US3225254A

US3225254A - Protective circuit

Info

Publication number: US3225254A
Application number: US114472A
Authority: US
Inventors: George L Clark; John J Hickey
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1961-06-02
Filing date: 1961-06-02
Publication date: 1965-12-21
Anticipated expiration: 1982-12-21

Description

Dec. 21, 1965 CLARK ETAL 3,225,254

PROTECTIVE CIRCUIT Filed June 2, 1961 2 Sheets-Sheet 1 TIP/665B 500905 BY W0 Dec. 21, 1965 5. CLARK ETAL 3,225,254

PROTECTIVE CIRCUIT Filed June 2, 1961 2 Sheets-Sheet 2 79/6659 SOUQCE 850965 4. CZAZQKZ JOE NJ AVG/Z54 INVENTORS a 9% BY Z Cyw/ United States Patent 3,225,254 PROTECTIVE CIRCUIT George L. Clark and John J. Hickey, both of Hawthorne, Caiifi, assignors, by mesne assignments, to TRW Inc., a corporation of Ohio Filed June 2, 1961, Ser. No. 114,472 13 Claims. (Cl. 315-163) This invention relates to circuits employing control devices, such as thyratron tubes, and more particularly to the provision of means for protecting a trigger source from high voltage pulses generated in a thyratron tube upon the firing thereof.

When a thyratron fires, the potential of the grid may momentarily jump to a value nearly equal to the anode potential. This pulse of voltage, commonly known as the grid spike, is transmitted through the grid coupling capacitor to the circuit which supplies the trigger to the thyratron grid. In the case of large thyratrons operating with anode potentials of -30 kilovolts and carrying hundreds of amperes of current, the grid spike can cause considerable damage to a trigger circuit which is not adequately protected.

The usual type of protection which is provided for the trigger circuit is a low pass filter inserted between it and the thyratron grid. Although the filter does cut down the amount of grid spike reaching the trigger source, it also reduces the size and rate of rise of the trigger pulse which is sent to the thyratron grid, thereby making it necessary to generate a larger trigger pulse than would be required in the absence of the filter. Since the grid spike is about 100 times as large as the trigger pulse, it is extremely difficult to filter out the grid spike and still trigger the thyratron reliably.

Accordingly, it is an object of this invention to protect a signal source from spurious high voltages generated in a control device that is fed from the signal source.

It is a more specific object of this invention to provide a protective circuit which operates to prevent the grid spike voltage of a thyratron tube from being transmitted to the trigger source supplying the thyratron with a trigger voltage pulse.

A further object is the prvoision of means for block ing the transmission of grid spike voltages from a thyratron to the source supplying a trigger pulse to the thyratron, without interfering with the transmission of the trigger pulse.

The foregoing and other objects are realized, according to one embodiment, in a protective circuit interposed between a trigger source supplying a trigger voltage pulse and a control device, such as a main thyratron tube, receiving said trigger pulse. The protective circuit includes a switching device or protective thyratron in shunt with the trigger source and normally biased nonconducting in the absence of a trigger pulse from the trigger source.

When a trigger pulse is supplied from the trigger source the pulse is received by the protective thyratron tube. However, since the protective thyratron tube is provided with means for delaying the rise of its grid voltage to the firing potential, it remains nonconducting for a short time sutiicient to permit transmission of the trigger pulse to the main thyratron tube. The protective thyratron tube is arranged to fire before the main thyratron tube fires. Thus, when the main thyratron tube fires, the grid spike voltage generated therein is shunted through the protective thyratron and is prevented from reaching the trigger source. A series impedance between the two thyratrons serves to limit the current drawn by the second thyratron as a result of its being subjected to the grid spike voltage.

3,225,254 Patented Dec. 21, 1965 In the drawings, wherein like numerals are used to represent like parts:

FIG. 1 is a schematic representation of one form of a protective circuit according to the invention; and

FIGS. 2, 3, and 4 are schematic representations of other forms of a protective circuit according to the invention.

