilized to selectively provide said cutting signal as well as said
coagulation signal to said handpiece means.
10. An electrosurgical unit as in claim 9, and further comprising a foot
pedal switch connected in parallel with said operating switch.
11. An electrosurgical unit as in claim 7, and further comprising cutting
indicator means and coagulation indicator means, and wherein said
switching circuit means further comprises a fourth switch having a first
position connecting to said cutting indicator means and a second position
connected to said coagulation indicator means, said fourth switch
operating in conjunction with said second switch.
12. An electrosurgical unit as in claim 1 and wherein said oscillator means
includes vacuum tubes having filaments and wherein said power supply means
comprises a high voltage output circuit means for connection to the output
of said oscillator means, a low voltage output circuit means for
connection to said switching means, and a filament voltage circuit means
coupled to the filaments of said vacuum tubes.
13. An electrosurgical unit as in claim 1, and wherein said filament
voltage circuit means includes an operating voltage level output and a
standby voltage level output less than said operating voltage level
output, and further comprising a standby switch and an operating switch
connected to aid power supply means, said standby switch selecting said
standby voltage level output and said operating switch selecting said
operating voltage level output, said operating switch also interconnecting
said high voltage output circuit means to said oscillator means.
14. An electrosurgical unit as in claim 13, and further comprising circuit
protection means for preventing the simultaneous operation of both said
standby switch and said operating switch.
15. An electrosurgical unit as in claim 13, and further comprising standby
indicator means and operating indicator means for respectively indicating
which of said standby switch and said operating switch has been activated.
16. An electrosurgical unit as in claim 1, and wherein said coagulation
output means includes a forceps having spaced apart ends across which said
coagulation signal is provided, and wherein said cutting output means
includes handpiece means and a return plate across which said cutting
signal is provided, said forceps including an integral switch in the arms
of the forceps which closes when the forceps arms are brought together.
17. An electrosurgical unit as in claim 16, and wherein said cutting output
means further includes a control switch means for permitting said cutting
output means to selectively provide said cutting signal as well as said
coagulation signal.
18. An electrosurgical unit as in claim 16, and wherein said return plate
includes interlock means for providing an indication when said return
plate is physically detached from said oscillator means.
19. An electrosurgical unit as in claim 16, and wherein said coagulation
output means further comprises two wires respectively connected to said
spaced apart ends and capable of interconnecting to said oscillator means,
and capacitor means respectively interconnected in each of said two wires.
20. An electrosurgical unit as in claim 16, and further comprising housing
means containing thereon a first plug means connected in said coagulation
output means for connecting thereto said forceps, a second plug means
connected in said cutting output means for connecting thereto said
handpiece means, a third plug means connected in said cutting output means
for connecting thereto said return plate, a first switch means connected
in said power supply means for activating said power supply means in a
standby condition, a second switch means connected in said power supply
means for activating said power supply means in an operating condition, a
first dial means for operating said variable coil and a second dial means
for operating said coagulation level control means.
21. An electrosurgical unit as in claim 20, and further comprising a foot
control unit, and wherein said housing means further contains thereon a
fourth plug connected in said switching circuit means for connecting
thereto said foot control unit.
22. An electrosurgical unit as in claim 20, and further comprising first
indicator means providing an indication when said unit is in a standby
condition, second indicator means for indicating when said unit is in an
operating condition, third indicator means indicating when said
coagulation output is being supplied, fourth indicator means for
indicating when said cutting output is being supplied and interlock
warning indicator means for indicating when said return plate has been
disconnected from said housing means. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to electrosurgical devices and more particularly to
an improved electrosurgical unit capable of providing an unmodulated
signal for cutting tissue and a modulated signal for coagulation.
