US2931983A - Method of charging, holding, and automatically resetting the voltage level on a condenser - Google Patents

Description

April 5, 1960 R. F. BLAKE METHOD OF CHARGING, HO

2,931,983 LDING, AND AUTOMATICALLY RESEITTING THE VOLTAGE LEVEL ON A CONDENSER Filed July :51, 1956 2 Sheets-Sheet 1 INVENTOR ATTORNEYJ Aprll 5, 1960 R. F. BLAKE 2,931,983

METHOD OF CHARGING, HOLDING, AND AUTOMATICALLY RESBTTING THE VOLTAGE LEVEL ON A CONDENSER Filed July 31, 1956 2 Sheets-Sheet 2 ivm INVENTOR RICHARD F. BLAKE W ATTORNEYJ United States Patent METHOD OF CHARGING, HOLDING, AND AUTO- MATICALLY RESETIIN G THE VOLTAGE LEVEL ON A CONDENSER Richard F. Blake, Mountain Lakes, NJ., assignor to the United States of America as represented by the Secretary of the Navy Application July 31, 1956, Serial No. 601,326

6 Claims. (Cl. 328-121) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates generally to condenser charging and discharging circuits and more particularly to a condenser charging and discharging circuit having quiescently an extremely high back impedance whereby a charge impressed on a condenser may be retained thereon over a long period of time.

In the prior art it is common to utilize a condenser as an information item storage unit, the condenser being impressed with a charge which is a measure of the information carried by a transient input signal. Once the charged state of the condenser has been created, discharge of the condenser is prevented for a selected interval of time during which interim, therefore, the originally transient information carried by the input signal is retained in the quasi-permanent form of the condenser charge and is available as an electrical signal for control purposes or the like. When a given information item stored by the condenser has performed its purpose the condenser may be discharged at will thereby erasing the given information item and preparing the way for the impression of fresh information upon the condenser.

In the prior art the impression of an information signal on the condenser and the prevention of subsequent loss by leakage discharge currents of this signal are commonly accomplished by charging the condenser through a rectifier or a device having a similar effect so far as charge and discharge currents are concerned. One type of element used is a gas filled cold electrode tube such as a neon tube. Employment of a cold electrode tube, however, involves the following disadvantages. First, since some time is required for tube ionization, with respect to the onset of the input signal the condenser charging is delayed. Second, upon termination of the input signal the cold electrode tube has a deionization period during which time discharge current will flow from the condenser and backward through the tube destroying the stored condenser signal. Third, particularly in the case of neon tubes or the like, during a quiescent storage period if the voltage differential between the tube electrodes is sufficiently great, since the tube conducts equally well in both directions, a back discharge will occur through the tube, the information stored on the condenser thus being destroyed.

As a result of the disadvantages of the cold electrode tube the prior art commonly employs instead a thermionic diode as a rectifier element. The thermionic diode, however, is characterized by the disadvantage that betweeen its cathode and its filament exists a relatively low impedance leakage path permitting discharge of the condenser and consequent destruction of the information stored thereon within a relatively short period of time. This disadvantage, of course, my be overcome by isolating the diode filament from ground but only at the price of unduly complicating the circuit and changing the condenser characteristics as an information storage unit.

To summarize the discharge current problem, whether a cold electrode discharge device or a thermionic diode alone is utilized the information signal stored on the condenser rapidly deteriorates with time. In addition, in the case of the cold electrode tube, because of the tube ionization time the circuit has a minimum delay which limits its accommodation to short duration input signals. Further in the absence of some sort of charging current linearizing device the magnitude of the condenser charge bears no simple relation to the information carried by the originative input signal.

It is therefore an object of this invention to provide a condenser charging and discharging arrangement whereby a charge once impressed on the condenser is retained substantially unchanged over a considerable period of time.

It is a further object of this invention to provide a condenser charging and discharging arrangement whereby the charge impressed on the condenser is a substantially unchanging and simple measure of the transient signal originative of the charge.

It is a further object of this invention to provide a condenser charging and discharging arrangement incorporating a gas filled cold electrode tube whereby the minimum delay caused by the tube ionization time may be decreased.

Other and further objects and features of the present invention will become apparent upon a careful consideration of the following description when taken together with the accompanying drawings which illustrate typical features of the invention and the manner in which the invention may be considered to operate.

In the drawings:

Figure 1 represents in schematic form one illustrative embodiment of the present invention;

Figure 2 represents wave forms associated with the embodiment of Figure l in the operating condition.