Referring to FIG. 1 a trigger source 10, shown in block form, is provided with a pair of

output terminals

12 and 14 across which is generated a trigger voltage pulse 16 to be fed to a control device or main thyratron tube 18. The trigger source may, for example, comprise a conventional blocking oscillator circuit and a cathode follower. The main thyratron tube 18 may, for example, be one of the kind, such as type 1907, handling 20 kilovolts and carrying several hundred amperes of current. Normally it is desired to apply the trigger pulse 16 to the control grid 20 of the main thyratron tube 18 to render the latter conducting, and thereby initiate some further action under the control of the main thyratron tube 18, such as generating a pulse to drive a high-power tube, or discharging a capacitor. In this case, the main thyratron tube 18 is used in a pulse forming circuit. The anode 22 of the main thyratron tube is connected to a high voltage source 24 through an anode resistor 26. The high voltage source 24 is of the order of 20 kilovolts. A pulse forming network, such as a delay line 28, is connected in the anode circuit to generate an output pulse of given time duration across an output pulse transformer 30 also connected in the anode circuit. The screen grid 32 and cathode 34 are maintained at ground reference potential.

When the main thyratron tube 18 is triggered by applying a positive pulse to the control grid 20, the region of the tube between the grid 20 and cathode 34 ionizes first, and then the region between the grid 20 and anode 22 ionizes. When the gas between the grid 20 and anode 22 becomes conducting, the potential of the control grid 20 rises momentarily to a value nearly equal to the potential of the anode 22 and then falls to a potential a little higher than that of the cathode 34. It is necessary to provide some means for preventing this increase in potential, or grid spike, as it is commonly known, from reaching the trigger source 10, to avoid damage to the latter. The grid spike is shown in FIG. 1 as a sharply rising pulse 36.

In accordance with the invention, a protective circuit is interposed between the trigger source 10 and the main thyratron tube 18. The protective circuit is arranged to transmit with little attenuation the trigger pulse 16 from the trigger source 10 to the control grid 20 of the main thyratron tube 18. However, once the trigger pulse 16 reaches the control grid 20 of the main thyratron tube 18 and remains there long enough to subsequently trigger the main thyratron tube 18, the protective circuit blocks the transmission of the grid spike 36 from the main thyratron tube 18 to the trigger source 10.

The protective circuit includes a first Series resistor 38 connected at one end to the output terminal 12 of the trigger source 10 and connected at its other end to the anode 40 of a switching device or protective thyratron tube 42. The protective thyratron tube ;42 may be a miniature thyratron such as type 2'D21 that includes a cathode 44, a control grid 46 and a screen grid 48, in addition to the anode 40. The screen grid 48 is preferably tied to the cathode 44, which is connected to ground. The control grid 46 is biased beyond cutoff by connection through a grid bias resistor 50 to a negative bias voltage source 52, so that the protective thyratron tube 42 is maintained normally in a nonconducting condition. A series circuit comprising variable resistor 54 and a DC. blocking capacitor 56 is connected between the control grid 46 and the anode 40 of the second thyratron tube 42, and a capacitor 58 is connected between the control grid 46 and cathode 44-.

A second series resistor 60 is connected between the anode 40 of the protective thyratron tube 42 and the control grid 20 of the main thyratron tube 18. A resistor 60, connected between ground and the anode 40 of the protective thyratron tube 42 serves to return the grid 20 of the main thyratron 18 and the anode 40 of the protective thyratron 42 to ground potential between trigger pulses.

In the operation of the circuit of FIG. 1, both thyratr-on

tubes

18 and 42 are normally nonconducting in the absence of the trigger pulse 16 from the trigger source 10. When it is desired to render the main thyratron tube 18 conducting the trigger pulse 16 is sent from the trigger source through the first series resistor 38, and the second series resistor 60 to the control grid 20 of the main thyratron tube 18. The three

resistors

54, 50, and 62 shunting the protective thyratron tube 42 are sufiiciently large in resistance so that they do not affect the transmission of the trigger pulse 16. The

series resistors

38 and 60 are much lower in resistance, the first series resistor 38 serving to protect the trigger source 10 from being short circuited when the protective thyraton tube 42 is rendered conducting, and the second series resistor 60 serving to limit the current drawn by the protective thyratron tube 42 when the grid spike voltage 36 is impressed upon the protective thyratron tube 42.