High frequency oscillations have been utilized for various
electrotherapeutic purposes. Some devices utilize the electrostatic field
produced by a high frequency oscillator for surgery, coagulation, or
sterilization of utensils. For example, U.S. Pat. No. 1,945,867 teaches
the creation of such high frequency oscillatory electrostatic field which
is utilized for electrotherapeutic purposes. More recently, use has been
made of the high frequency electrical current produced. Electrosurgery has
been carried out based upon the ability to localize and control the
heating effect from such high frequency electrical current. Such electric
current is localized at a sharp point, usually by means of a pointed
electrode, to create a high current density which provides the intense
localized power needed for tissue effect. A return electrode, usually a
large plate positioned under the patient, returns the current back to the
electrosurgical unit. By having a rather large return plate, the current
density is dispersed, causing a low current density at the contact with
the return plate.
It has been found that tissue cutting can be produced by utilizing an
undamped signal, while coagulation can be achieved by utilizing a damped
frequency signal. Spark gap oscillators generally produce damped waveforms
while vacuum tube oscillators produce undamped waveforms. As a result,
many electrosurgical devices providing both tissue cutting and tissue
coagulation outputs will utilize a spark gap generated waveform for
coagulation, and a vacuum tube oscillator for tissue cutting. Typical of
such unit is described in U.S. Pat. No. 3,058,470. Other electrosurgical
units will rather utilize a single oscillator which alternates between
damped and undamped signals. For example, U.S. Pat. No. 3,261,358 provides
such alternating output. Also, U.S. Pat. No. 3,478,744 provides a
modulated output which finds use for both cutting and coagulation. While
vacuum tube oscillators are generally preferred, many units do not use
them because they are usually bulky and heavy and require a long amount of
warm up time during the turn-on periods.
In general, all electrosurgical units provide a return electrode,
frequently called the dispersion electrode, the indifferent plate, or the
butt plate. Most such prior art units extract the oscillator output across
an output transformer which has either one end, or a midpoint grounded,
whereby the return plate operates as a ground plate. The use of this type
of grounded output unit has created many surgical problems and numerous
patient injuries. The main purpose of the return plate is to disperse the
current and create a low current density contact between the patient and
the return path. If the patient accidentally touches a piece of grounded
metal, such as the operating table, there will occur a grounded return
path at that point of contact. However, the contact point will be very
small which will result in a high density current causing a burn at the
contact point. Even if precautions are taken to prevent contact between
the patient and the operating table, it is practically impossible to avoid
complete contact, because of conductive paths provided by the spillage of
blood, or saline solutions. Additionally, there generally exists
capacitive paths between the patient and ground which can also cause
return paths to ground with possible burns at the points of close contact
between the patient and ground. In recent times where numerous monitoring
units, such as EKGs, ECGs, etc., are connected to the patient during a
surgical operation, the point of contact between such peripheral
electrical equipment and the patient also causes the possibility of high
current densities flowing at such points of contacts which may also cause
burns as the current flows through the equipment to ground. A further
hazard can result if the return cable to the electrosurgical unit breaks
or if the return plate accidentally becomes disconnected. The current will
then seek alternate ground paths through the patient. Such alternate
contacts will frequently be over a very small area causing severe patient
burns.
A solution has been presented to provide an isolated output unit where the
return plate is ungrounded and is in fact isolated from ground. In this
way, the current will not seek ground contacts as return paths since the
electrosurgical unit is isolated from ground. However, it has heretofore
not been possible to obtain a very good isolated output and frequently,
stray pathways to ground within the unit defeated the attempted isolation.
Since the accidental disconnection of the return plate can cause burns in
the patient as well as other hazardous conditions, many prior art units
contain sensory warning devices to give an indication when such
disconnection occurs or when the cable is broken. However, in most prior
art units, even though the return plate is disconnected, the probe will
still provide the high current density and continue cutting tissue,
thereby continuing the possibility of burns.
Prior art electrosurgical units have also presented other problems. In many
cases it is desired to provide alternately either a coagulation signal or
a cutting signal. Some units have provided two separate output probes, one
for coagulation and one for cutting. However, frequently, both units are
simultaneously activated so that while one of the probes is being used,
someone may accidentally pick up the other probe and burn their hand. Some
electrosurgical units only provide a single probe for alternately
supplying a cutting or a coagulating output. With these units, however,
when the surgeon is utilizing the single probe, it is not possible for an
assistant to provide coagulation support to the surgeon.