Briefly the objects of invention set forth above are achieved by employing the series combination of a thermionic diode and a gas filled cold eelctrode tube to charge the condenser. During a quiescent period when information is stored on the condenser the cold electrode tube prevents loss of the stored signal by discharge current flow from the condenser and through the leakage path between the cathode and the filament of the thermionic diode, the cold electrode tube furnishing an open circuit between the condenser and the leakage path. Conversely the thermionic diode prevents loss of the stored signal through the cold electrode tube during the deionization period of the same or as a result of a breakdown voltage differential between the tube electrodes. The charging current is applied to the series combination mentioned by a boot strap amplifier responsive to the input signal. By utilizing a boot strap circuit the ionization delay time of the cold cathode discharge device is reduced permitting accommodation of short duration signals, and the charge on the condenser is made to bear a simple relation to the characteristic of the originative input signal. The condenser may be discharged and the stored information signal thus erased by a gate signal applied to a normally non-conducting tube connected between the condenser plates.

Referring now to Figure 1, the numeral 20 designates an input signal amplifier tube having a plate 22, grid 24 and cathode 26. Plate current is fed to tube 20 from a B supply at volts through a dry rectifier 28 and a plate resistor 30. The cathode 26 of tube 20 is directly connected to a -l50 volt supply. Information carrying input signals are coupled from a source not shown to the grid 24 by a coupling condenser 32 connected between the source and grid 24. A grid leak resistor 34 conmeted between grid 24 and cathode 26 maintains the grid quiescently at the voltage of the cathode.

Associated with input signal amplifier tube 20 is a boot strap tube 36 having a plate 38, grid 40' and cathode 42. The plate 38 is directly connected to the B supply of +150 volts. The cathode 42 is connected to ground through a load resistor 44, and to the junction of rectifier 28 and plate resistor 30 by means of a coupling condenser 46. The grid 4t) is coupled to the plate 22 of tube 20 by means of a resistance 48.

The plate signals of input signal amplifier tube 20 are applied to a thermionic diode 50 having a plate 52, cathode 54 and filament 56. The plate 52 of diode 50 is directly connected to the plate 22 of amplifier 29. Heating current is supplied to the filament 56 of diode Si) by a current supply 58, typically a battery, one terminal of which current supply is connected to ground. Between cathode 54 and filament 56 inherently there exists a relatively low impedance leakage path, typically of only a few megohms, which leakage path is represented in Figure 1 as the dotted resistor 60.

Connected in series with diode 50 is a gas-filled cold electrode discharge tube 62, typically a neon glow-tube, having a pair of electrodes 64 and 66. Between electrodes 64 and 66 inherently there exists an extremely high leakage impedance path the significance of which will be later discussed. This high impedance leakage path is represented in Figure l by the dotted resistor 65. Other types of gas filled cold electrode tubessuch as cold cathode rectifiers may also be employed.

Electrode 66 of tube 62 is connected to the high side of condenser 68, the low side of which is connected to ground. Condenser 68 performs the function of retaining the information carried by the transient input signals by assuming a quasi-permanent charge which is a measure of a given information item. The information while so stored may be utilized for control purposes or the like by means of a cathode followertube 70 having a plate 72, grid 74 and cathode 76. I The plate 72 is directly connected to a B supply of +105 volts. The cathode 76 of tube 70 is connected to groundthrough a load resistance 78 across which load resistance a utilizable output signal appears between terminal 89 connected to the ungrounded side of resistor 78 and terminal 82 connected to ground. The grid 74 of tube 70 is connected to the high side of storage condenser 68 by means of a high impedance coupling resistor 84.

Information stored upon condenser68 may be removed by erasure tube 86 having a plate 88, grid 90 and cathode 92. The plate 88 is directly connected to the high side of condenser 68 and the cathode 92 is connected to ground. The grid 90 of erasure tube 86 is normally held strongly negative by connection to a --50 volt supply through resistor 94. Erasure gate signals may be impressed upon grid 98 by means of condenser 96 which couples grid 90 to an erasure gate signal source not shown. Between plate 88 and cathode92 of the erasure tube inherently there exists an extremely high impedance leakage path the significance of which will be later discussed. In Figure '1 this leakage path is represented by the dotted resistor 98. a

Typical values for the circuit elements of the embodiment of Figure 1 are indicated in the drawing.