The trigger pulse 16 from the trigger source 10 is impressed across the anode 40 of the protective thyratron tube 42 and the control grid 20 of the main thyratron tube 18 almost simultaneously. When it is impressed upon the anode 40 of the protective thyratron tube 42, the capacitor 58 charges up through the variable resistor 54 and capacitor 56, thereby raising the control grid potential of the protective thyratron tube 42. Capacitor 56 is much larger than capacitor 58, so that a negligible portion of the trigger pulse 16 appears across capacitor 56. After a certain amount of delay, which is determined by the time constant of the capacitor 58 and variable resistor 54, the latter being variable to adjust the time constant, the potential on the control grid 46 of the second thyratron tube 42 reaches the firing potential, whereupon the protective thyratron tube 42 is rendered conducting. The delay in the firing of the protective thyratron tube 42 is arranged to be shorter than the delay in the firing of the main thyratron tube 18. Normally large thyratrons such as the main thyratron tube 18 have a delay time in excess of 70 nanoseconds, whereas miniature thyratrons such as the protective thyratron tube 42, in the absence of special delay circuitry, have delay times less than 70 nanoseconds.

When the protective thyratron tube 42 fires, it shunts and cuts off the trigger pulse 16 if the latter has not already been terminated. The duration of the trigger pulse 16 need only be long enough to cause the eventual firing of the main thyratron tube 18. After a certain length of time following the firing of the protective thyratron tube 42, the main thyratron tube 18 fires, whereupon an output pulse is generated across the transformer 30 and a grid spike appears at the control grid 20. The grid spike voltage 36 is transmitted through the second series resistor and is shunted by the low resistance of the protective thyratron tube 42 that is now conducting. The grid spike voltage 36 is thereby prevented from reaching the trigger source 10.

The second series resistor 60 insures that the protective thyratron tube 42 will not be overloaded by current during the occurrence of the grid spike 36. The coupling capacitor 56 provides D.C. isolation between the control grid 46 of the protective thyratron 42 and the rest of the circuit.

In one operative embodiment, circuit values were as follows:

Resistor 50 10,000 ohms.

Capacitor 58 micro-microfarads. Source 52 50 volts.

Resistor 54 10,000 ohm potentiometer. Tube 42 Type 2D21 thyratron. Tube 18 Type 1907 thyratron. Capacitor 56 0.02 microfarad.

Resistor 62 10,000 ohms.

Resistor 60 ohms.

Resistor 38 50 ohms.

In some instances, the inherent delay in the firing of the protective thyratron tube 42 may be sufficient to permit the trigger pulse 16 to last long enough to energize the main thyratron tube 18. In such instances, the delay circuit of FIG. 1 may be dispensed with. As shown in the embodiment of FIG. 2 the capacitor 56, capacitor 58, the bias source 52, and grid bias resistor 50 are eliminated, and the variable resistor 54 is replaced by a fixed resistor 64. Otherwise, the circuit is the same as that of FIG. 1. Depending upon the amount of trigger voltage provided by the trigger source 10, which voltage may be between 200 and 1000 volts, a delay of from 0.05 to l microsecond may be realized in the firing of the protective thyratron tube 42. A trigger pulse of this duration is sufiicient to trigger the main thyratron tube 18. A longer delay occurs in the firing of the main thyratron tube 18 in order to assure that the protective thyratron tube 42 fires before the main thyratron tube 18, so that the former will be conducting when the grid spike voltage occurs and will thereby be in condition to shunt the grid spike voltage.

It is generally desirable in thyratron control circuits to utilize a rather long duration trigger pulse to trigger a thyratron, such as the main thyratron tube 18, in order to reduce the jitter time, or variation in the switching time of the tube. Furthermore, it may be desirable to utilize the same tube type for each of the main thyratron and protective thyratron tubes. One form of pro tective circuit which fulfills these requirements is shown in FIG. 3. In this circuit a protective thyratron 70 is used in conjunction with a main thyratron tube 72 which may be of the same tube type as the protective thyratron tube 70. For example, each of the

tubes

70 and 72 may be a type 4C3 5 A delay network 74, comprising a plurality of series inductors 76 and shunt capacitors 78, is connected between the anode 80 of the protective thyratron tube 70 and the junction of the

resistors

60 and 62. The control grid 82 of the protective thyratron tube 70 is connected to the output of the delay network 74 at the junction of the

resistors

60 and 62.