Another difficulty with prior art electrosurgical units is in connection
with the magnitude of the coagulation or cutting output voltage. It is
necessary to control the magnitude of these outputs depending upon the
depth of cut, the impedance provided by the patient, and various other
factors. Most units do provide some type of intensity control. However,
the intensity control set for a cutting procedure may not be suitable for
a coagulation procedure. As a result, it is necessary for the surgeon to
reset the unit as he alternates between coagulating and cutting.
Furthermore, in providing the modulated output for coagulation purposes,
most electrosurgical units provide a single level of modulation, and
usually use a standard 60 cycle per second output. However, it has been
found that the patient acts as a rectifier for such 60 cycle modulation
and muscle spasm will result during the coagulation procedure.
Still a further problem with many eklectrosurgical units is that the
switching between the coagulation and the cutting takes place at a high
voltage. As a result, the possiblity of sparking exists and when using
explosive chemicals there is the dangerous possiblity of an explosion
occurring in the operating room.
SUMMARY OF THE INVENTION
It is accordingly an object of the present invention to provide an
electrosurgical unit which avoids the aforementioned problems of prior art
devices.
It is another object of the present invention to provide an electrosurgical
unit with a highly isolated output.
Yet a further object of the present invention is to provide an
electrosurgical unit which utilizes two vacuum tubes in push-pull
arrangement, in conjunction with a balanced transformer coil, to thereby
produce a high isolated output.
Still a further object of the present invention is to provide an electrical
surgical unit which provides a forceps for sealing blood vessels and a
handpiece for cutting.
Another object of the present invention is to provide an electrosurgical
unit which utilizes a handpiece to selectively provide either a
coagulation or a cutting output voltage.
Still another object of the present invention is to provide an
electrosurgical unit which provides independent control for the level of
the cutting voltage and the level of the coagulation voltage.
A further object of the present invention is to provide an electrosurgical
unit which includes a standby circuit for providing low standby power to
heat the filaments of the oscillator vacuum tubes thereby minimizing the
warm up period.
Still another object of the present invention is to provide an
electrosurgical unit which includes an interlock coupled to the return
plate for stopping the cutting output and providing a warning signal when
the return plate is disconnected.
Still a further object of the present invention is to provide an
electrosurgical unit which contains a standby switch and an operating
switch and which includes a preventive circuit to prevent simultaneous
activation of both these switches.
Another object of the present invention is to provide an electrosurgical
unit which contains output indicators for coagulation, cutting, standby,
and operating conditions.
Yet a further object of the present invention is to provide an
electrosurgical unit which utilizes an improved transformer unit which
prevents unbalance and reduces capacitive leakage to ground.
Still another object of the present invention is to provide an
electrosurgical unit which utilizes low voltage switching to prevent the
possibility of explosions.
Briefly, the invention provides an electrosurgical unit which includes a
vacuum tube oscillator means for providing an ungrounded output of a given
frequency. A power supply means supplies the oscillator with a voltage. A
coagulation output means is available for coupling to the oscillator means
and providing a coagulation voltage for use in coagulation procedures. A
cutting output means is also available for coupling to the oscillator
means and providing a cutting voltage for use in cutting procedures. A
switching circuit means can selectively connect the coagulation output
means or the cutting output means to the oscillator means. An intensity
control means is also available for coupling to the power supply and
varying the voltage provided to the oscillator means. A modulation means
is also available for coupling to the oscillator means and providing
intermittent operation of the oscillator means. The switching means
interconnects the intensity control means to the power supply means only
when the cutting output means is connected to the oscillator means, and
interconnects the modulation means to the oscillator means only when the
coagulation output means is connected to the oscillator means.
The cutting output means further includes a handpiece unit for applying the
cutting voltage as a high density RF voltage and a return plate for
returning the current to the oscillator means. Neither the handpiece unit
nor the return plate are grounded, and both are highly isolated from
ground.