Considering now the mode by which the device may be thought to operate reference must-be made to both Figures 1 and 2. In Figure 2 the diagramsI, II, III and IV represent concurrent wave forms, spaced along a time axis, of the signalsassociated with thecondenser charging and discharging circuit. Diagram I depicts the input signals to the device; diagram 11 the voltage at the 'plateof input signal amplifier tube 20; diagram III the charge impressed on the storage condenser 68. or alternatively the, potential exhibited by this condenser and diagram IV depicts the erasure gate signal.

Initially assume that thecircuit of Figure 1 is in a quiescent erasure condition. In such state, at most, a negligible residual charge exists on condenser 68 representative of zero information. Erasure tube 86 is maintained in a condition considerably below cutofl as a result of the heavy negative bias impressed on its grid. The input signal amplifier tube 20 is maintained in a conducting condition with its plate being at approximately 30 volts. As a result of this negative plate voltage with respect to ground, no charging current can pass through diode 50 and discharge tube 62 to effect a change in the charge on condenser 68. Also as a result of this negative plate voltage the boot strap tube 36 is held at below cutoff.

Referring to diagram I of Figure 2, assume now that an input signal S is impressed on the grid 24 of tube 20 at the time a. The input signal S has the form of a negative pulse with the width W of the pulse being a measure of an item of information carried and the magnitude of the pulse being suflicient to cut off tube 20. In diagram I, the width W of the signal 'S has an illustrative value of 50 microseconds. When tube 20 is cut oil at time a, its plate voltage rises extremely sharply as shown by the corresponding wave form in diagram II. The discharge tube 62 has a short but appreciable necessary ionization time during which no current can be conducted through tube 62. As a result there is no passage of current from the plate 22 to condenser 68 which alleviates the sharp rise in potential of plate 22. Plate 22 thus remains at a high potential for the interval between time points a and b, at the end of which interval tube 62 breaks down and conducts.

The ionization time for the tube 62 varies roughlyim versely with the voltage applied between the tube electrodes. In the absence of the boot strap tube 36 the tube 62 in the embodiment of Figure 1 would break down in about 25 microseconds. It has been found, however, that by the use of the boot strap tube 36 thefiring delay for tube 62 can be decreased to about 10 microseconds, this mentioned value representing the lower limit for the width of input pulses utilizable with the presently disclosed embodiment. The boot strap tube 36 operates as follows. As the plate voltage of tube 20 rises this rise is communicated through resistance 48 to the, grid 40 of the boot strap tube, the positive signal on grid 40 induces a flow of current through load resistor 44, the cathode 42 of tube 36 thus being raised positively with respect to ground. The positive rise of cathode 42 in turn is communicated to the junction point of rectifier 28 and resistor 30'by means of the condenser 46. Condenser 46 thus acts as a transient auxiliary supply of plate current for the tube 2.0, the auxiliary supply being at a morepositive voltage than the normal supply. Since the described interaction between tubes 28 and 36 is obviously cumulative both the plate 22 of tube 20 and the cathode 42 of tube 36 will rise in potential up to the point where tube 36 no longer effectively operates as a cathode follower. In the embodiment shown in Figure 1 this point is reached when the plate 22 attains a potential of about +250 volts. With this high transient voltage available at plate 22 the ionization time necessary for discharge tube 62 is shortened to ten microseconds as previously stated.

As mentioned heretofore, at time b, the discharge tube 62 breaks down and current commences to flow from the plate terminal of tube 20 into the storage condenser 68. Since the tube 62 self-maintains a discharge sustaining potential between its two electrodes, typically 75 volts, which potential permits the continued flow of current through the tube, at point b the plate voltage of tube 20 will not drop to ground but rather to'a level above ground equal to the ionizing potential of the tube 62. As shown by diagram III, however, commencing with point b the charge across the plates of condenser 68 will start to build up from its residual value.

The interval between the time points b and c' IBPIB".

conduction through tube 62 can occur.

sents the charging period for the condenser 68. In the absence of the boot strap circuit the charging characteristic of the condenser would be exponential rather than linear with the magnitude of charging current decreasing over a period of time compared to an initial heavy value. By utilizing the boot strap tube 36, however, in a manner well known in the art, a constant charging current and resultant linear charging characteristics for the condenser is obtained as shown in diagram III in which, during the interval be, the amount of charge impressed on the condenser 68 is in direct ratio to the time of charging.