In operation, the trigger pulse 16 is delayed from reaching the control grid 82 of the protective thyratron 80 and the control grid 84 of the main thyratron tube 72. Assuming the

tubes

70 and 72 fire simultaneously, it can be seen that the grid spike 36, which must pass through the delay network 74 before reaching the anode 80 of the protective thyratron, will be delayed by the network 74. Thus, the delay network 74 assures that the protective thyratron tube 70 will be conducting prior to reception of the grid spike 36. Accordingly, the grid spike 36 is shunted through the protective thyratron 70 before it can reach the trigger source 10.

It can be seen that the amount of delay produced by the delay network 74 can be adjusted to render the protective circuit of FIG. 3 effective even in instances where the protective thyratron tube 70 fires later than the main thyratron tube 72. Complete protection can be realized in this instance if the delay of the delay network 74 is greater than the difference in the firing times of the two thyratrons 70 and 72. Accordingly, even ditferent tube types can be used for the thyratrons 70 and 72, provided the above requirement is met. For example, a tube type can be used for the protective thyratron 70 that has an inherent delay greater than that of the tube type used for the main thyratron 72.

Another circuit which is useful for extending the duration of the trigger pulse and for affording complete protection despite a longer inherent delay in the firing of the protective thyratron relative to the main thyratron is shown in FIG. 4. In this embodiment, a trigger source 90 provides a main trigger pulse 16 for energizing the main thyratron tube 72 and a separate auxiliary trigger pulse 92 for energizing the protective thyratron tube 70. The auxiliary trigger pulse 92 is delayed with respect to the main trigger pulse 16, the delay being built into the trigger source 90. The auxiliary trigger pulse 92 is fed through a coupling capacitor 94 to the control grid 82 of the protective thyratron tube 70. A negative bias voltage applied to the control grid 82 through a bias resistor 96 from a bias source 98 maintains the protective thyratron tube 70 normally nonconducting until the auxiliary trigger pulse 92 renders it conducting.

A delay network 74 is connected between the series resistor 60 and the shunt resitor 62 to delay the arrival of the grid spike voltage relative to the firing of the protective thyratron 70.

The

separate trigger pulses

16 and 92 should be spacedapart in time only as long as is necessary to provide a main trigger pulse 16 of suflicient duration to trigger the main thyratron tube 72. The duration of the main trigger pulse 16 is equal to the sum of the delay between the two

trigger pulses

16 and 92 plus the inherent delay in the firing of the protective thyratron tube 70. The relationship between the inherent delay (A) in the firing of the protective thyratron 70, the inherent delay (B) in the firing of the main thyratron 72, the delay (5) provided by the delay network 74 and the delay (T) between the two

trigger pulses

16 and 92 is determined by the following expression:

Adherence to the foregoing relationship will assure that the arrival of the grid spike voltage will be delayed until after the firing of the protective thyratron tube 70.

It is now apparent that the invention provides an effective means for transmitting operative signal pulses from a signal source to a control device, while shunting spurious signal from the control device through a switching device, thereby blocking the spurious signals from the signal source. The one-way transmission is elfected through the use of a switching device that operates on a time delay basis, rather than on a threshold voltage basis, as in some prior art devices. A switching device in the form of a thyratron makes the invention particularly effective in applications requiring very high current and voltage handling capabilities.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination, an electrical control device having an input circuit and adapted to change in electrical properties after the application to the input circuit thereof of an electrical control signal of at least a predetermined time duration, thereby giving rise in said input circuit to a spurious signal; a terminal for receiving said control signal; means for coupling said control signal from said terminal to the input circuit of said control device; and means for blocking the transmission of said spurious sig nal from said input circuit of said control device to said terminal, said last mentioned means comprising a switching device connected in shunt relation to said terminal, a current limiting impedance connected in series between said switching device and said input circuit of said control device, time delay means for maintaining said switching device at a relatively high impedance, after application to said terminal of said control signal, at least for said predetermined time duration, means for transforming said switching device to a relatively low impedance, and means for delaying the arrival of said spurious signal at said terminal until after the transformation of said switching device.