BRIEF DESCRIPTION OF THE DRAWINGS
With the above and additional objects and advantages in view, as will
hereinafter appear, this invention comprises the devices, combinations and
arrangements of parts hereinafter described by way of example and
illustrated in the accompanying drawings of a preferred embodiment in
which:
FIG. 1 is a schematic drawing of a grounded output electrosurgical unit
connected to a patient;
FIG. 2 is a schematic drawing of an isolated output electrosurgical unit
connected to a patient;
FIG. 3 is a circuit drawing of an electrosurgical unit, in accordance with
the present invention;
FIGS. 4A, 4B and 4C show side elevational views of the sections of the coil
assembly, in accordance with the present invention;
FIGS. 5A, 5B and 5C show end views of the sections of the coil assembly, in
accordance with the present invention;
FIG. 6 shows an end view of the assembled coils, in accordance with the
present invention;
FIG. 7 shows a sectional view taken along line 7--7 of FIG. 6;
FIG. 8 shows a schematic circuit drawing of the transformer coil, in
accordance with the present invention; FIG. 9 shows an isometric view of
the electrosurgical unit in accordance with the present invention; and
FIG. 10 shows a schematic drawing of an embodiment of the forceps for use
in a coagulation procedure.
In the various figures of the drawings, like reference characters designate
like parts.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Electrosurgical units generally contain an RF oscillator which provides a
high frequency output electrical current which is extracted across an
output transformer. The current is applied to the patient from an
electrode in a handpice unit and returns from a wide area return plate,
hereinafter referred to as the indifferent plate.
Referring now to FIGS. 1 and 2 there will be compared the non-isolated or
grounded output unit, shown in FIG. 1, with the isolated output unit,
shown in FIG. 2. In FIG. 1, the electrosurgical unit, shown generally at
10 indicates an output transformer 12 having one end thereof grounded at
14. The high frequency output current is applied by means of a handpiece
electrode 16 to the patient 18. The current path returns through the
indifferent plate 20, generally placed under a wide area of the patient
such as the buttocks. It will therefore be appreciated that the return
plate is in fact grounded. The indifferent plate is generally made large
to permit a broad area of contact with the patient thereby providing a low
current density path for the return current. In fact, the only difference
between the handpiece 16 and the plate 20 is that the handpiece provides a
high density current which thereby causes the burning or cutting of the
tissue, while the low density plate 20 will not cause any such burning or
cutting.
Frequently there are connected additional peripheral electrical equipment
21 such as monitoring units, EKG units, etc. These equipment units are
grounded and are connected to the patient by means of contacts, for
example, contacts 22. As a result these contacts also provide a path
leading to ground. Additionally, there exists capacitive paths, shown by
the dotted lines 24, between the patient and ground. As a result, the main
current path from the electrosurgical unit will pass through the probe 16
and return through the indifferent plate 20, as shown by path 26. However,
additional current paths 28 will also exist through the contacts 22 to the
peripheral equipment 21, as well as current paths 30 through the
capacitive coupling to ground. At any of these additional current paths a
burn could result if there is a small enough area of contact. Such small
are of contact would duplicate the probe electrode 16 and cause a high
density RF current to flow which will burn, and may cut the tissue.
An even more serious problem can occur if the return cable 32 were to be
severed, as for example at point 34. In this case there no longer exists
the wide area of the indifferent plate to provide a low density return
path. The current will therefore seek the alternate paths to return to
ground and will cause severe burns, as shown at contact 36.
In the isolated system, shown in FIG. 2, the output transformer 12 is not
grounded and is therefore isolated from ground. As a result, neither the
handpiece 16 nor the indifferent plate 20 is grounded and there only
exists the single current path 26 flowing from the handpiece, througe the
patient, the back to the electrosurgical unit through the indifferent
plate 20. While the electrosurgical unit itself may be grounded at 14, the
output transformer is isolated from ground and prevents any current flow
path to ground. As a result, even though peripheral equipment 21 may be
connected to the patient through contacts 22, and although capacitive
paths 24 may exist to ground, these do not provide any current flow path
back to the electrosurgical unit. Furthermore, even if the return cable 32
should be severed at 34, no burns would occur at any of the contacts 22,
and in fact, no further current would flow from the electrosurgical unit
10 to the handpiece 16.