Of course if it is not desired to utilize the presently disclosed invention with input signals of short duration relative to the circuit parameters, the boot strap tube 36 may be eliminated. In such case, for many applications sufficient linearity is achieved by utilizing only the initial portion of the exponential condenser charging curve. Conversely if it is desired to extend downward the lower limit of input signal pulse width accommodation, a higher voltage input signal amplifier tube and boot strap tube may be employed, thereby further decreasing the ionization time of the tube 62.

The same linear relation Will occur in the case of a signal S of greater duration W Since in both cases the potential exhibited across the plates of condenser 68 is directly proportional to the charge impressed on the condenser, diagram III may be considered to represent condenser potential as well as condenser charge, the condenser potential at times and k being V and V respectively. It will be noted, however, that whereas the input signals S and S commence at time a and i respectively, the charging of the condenser does not commence until times b and j respectively. Accordingly, the signal width is not directly proportional to the maximum condenser potential but it may be expressed by the following formula:

W=K (V +K where W is the signal Width, V the potential to which the condenser has charged by the end of the signal, and K and K are constants derived from the discharge tube ionization time and the RC time constant of the charging path respectively. By employing this formula the potential on the storage condenser 68 may be utilized as a measure of the width of the originative input signal which in turn is indicative of the information carried by the signal.

Obviously the maximum width signal which may be accommodated by the present system using a fixed value 'for resistance 30 depends upon the capacitance of condenser 68 since a point will be reached in charging the same where the condenser potential will attain so high a level that the cathode follower section of the boo-t strap amplifier can no longer operate linearly. Beyond this point no further charge can be accumulated by condenser 68 without distortion. Of course, by utilizing a larger charging resistance or storage condenser, the rate of potential increases may be retarded. Thus by increasing the RC product the rate at which the condenser potential approaches the distortion level is decreased. In this manner the ability of the circuit to handle greater width pulses may be increased almost indefinitely.

Upon reaching the time point c the input signal S terminates and tube 20 is once again rendered conducting, its plate 22 returning to a level of about 30 volts. A considerable amount of time, typically on the order of several hundred microseconds, must be allowed for the discharge tube 62 to deionize, during which period back For example, referring to Figure 1, if the electrode 64 of tube 62 were connected directly to the plate 22 of tube 20 instead of being connected as shown through the thermionic diode 50, during the deionization period a large part of the charge stored in condenser 68 would escape from the same by conduction through tube 62 to the plate of tube 22. The thermionic diode 50, however, is incapable of conducting from cathode to anode the instant its plate potential becomes lower than that of its cathode. Accordingly, during the deionization period of discharge tube 62 the thermionic diode 50 prevents the flow of these back currents which would rapidly discharge the condenser.

In Figure 2 the interval betwen time points a and e as shown by the dotted lines of the diagrams represents an indefinite but selectable period of time. The interval between 0 and 1, therefore, represents a period of no fixed duration during which a condition of quiescent storage of information exists in the arrangement of Figure 1. During quiescent storage the charge impressed on the condenser must be retained thereon substantially without change for the storage period. It follows that leakage discharge currents from condenser 68 must be eliminated as completely as possible. As mentioned heretofore, in the ordinary thermionic diode there exists a low impedance leakage path typically of a few megohms between the cathode and the filament. This leakage path is shown in Figure 1 by the dotted resistor 60. With filament 56 of diode 50 directly connected to the low side of condenser 68 as shown, in the absence of glow tube 62 the charge stored on condenser 68 would rapidly leak off through leakage path 60, thus rendering the condenser impracticable as an information storage device over any substantial period of time. If, to avoid this the filament is maintained at the voltage of the high side of condenser 68, a floating supply for the filaments is then required which is more expensive, complicated and introduces other losses through the power supply. In the embodiment shown in Figure 1, however, the gas filled, cold electrode discharge tube 62 is interposed as substantially an open circuit between the leakage path 60 and the high side of condenser 68. As a result the leakage discharge current is negligible through the path between the cathode and filament of diode 50.

Another possible leakage path by which condenser 68 might rapidly discharge is through the erasure tube 86. If erasure tube 86 were maintained in a condition only slightly below cutoff sufficient discharge current would flow internally through the tube to render condenser 68 impracticable as an information storage device. In the operation of the present embodiment, however, the erasure tube during quiescent storage is maintained in a condition so far below cutoff that the internal current through the tube is negligible.

