Note: Descriptions are shown in the official language in which they were submitted.
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This invention pertains generally to high energy switch-
ing devices and more particularly to high energy swltching
devices utilizing gas discharge tubes.
Heretofore, Xenon flash tubes and other gas discharge
tubes have been utilized for switching large amounts of power
by connecting the electrodes of the tube electrically in series
with a source and load and applying a trigger pulse to a control
terminal to break down the gas and initiate the main discharge
to transfer energy from the source to the load. Such switches
will handle voltages on the order of lOKV and currents on the
order of several hundred amperes, and they can be switched at
very high speeds. - ;
In the past, high energy switches utilizing gas dis-
charge tubes have had a serious limitation in that the tubes have
erratic hold-off potentials which typically range from less than
8KV to more than 25KV. Consequently, if the source voltages is
8KV or more, the device may fire by itself, and this sel~-firing
cannot be tolerated in many applications, for example, de~ibril-
lators where the device is used to control the application of a
defibrillating pulse to a patient's heart.
In the high energy switching device of the invention,
an electrostatic shield is placed about the output electrode
of the gas discharge tube. It has been observed that this
shield results in a highly reliable hold-off potential on the
order of 15 KV with Xenon flash tubes which are available com~
mercially.
It is in general an object of the invention to provide
a new and improved high energy switching device.
Another object of the invention is to provide a switch-
ing device of the above character which has a highly reliablehold-off potential and i9 suitable for use in a defibrillator.
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Another object of the invent.ion is to provide a switch- . .
ing device of the above character which is fail safe in
operation. .
According to the invention there is provided in a high
energy switching device for delivering energy from a source to
a load: a gas discharge tube having first and second electrodes
connected electrically in series with the source and the load,
a control element connected to the first electrode, means for
applying a trigger signal to the control element to initiate
firing of the tube to pass energy from the source to the load,
and means comprising an electrostatic shield disposed about the .
second electrode for preventing the tube from firing in the ~;
absence of a trigger signal when the potential between the ~ :
electrodes is below a predetermined level.
Additional objects and features of the invention
will be apparent from the following description in which the .
.preferred embodiment is set forth in detail in conjunction
with the accompanying drawing5,
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Figure 1 is a vertical sectional view of one
embodiment of a high energy switching device according to
the invention.
Figure 2 is a cross-sectional view taken along
line 2-2 in Figure 1.
Figure 3 is a circuit diagram of the switching
device of Figure 1 employed in a defibrillator.
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~igure 4 i~ a graphical representation Or the out-
put voltage produced by ths switching device in the circuit
Or Figure 3
Description of the Pref~rrecl Embodiment
,
The switching de~rice includes a gas discharge tube
10 having an elongated glass envelope 11 in which axi~lly
sp~ced electrodes 12 and 13 are enclosed. The envelope is
filled with a suitable gas such as Xenon. In the pre~erred
embodiment the tube is a Xenon ~lash tube, and the pressure -~
within the tube is slightly leas than atmospheric pressure,
for example
A trigger element 16 is provided ~or breaking down
the gas in tube 10 and initiating the main discharge between
electrodes 12 and 13. The trigger element comprises oonduc~
tive loops 16a and 16b which are disposed coaxially of the
electrodes and connected to electrode 12. The loops are
sp~ced ax;ally apart, with loop 16a adjacent to el'ectrode 12
and loop 16b midway between the electrodes.
Tube 10 and trigger element 16 are enclosed within ~;
20 a generally cyclindrical housing 21 ~abricated of an elec-
trically insulative material such as plastic. The tube and
- trigger element are disposed coaxially o~ the housing and sup-
ported by suitable means, not shown. In the preferred embodi-
ment, the tube has a diameter on the order o~ 1/4 to 3/8 inch,
25 the housing has a diameter on the order of 1 inah, and loops
16a~ 16b are slightly smaller in diameter than the housing.
An electrostatic shield 23 is disposed coaxially
about electrode 13. The shield i9 f`abricated of an elec-
trically conductive material, and in the preferred embodiment
30 it consists of a layer o:~ brass ~oil having a width of 1 inch
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and a thickness on the order of .002 to .003 inch wr~ppod
about the end of housing 21 at which electrode 13 is locatsd.
The shield i9 provided with an air gap 24 to prevent it ~rom
being a shorted turn. I~ desired, the shield can be covered
by suitable insulating material such as a rubber boot.
In Figure 3, the switching device i9 illustrated
in connection with a de~ibrillator having output terminals or
paddles 26,27 adapted to ba placed in contact with a patient's
body for delivering a defibrillating pulse to the patient's
heart. Paddle 26 is connected to electrode 13, which serve~
as an output electrode, and paddle 27 is connected to shield
24 and to a capacitor 31.