While in theory the isolated output units provide beneficial results, in
practice it has heretofore not been possible to achieve such perfect
isolation and even isolated output units have caused burns, and other
problems to both patients and peripheral equipment.
The present invention is of the isolated type system, heretofore described,
but is capable of providing an exceedingly high isolation from ground due
to the use of an improved RF generating circuit in conjunction with low
capacitance interconnecting cables and an improved design for the output
coil. Although prior art electrosurgical units have utilized vacuum tubes,
the trend has been to eliminate such use because of the bulky size
involved as well as the long delays which occur during warm-up of the
filaments when the vacuum tubes are turned on. These problems have been
avoided in the present invention by the unique design of the
electrosurgical unit to be described in conjunction with FIG. 3.
The RF electrical current is provided by means of the oscillator 40 which
includes two vacuum tubes 42, 44 connected in push-pull relationship. The
cathodes 46, 48 of the tubes are interconnected along line 50. A first
control grid 52, 54 of each tube are interconnected through respective
resistors 56, 58 to opposite ends G, G' of the grid coil 60 of the
transformer to be hereinafter described. The centerpoint Gc of the grid
coil is connected through resistor 62 to ground 64. The second grid 66, 68
of each tube are interconnected along line 70 and coupled through resistor
72 to the power supply 74 to be hereinafter described. Capacitor 76
connected to ground, filters the high voltage supplied to the tubes.
A third control grid 78, 80 of each tube, is respectively connected to the
filaments 46, 48 of the tubes through the capacitors 82, 84. The plates of
the tube 86, 88 are interconnected to the end points P, P' of the plate
coil 90 of the transformer. The centerpoint Pc of the plate coil is also
connected to the main power supply 74.
The output coil 92 of the transformer has its ends 0, 0' connected across a
tuning capacitor 94 and connects to the centerpoints of switches 96, 98 of
the switching circuit 100. Switches 96, 98 are normally in their cutting
position a which respectfully interconnects to the cutter handpiece shown
generally at 102, and the indifferent plate or dispersion electrode shown
generally at 104. When the switches 96, 98 are connected to their
positions b, they interconnect to the coagulation forceps, shown generally
at 106.
The forceps 106 provide the means for sealing blood vessels and includes
the two arms 108, 110 connected to the main unit by means of a plug and
jack 112. The outer shield 114 of the forceps is also connected through
the plug and jack to a ground through line 116. When the two ends of the
forceps are brought together, an integral switch 118 is closed contacting
onto point 120 which is connected to ground 116 through resistor 122.
Closing of the switch 118, energizes the relay coil 124 whose other end is
connected to the +V supply, which is a low voltage supply.
The cutter handpiece 102 includes an electrode 126 coupled through the plug
and jack arrangement 128 to the position a of switch 98. The shield 130 of
the probe electrode is coupled to the shield 132 of the indifferent plate
134. A coag/cut switch 136 is contained on the probe so that the user can
provide both a cutting signal and a coagulation signal by using the same
probe. When placed on its coag position, it interconnects point 138 with
point 140. Switch 142 is available to connect ground 144 to point 138 or
point 140. When switch 136 is placed in its cut position, it connects
point 138 to point 146.
A foot pedal, shown generally at 150, operates in parallel with the switch
136 whereby the user can either manipulate the hand switch 136 to obtain
the coag or cutting signal from the handpiece 102 or can utilize the foot
pedal to obtain the same results. The foot pedal includes two separate
switches 152 and 154. When desiring a coagulation output, switch 152 is
moved from its normal position C to position d which interconnects point
138 with point 140, similar to the action of switch 136. When a cutting
signal is desired, switch 154 is moved from its normal position f to
position e which interconnects point 138 to point 146, similar to the
action of switch 136. The foot pedal 150 is connected to the main unit
through the plug and jack arrangement 156. If both foot pedal switches are
inadvertently activated at the same time, only the coagulate function is
provided.
