US2930935A - Gas tube circuits - Google Patents

Description

March 29, 1960 w. E. MARTIN 2,930,935.

GAS TUBE CIRCUITS Filed Dec. 5, 1956 INVENTOR. F 7 WILLIAM E. Man-rm j l fzzcar MIA - 2,930,935 GAS roan cmcurrs William E. Martin, Pennsauken, N.J., assignor to Radio Corporation of America, a corporation of Delaware The present invention relates to improved circuits for quickly and reliably deionizing a gas tube. The invention is particularly applicable to the type of pulse generator or relaxation oscillator in which the charge stored in a condenser is periodically discharged through a gas tube.

The use of 'a' gas tube in the discharge circuit of a storage condenser is advantageous as the gas has a low impedance, when ionized, and it is capable of conduct- 'ing a substantial amount of current, when ionized. However, once ionized, the gas may remain ionized for a relatively long interval of time. The interval may not be the same, cycle-to-cycle. 7 During the ionization interval, the storage condenser cannot again begin to charge. This limits the condenser discharge repetitionfrequency. Moreover, in those cases in which the circuit is free running, the recurrence frequency of the output pulses or waves may be erratic due to the varying deionization times of the gas in the gas tube.

An object of this invention is to provide an improved circuit for quickly and reliably deionizing a gas tube within a predetermined interval of time after it has fired.

Another object of this invention is to provide an improved relaxation oscillator or pulse generator type circuit in which a charge stored in a storage means is discharged through a gas tube such as a thyratron, in which the frequency of the output wave can be substantially increased over' that of known circuits employing similar elements.

A more specific object of this invention is to provide an improved pulsed light source which is suitable for .stroboscopic, photographic or pulsed light radar systems.

According to the invention, a charge storage means such as a condenser is charged-through an impedance element which is capable of assuming a relatively low or a relatively high value of impedance. One such element, for example, is a gridcontrolled vacuum tube such as a triode, pentode or the like. The discharge path of the storage condenser includes a tube such as a thyratron having an ionizable vapor. The thyratron is also in series with the impedance elementand the charging voltage source.

During the charging interval, the impedance element is at its' low value, whereby the storage condenser charges at a relatively rapid rate. .Upon discharge of the condenser, the impedance. elementv assumes its high value and substantially the entire. source voltage develops across this element. This reduces thevoltage across the thyratron to substantially zerovolts,v audits gas deionizes.

In. the embodiment of the invention in which the impedance element is a triode, it is driven beyond cut oif upon discharge of the condenser. This changes the triode impedance from its low'to its high value. The cut off bias is developed in the condenser discharging circuit and is applied to the triode control grid.

The invention will be described in greater detail by reference to the following description taken in connection l. with the accompanying drawingin which:

' thereafter loses control.

' Figure l is a schematic circuit 'diagramof one of the present invention; I

- Figure 2 shows waveforms present at variouspoihtsin the circuit of Figure l; and

Figure 3 is a schematic circuit diagram of a lightlpu'lser circuit according to thisinve'ntion.

Referring to- Figure 1-, a directvoltage source (not shown) is adapted to be connected to-terminals I03 The charge storage means comprise a storage condenser- 12. it is connected in series wit-lithe primary-winding -14 of a pulse transformer or load circuit 16 and the anode-tocathode'circuit of an impedance element 18. The inrpedance element is capable of assuming a relatively low or relatively high value of impedance and, in the eninected through resistor 34 to 13+ and control electrode,

3 6 isdirectly connected to the junction of resistors' '3'8 and 40. Control electrode 36 is also coupled through capacitor 42 and resistor 44 to the junction of resistors 33 and 46.

Rotary switch 48, which is connected in shunt with resistor 46, is continuously driven by drive means 49. The latter may be an electric motor or the like. When switch 48 makes contact, resistor 46 is shortedout. This substantially increases the voltage at the junction of resistors 38 and 46, and this increased voltage is coupled to the thyratron control grid through th'e'couplingnetwork of resistor 44 and condenser 42.

The operation of the circuit shown in Figure 1 can be better understood by referring to the-waveforms of'F igure 2. Assume that the direct voltage supply has just been connected to terminals 10. Triode 18 conducts and condenser 12 charges through the path including the primary winding 14 of transformer 16 and the anode-tocathode circuit of triode 18. The condenser charging path is indicated schematically in Figure l by arrow 48. The voltage at. point 50 in the circuit of Figure 1 is shown in Figure 2a. The initial charge of the-condenser occurs between points 52 and 54 ofthe curve. s

When condenser has fully charged, triode I8 substantially stops conducting because of a low plate to cathode potential, and point 50 inthe circuit remains-within afew volts of theB+ potential; This isiillustrated in Figure 2a by the portion of the curvebetween points 54-and- 56.