Capacitor 31 and the secondary winding 32 of a
transformer are connected electrically in series between pad-
15 dle 27 and input electrode 12. Capacitor 31 serves as a stor-
age capacitor for the energy to be delivered to the patient' 9
heart, and it i9 cbarged from a 24 volt battery 33 to a level
on the order of 3KV to 7KV by a DC-to-DC converter 34 through
a diode 36.
Winding 32 is the secondary winding of a transformer
37 which also ha3 a primary winding 38. In the preferred em-
bodiment, primary winding 38 oonsists of four to five turns
o~ copper sheet, aecondary winding 32 consists of several
hundred turns of copper wire, and the transformer is enclosed
in a cylindrical aase 39 and mounted coaxially of housing 21
adjacent to input electrode 12 and trigger element 16.
Means is provided for applying trigger pulses to
trigger element 16 through transformer 37. This means in-
cludes a capacitor 41 and a silicon controlled rectifier 42
which are cormected electrically in series with primary wind-
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ing 38~ Capacitor 41 is charged to a suitable triggering
level, such as 200 volts, rrom battery 33 b~ a converter ~3
similar to converter 34. The gate of SCR 42 is connected to
a suitable control device such as a manually operated switch.
Operation and use Or the switching device can be
described briefly. Capacitor 41 is charged to a level on the
order of 200 volts by converter 43 when the unit is turned on.
When converter 34 is energized, capacitor 31 is charged to a
level on the order of 3KV to 7KV. Paddles 26 and 27 are
placed in contact with the patient's body, and SCR 42 is
fired when a defibrillating pulse is desired. When thé SCR
fires, capacitor 41 discharges through primary winding 38,
producing a trigger pulse on the order of 25KV and 1-2 micro-
seconds duration in secondary winding 32. This pulse ionizes
tbe ga9 in tube 10, initiating the main diacharge bet~een elec-
trodes 12 and 13 to deliver the energy stored in capacitor ~1
to paddles 26 and 27.
Capacitcr 31, winding 32 and the patient's bod~
eonstitute an LCR circuit which produce~ the waveform shown
in ~igure 4 when capacitor 31 discharges. This waveform is
oommonly known as a Lown waveform, and it is a slightly under-
dampea sinusoidal wave~orm wbich decreases in magnitude at a
rate on the order of 82% per hal~ cycle, Tubs 10 requires a
certain minimum current to sustain ignition, and it extin-
guishes on the zero cros~ing at the end of the first or sec-
ond half cyele of the output voltageg depending on the level
to which capacitor 31 is initially charged. With an initial
eharge of 7KV, for exampleJ the voltage drop at the first zero
erossing i9 very rapid, and the tube does not turn off until
the second zero crossing. With a smaller initial charge,
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e.g., 3KV, the tube extinguishes on the ~ir~t zero crossing.
The pulse delivered to thepatient 1 9 heart has a duration on
the order of 5 to 8 milliseconds ancl a level depending upon
the initial charge on capacitor 31.
It has been ~ound that electro~tatic shield 2~ i9
very erfective in preventing the discharge tube from firing
before the trigger pulse is appliedq With the shie~d, tubes
which otherwise would break down with potentials as low as
8KV will consistently and reliably hold off potentials on the
order of 15KV. Prior to ignition, the ~hield is maintained
at substantialIy the same potential as the output electrode
and it is believed to perform its functïon by shaping the
electrostatic field in the region o~ the output electrode and `
relieving the field gradient concentration which would other-
wise occur in tbi~ region due to the sharpness of the elec-
trodeO This ahield has been found to perform its function even
when the tube i9 fired in the presence of external metal ob-
~ects.
The switching device is fail safe in operation in
th~t it will not ~ire in the event that the envelope is bro-
ken, Trigger element 16 is 3paced a substantial distance from
electrode 13 and shield 24, and it cannot arc over to either
of tbem in the event the tube should break in such a manner
that the electrode 13 is left exposed, In the event of a
leak or crack in the envelope, the negative pressure to which
the tube is filled will cause the Xenon gas to mix with the
incoming air which will cauae the hold off voltage to increase.
It i9 apparent from the foregoing that a new and im-
proved high energy switching device has been provided. While
only the preferred embodiment has been described, as will be
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apparent to thos~ ~Dmiliar with -the ~rt, cert~in ch~nges and
modi~ications c~n be made without departing ~rom the scope
~r the invention ~s de~ined by the ~ollowing cl~ims~
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