The indifferent plate 134 is connected through a plug and jack arrangement
158 to position a of switch 96. An interlock circuit 160 is also connected
to the plug and jack to provide a visual indication when the plug and jack
arrangement 158 has been opened. A low voltage supply +V passes through
resistor 162 and is normally shorted to ground 166 though the shorting
wire 164 on the plug side of the plug and jack arrangement 158. However,
when the plug has been displaced from the jack, the current will pass
through the light emitting diode 168 to provide a visual indication of the
disconnection. At the same time, when the indifferent plate 134 has been
removed from the circuit, no voltage will pass through the probe unit 102
since both the probe and the indifferent plate are connected to opposite
ends of the output coil 92 and no return path to the unit will be then
provided.
A multivibrator unit, shown generally at 170 is adapted to be coupled to
the input of the oscillator unit 40 to effectively pulse modulate the
operation of the oscillator. The multivibrator 170 includes transistors
172 and 174 whose emitters are coupled together along line 176 and whose
collectors are respectively connected to resistors 178 and 180. The base
of transistor 172 is interconnected to the collector of transistor 174
through capacitor 182 while the base of transistor 174 is connected to the
collector of transistor 172 through capacitor 184. The bases of the two
transistors are interconnected through the fixed base resistors 186 and
188 and through the variable resistor 190 which is controlled by means of
the variable control 192 connected at the end of resistors 178 and 180
which is also connected to the +V supply.
By means of the variable control 192, it is possible to control the duty
cycle of the multivibrator and thereby control the pulse width of the
output pulses produced.
One of the output pulses of the multivibrator 170 is applied to the
amplifier switch circuit 194. This circuit includes a first transistor 196
having its emitter coupled to the +V supply and its collector coupled
through a voltage divider comprising resistors 198, 200 to the base of a
grounded emitter transistor 202. A diode 204 is connected across the
collector-emitter path of transistor 196. The base of transistor 196 is
connected through the resistor 206 to one of the outputs of the
multivibrator 170 and specifically to the collector of transistor 172. It
is also coupled to point 146 and also connected through capacitor 208 to
ground. The collector of transistor 202 is connected to line 50 of the
oscillator unit and the emitter of transistor 202 is grounded.
When the multivibrator 170 is operating, one polarity of the output pulses
will be applied through the transistor 196 and inverted by the transistor
202 to control the oscillator 40. Each time that pulse appears, the
oscillator will be operative. By means of the coag level control 192, the
on time of the oscillator can be controlled. At the same time, when the
point 146 is grounded by placing the switch 136 or the foot pedal 150 in
the cut position, the transistor 196 will cause the oscillator to be
continuously operative.
Power is supplied to the unit through the main power supply 74 which is
interconnected to an AC source 208. The current passes through a circuit
breaker 210 and through the series of cross connected switches 212 to the
transformer 214. The output of transformer 214 is rectified by means of
the rectifier 216 and filtered by the filter 218 to provide a high voltage
output at line 220. This high voltage output is fed through the switch
222, which is normally in its open position g. When the operating switch
224 is moved upward to its run position, switch 222 is caused to move to
position h which interconnects the high voltage to the point Pc of the
plate transformer coil 90, as well as to the screen grids 66 and 68 of the
tubes 42 and 44 of the oscillator.
The primary of the transformer 214 is connected to a variac coil 226. The
variac can be controlled by means of the control arm 228. The variac is
interconnected to the primary by means of the switch 230 being in position
i. When moved to position j, the variac is removed from the circuit. In
position j, the maximum output voltage is provided from the power supply,
while in position i, the control arm 228 can vary the output from a low
value up to its maximum value.
An auxillary and filament supply circuit 232 is provided for forming the
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