2. The invention according to claim 1, wherein said switching device includes an input circuit, and said time delay means comprises a delay network connected in series with said current limiting impedance, with the end of said delay network remote from said switching device connected to said input circuit.

5. In combination, a main thyratron tube having an input circuit and adapted to change in electrical properties after the application to the input circuit thereof of an electrical control signal of at least a predetermined time duration, thereby giving rise in said input circuit to a spurious signal; a terminal for receiving said control signal, means for coupling said control signal from said terminal to the input circuit of said main thyratron tube; and means for blocking the transmission of said spurious signal from said input circuit of said main thyratron tube to said terminal, said last mentioned means comprising a protective thyratron tube connected in shunt relation to said terminal, a current limiting impedance connected in series between said protective thyratron tube and said input circuit of said main thyratron tube, time delay means for maintaining said protective thyratron tube at a relatively high impedance, after application to said terminal of said control signal, at least for said predetermined time duration, means for transforming said protective thyratron tube to a relatively low impedance, and means for delaying the arrival of said spurious signal. at said terminal until after the transformation of said protective thyratron tube.

6. The invention according to claim 5, wherein said main thyratron tube has an inherent switching time that is greater than that of said protective thyratron tube.

7. The invention according to claim 6, wherein said protective thyratron tube includes a control grid and an anode coupled separately to said terminal, and said time delay means comprises a resistor-capacitor charging circuit coupled between said terminal and the control grid of said protective thyratron tube.

8. The invention according to claim 6, wherein said protective thyratron tube includes a control grid and an anode coupled separately to said terminal, with a resistor connected between said control grid and said anode.

9. In combination, a main thyratron tube having an input circuit and adapted to switch within a first time period after the application to the input circuit thereof of an electrical control signal of at least a predetermined time duration, thereby giving rise in said input circuit to a spurious signal; a terminal for receiving said control signal, means for coupling said control signal from said terminal to the input circuit of said main thyratron tube; and means for blocking the transmission of said spurious signal from said input circuit of said main thyratron tube to said terminal, said last mentioned means comprising a protective thyratron having an anode and a control grid and connected in shunt relation to said terminal, said protective thyratron having an inherent switching time smaller than that of said main thyratron tube, a current limiting impedance connected in series between said protective thyratron and said input circuit of said main thyratron tube, time delay means including a series resistance capacitance network connected between the anode and control grid of said protective thyratron for maintaining said protective thyratron at a relatively high impedance, after application to said terminal of said control signal, for a time at least equal to said predetermined time duration, and means for transforming said protective thyratron to a relatively low impedance, the switching time of said main thyratron tube being greater than the sum of the switching time of said protective thyratron tube and the time delay of said time delay means, thereby to delay the arrival of said spurious signal at said terminal until after the transformation of said protective thyratron.

10. In combination, a main thyratron having an input circuit and adapted to switch to a conducting state after the application to the input circuit thereof of an electrical control signal of at least a predetermined time duration, thereby giving rise in said input circuit to a spurious signal; a terminal for receiving said control signal, means for coupling said control signal from said terminal to the input circuit of said main thyratron tube; and means for blocking the transmission of said spurious signal from said input circuit of said main thyratron tube to said terminal, said last mentioned means comprising a protective thyratron connected in shunt relation to said terminal, a current limiting impedance connected in series between said protective thyratron and said input circuit of said main thyratron, means for applying a separate trigger pulse independent from said control signal to said protective thyratron, with said trigger pulse being delayed with respect to said control signal to maintain said protective thyratron at a relatively high impedance, after application to said terminal of said control signal, at least for said predetermined time duration, means for transforming said protective thyratron to a relatively low impedance, and means for delaying the arrival of said spurious signal at said terminal until after the transformation of said protective thyratron.