Drive means 49 continuously rotates rotary switch 48. When the switchmakes contact, a positive pulse, as shown in Figure 2b, is applied to the control grid 36 of the thyratron. This fires the gas tube, and the control grid Condenser 12 now quickly discharges through the low impedance path including primary winding 14 and the anode-to-cathode circuit of thyratron 26. The discharge path is shown in Figure 1 by arrow 58. The voltage at point 50, during the condenser discharge. interval, is indicated in Figure 2d between the points.56.and'58. i

At point.58, condenser 12 is completely discharged; that is, substantially no potential exists between its pl'ates. One would think that the discharging actionof the condenser would cease at this point, and it would if there were no inductance in the circuit. However, the primary winding 14 of transformer 16 prevents the condenser from stopping'discharging. During the dischargmg interval, primary winding 14 accumulates a substantial amount of energy and, when condenser -1-2- is com- Pdated Ma 2 .12 9

is across the thyratron.

a ringing type oscillation as shown in Figure 20.

' 7 Referring 'still to Figure 1, point 50 in the circuit is connected to the control grid 62 of the triode. During the condenser charging interval, the rising positive voltage at point 50 is effectively coupled to the control grid through coupling condenser 64 and causes the triode to conduct very heavily. However, during the condenser discharge interval, that is, during the portion of the discharging interval in which winding 14 supplies energy tothe condenser, point 50 is driven substantially below the cutoff voltage of the triode. Figure 2a where it is seen that point 60 is substantially below e the triode cut off voltage. Thus, the triode is cut-off near the end of the condenser discharging interval.

The triode is in series with the thyratron. When it begins conducting, substantially the entire line voltage After the condenser has discharged, the line voltage is across the thyratron and the load in series with the thyratron. If the load had a low impedance, the thyratron would hang. In other words, the thyratron would not deionize immediately upon the termination of the condenser discharge. However, in the circuit of Figure 1, upon the termination of the condenser discharge, the triode is cut off. This increases the triode impedance from about 8,000 ohms to about 50,000 ohms, in one practical form of the invention.

vthe triode, and substantially no voltage is across the thyratron. With no voltage across the thyratron, its gas quickly deionizes, and the condenser charging cycle again begins.

The above sequence of events is illustrated in Figure 2a. At point 60, the primary winding has discharged its entire energy into the storage condenser, and point 50 has reached its maximum negative value-one well below the triode cut off value. The thyratron is extinguished. As a matter of fact, it became extinguished at point 66, slightly after the triode was cut 01f. During the period from 60 to 68, condenser 64 discharges through resistor 70 (see Figure 1), and both point 50 and control grid 18 become more and more positive. At about point 68, the cut-off voltage of triode 18 is reached, and the latter begins to conduct. Storage condenser 12 now again charges along path 68, 70 (see Figure 2a), and the cycle of operation is repeated.

The voltage developed across the secondary winding 72 of output transformer 16 (see Figure 1) consists of The first cycle of the ringing is a positive-going pulse 74, and its width is determined by the frequency of the ringing oscillation. The frequency, in turn, depends upon the lumped and distributed inductance and capacitance in the output circuit. The ringing is highly damped and, in a practical circuit, it was found to be unnecessary to eliminate the negative swings 76. However, these swings can be eliminated, if desired, by using diode clamps in a well known manner. The output pulse of Figure 2c is applied to the load 78.

The circuit of Figure 3 operates in a manner similar to the one of Figure 1. Similar reference numerals have been applied to similar parts. In the circuit of this figure, the pulse transformer consists of an autotransformer 80. One output pulse is developed across portion 82 of the transformer, and a second, higher amplitude output pulse is taken from the entire transformer 84.

The load circuit consists of a gas tube 86 which is capable of producing a tremendous flash of light when pulsed. The gas in the tube is normally not ionized, however, when transformer 84 applies its output pulse to external probe 88, the gas is ionized, and the charge This is indicated in storing condenser 90, which is connected across the gas tube, dumps its charge into the gas tube. Resistor 92 is in the charging circuit of condenser 90 and radio frequency choke 94 in its discharging circuit.

The amplitude of the pulse applied from autotransformer to plate 88 is sufiicient to ionize the gas in gas tube 86, even with the inductance of choke 94, which opposes this ionization, in the circuit. The field built up in inductance 94 continues to discharge condenser even after zero potential exists across the condenser plates. This collapsing field continues to drive the condenser sufiiciently beyond its fully discharged condition (zero volts across the condenser plates) to extinguish the gas tube 86, upon the termination of the pulse from the autotransformer.

The pulse circuit shown in Figure 3 is especially useful in connection with pulsed-light radar systems. It is also useful in photographic applications where pulses having predetermined durations and high repetition rates are required.

Although no means are shown in Figure 3 for changing the light duration, this can be accomplished by changing the constants of the circuit. One way of doing this, for example, is to alter the inductance of radio frequency choke 94.

Although the embodiments of the invention illustrated are synchronized by a mechanically driven switch, it should be understood that the invention is equally applicable to circuits synchronized by electronic means, such as by a free running blocking oscillator, and to free running circuits. In circuits of the latter type, switch 48 is eliminated and control grid 36 is biased to a value such that it fires the thyratron when the thyratron anodeto-cathode voltage reaches a given value.