11. The invention according to claim 10, wherein the means for delaying the arrival of said spurious signal comprises a delay network connected in series with said current limiting impedance.

12. In combination, a main thyratron having an input circuit and adapted to change in electrical properties after the application to the input circuit thereof of an electrical control signal of at least a predetermined time duration, thereby giving rise in said input circuit to a spurious signal; a terminal for receiving said control signal, means for coupling said control signal from said terminal to the input circuit of said main thyratron; and means for blocking the transmission of said spurious signal from said input circuit of said main thyratron to said terminal, said last mentioned means comprising a protective thyratron connected in shunt relation to said terminal and including a control grid, a current limiting impedance connected in series between said protective thyratron and said input circuit of said main thyratron, a delay network connected in series with said current limiting impedance with the end of said delay network remote from said protective thyratron connected to said control grid, thereby to maintain said switching device at a relatively high impedance, after application to said terminal of said control signal, at least for said predetermined time duration, means for transforming said switching device to a relatively low impedance, and means for delaying the arrival of said spurious signal at said terminal until after the transformation of said switching device.

13. The invention according to claim 12, wherein the sum of the inherent delay of said main thyratron and the delay introduced by said delay network is greater than the inherent delay of said protective thyratron tube, whereby said delay network serves to delay the arrival of said spurious signal at said terminal until after the transformation of said protective thyratron.

References Cited by the Examiner UNITED STATES PATENTS 2,129,088 9/1938 George 328l0 2,342,673 2/1944 Klemperer 315-272 X 2,572,832 10/1951 Bernard 328-10 2,642,552 6/1953 Sager 315272 X GEORGE N. WESTBY, Primary Examiner.

RALPH NILSON, Examiner.

CHARLES R. CAMPBELL, Assistant Examiner.

Claims

10. IN COMBINATION, A MAIN THYRATRON HAVING AN INPUT CIRCUIT AND ADAPTED TO SWITCH TO A CONDUCTING STATE AFTER THE APPLICATION TO THE INPUT CIRCUIT THEREOF OF AN ELECTRICAL CONTROL SIGNAL OF AT LEAST A PREDETERMINED TIME DURATION, THEREBY GIVING RISE IN SAID INPUT CIRCUIT TO A SPURIOUS SIGNAL; A TERMINAL FOR RECEIVING SAID CONTROL SIGNAL, MEANS FOR COUPLING SAID CONTROL SIGNAL FROM SAID TERMINAL TO THE INPUT CIRCUIT OF SAID MAIN THYRATRON TUBE; AND MEANS FOR BLOCKING THE TRANSMISSION OF SAID SPURIOUS SIGNAL FROM SAID INPUT CIRCUIT OF SAID MAIN THYRATRON TUBE TO SAID TERMINAL, SAID LAST MENTIONED MEANS COMPRISING A PROTECTIVE THYRATRON CONNECTED IN SHUNT RELATION TO SAID TERMINAL, A CURRENT LIMITING IMPEDANCE CONNECTED IN SERIES BETWEEN SAID PROTECTIVE THYRATRON AND SAID INPUT CIRCUIT OF SAID MAIN THYRATRON, MEANS FOR APPLYING A SEPARATE TRIGGER PULSE INDEPENDENT, FROM SAID CONTROL SIGNAL TO SAID PROTECTIVE THYRATRON, WITH SAID TRIGGER PULSE BEING DELAYED WITH RESPECT TO SAID CONTROL SIGNAL TO MAINTAIN SAID PROTECTIVE THYRATRON AT A RELATIVELY HIGH IMPEDANCE, AFTER APPLICATION TO SAID TERMINAL OF SAID CONTROL SIGNAL, AT LEAST FOR SAID PREDETERMINED TIME DURATION, MEANS FOR TRANSFORMING SAID PROTECTIVE THYRATRON TO A RELATIVELY LOW IMPEDANCE, AND MEANS FOR DELAYING THE ARRIVAL OF SAID SPURIOUS SIGNAL AT SAID TERMINAL UNTIL AFTER THE TRANSFORMATION OF SAID PROTECTIVE THYRATRON.