The maximum pulse repetition frequency of the circuits shown in Figures 1 and 3, in those cases in which it is desired to use the full charge storing capability of the storage condenser, is determined by the period of time between points 56 and 70 of the wave shown in Figure 2a. This time can be reduced to a minimum by proper component design. The pulse transformer lumped constants chosen should be those which will drive the storage capacitor and triode control grid to a point just sufiicient to cut oil the triode and deionize the thyratron. The triode bias will then quickly increase to a value above the triode cut off voltage and the condenser charging cycle will again begin.

In a practical circuit according to this invention, the following circuit elements and voltages may be employed. It is to be understood that the elements and voltages listed are illustrative of the invention and not meant to be limiting.

Source voltage 600 volts D.C. Resistor 46 2,200 ohms. Resistor 38 150,000 ohms. Resistor 40 120,000 ohms. Resistor 44 150,000 ohms. Resistor 34 l0 megohms. Resistor 24 3.900 ohms. Resistor 70 100,000 ohms. Condenser 42 .01 microfarads. Condenser 12 .47 microfarads. Condenser 64 .001 microfarads. -Thyratron 26 Type 0A5. Light tube 86 Type G.E. Ft 118 or equivalent.

What is claimed is:

1. In combination, charge storage means; a charging circuit for said storage means including a load circuit and an element the impedance of which is capable of being changed from a relatively low value to a relatively high value connected in series with said storage means, said charging circuit and said load circuit, said element having a relatively low impedance value during the charging time of said storage means; a discharging circuit for said storage means including a gas discharge tube connected across said storage means and said load circuit; means coupled to said discharge tube for firing the latter so as to discharge said storage means; and means coupled to said element for sensing the discharge of said storage means and for changing the impedance of said element from its low to its relatively high value in response to said discharge of said storage means.

2. In the combination as set forth in claim 1, said element comprising a grid controlled vacuum tube.

3. In combination, a storage condenser; connections for a source of direct potential; a load circuit; an element the impedance of which is capable of being changed from a relatively low value to a relatively high value connected in series with said condenser, said connections and said load circuit, said element having a relatively low value during the charging time of said condenser; a gas discharge tube etfectively connected across said storage capacitor and said load circuit; means coupled to said discharge tube for firing the latter so as to discharge said condenser; and means for sensing the discharge of said condenser for changing the impedance of said element from its low to its relatively high value in response to said discharge.

4. In combination, charge storage means; a charging circuit for said storage means including a load circuit and also including a grid controlled vacuum tube, the anode-to-cathode circuit of said vacuum tube being connected in series with said storage means and said load circuit, said vacuum tube being biased to a value such that it conducts during the charging time of said storage means; a discharging circuit for said storage means including a gas discharge tube connected across said storage means and said load circuit; means coupled to said discharge tube for firing the latter so as to discharge the storage means; and a feedback circuit connected between said charge storage means and the grid of said vacuum tube for applying a voltage to said grid of the correct polarity and amplitude to drive said vacuum tube beyond cut off in response to discharge of said storage means.

5. In combination, charge storage means; a charging circuit for said storage means including an output transformer and also including a grid controlled vacuum tube, the anode-to-cathode circuit of said vacuum tube being connected in series with said storage means and said transformer, said vacuum tube being biased to a value such that it conducts during the charging time of said storage means; a discharging circuit for said storage means including a gas discharge tube connected across said storage means and said transformer; means coupled to said dischargetube for firing the latter so as to discharge the storage means; and a feedback circuit including the primary winding of said transformer connected both to the grid of said vacuum tube and said storage means for applying a voltage pulse to said grid of the correct polarity and amplitude to drive said vacuum tube beyond cut off in response to discharge of said storage means.

6. In combination, charge storage means; a charging circuit for said storage means including a voltage responsive element which is capable of being changed from a relatively low value of impedance to a relatively high value of impedance connected in series with said storage means, said element having a relatively low value during the charging time of said storage means; a discharging circuit for said storage means including an inductance and a gas discharge tube connected in series with each other and across said storage means; means coupled to said discharge tube for firing the latter so as to discharge said storage means; and means including said inductance in the discharge circuit of said storage means coupled to said impedance element for applying a voltage thereto for changing its impedance from its low to its relatively high value in response to discharge of said storage means.

7. In combination, a gas tube, a circuit element having a non-linear impedance characteristic, a condenser, connections for a source of direct potential, a load circuit including a transformer having a primary winding, said connections, said condenser, said circuit element, and said primary winding being connected in series with each other, said gas tube being connected across said condenser and saidprimary winding, and a secondary winding of said transformer connected to a load, means for ionizing said gas tube to discharge said condenser through said transformer, and means responsive to the discharge of said condenser for substantially increasing the resistance of said non-linear resistance element so as to decrease the deionization time of said gas tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,310,092 Knowles Feb. 2, 1943 2,342,257 Edgerton Feb. 22, 1944 2,426,602 Edgerton Sept. 2, 1947 2,478,901 Edgerton Aug. 16, 1949 2,478,907 Edgerton Aug 16, 1949 2,606,308 Parker Aug. 5, 1952 a

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