While the circuit is in the quiescent storage condition the potential exhibited by condenser 68 may be utilized for control purposes or the like. A control signal is extracted from the system without destruction of the information signal stored upon the condenser by utilizing the potential across the plates of condenser 68 to control the flow of current through the cathode follower 70. The control signal appearing across the load resistor 78 of cathode follower 70 may be coupled by terminals 80 and 82 to a utilization device not shown.

In utilizing the cathode follower tube 70 to obtain an output signal from the condenser 68 it has been found that the following phenomenon takes place. A fraction of the primary electrons speeding from the cathode 76 to the plate 72 will strike the grid 74 causing secondary emission of electrons. These secondary electrons, of which there are a greater number than the fraction of primary bombarding electrons, will also be drawn towards the plate 72. There results in effect a charging current flowing from plate 72 through resistance 84 and into condenser 68, this current being of a very low value and equivalent to the current which would be produced through an impedance of some thousands of megohms shunted between plate 72 and grid 74. Now as a result of the inherent imperfection of electrical devices some leakage paths, albeit extremely high impedance ones, exist for discharge currents from the condenser 68. One of these extremely high impedance leakage paths is represented in Figure 1 by the dotted resistor 65. An-

other such path is represented in Figure'l by the dotted .resistor 98; Since these high impedance leakage discharge paths exist and'are ineradicable, the minute charging current for condenser '68 derived from the cathode follower 70 represents an asset rather than a liability.

In the present embodiment shown in Figure 1 the charging current to condenser 68 from cathode follower 70 largely offsets the discharge current from condenser 68 through the high impedance leakage discharge paths, for example paths 65 and 98, and this offsetting action occurs for the range of potentials exhibited by the condenser as stored information signals. It will thus be seen that by this offsetting action of the charge and discharge currents the effective storage time of the circuit is substantially increased.

One other function of the thermionicdiode'St) should be mentioned. It will be recollected that during the quiescent storage period the plate of input signal amplifier tube is maintained at a level of about-30 volts. On the other hand the high side of condenser 68 may be maintained at a substantially positive voltage. In such case if electrode 64 of neon tube 62 were directly connected to the plate 22 of input signal amplifier tube 20, as a result of the high voltage differential existing between the electrodes of tube 62 a back discharge might occur through the tubedestroying the signal stored on condenser 68. A back discharge is so possible for the reason that aneon or similar glow tube, in contrast to other gas filled cold electrode tubes is bilaterally conducting.

With the connections shown in Figure 1, however, the

thermionic diode 50, by reason of its high inverse voltage rating, effectively prevents the occurrence of such a back discharge through the tube 62. o

By virtue of the arrangement presently disclosed the leakage impedance for discharge currents from condenser 68 has the unusually high value of about 500,000 meghoms. As a result of this extremely high value for the leakage paths the signals stored on a .015 mfd. condenser typically drift at a rate of no more than /3 of a volt per minute, permitting an effective information storage time of up to one hour for this size condenser. 7

Referring again to Figure 2 assume that the time point f has been selected for the erasure of the signals stored on condenser 68. At the time 1'' an erasure gate signal as shown in diagram IV is impressed on the grid 90 of erasure tube 86. This erasure gate signal may occur as a result of either manual actuation or automatically in response to a received erasure signal by means of circuits not shown. The erasure gate signal drives the grid 90 "from a level far below cutoff to a level considerably above cutofif. Erasure tube 86 thereby in effect becomes a low impedance shunt across the plates of condenser 68. Condenser 68 accordingly will rapidly discharge itself through the tube 86 by a heavy discharge current, the condenser being substantially discharged in ten microseconds. The circuit as a whole has now been converted into the quiescent erasure condition in which it remains until the reception of a fresh information carrying input signal. Such a signal is shown by the portion S in diagram I of Figure 2, S being an input pulse of about one hundred microseconds Width. a

It is therefore seen that the present invention. represents a charging and discharging circuit for an information storage condenser characterized by the fact that the signal stored by the condenser may be retained thereon without deterioration for a substantial period of time. In addition, the present invention represents a charging and discharging circuit for an information storage condenser adaptable for use with short duration input signals and productive of stored signals which, as measures of information received, bear a simple relation to the widths'of the input signals. Many applications of the present invention will occur to those familiar to the art. In particular the present invention is ideally suited for incorporation with electrically controlled devices where continuous electrode type, and a storage condenser serially connected in the order named to provide a unilateral conducting path, means for applying the signal to be stored across the terminal points of said series circuit as represented by the remote terminal of the vacuum rectifier and the remote terminal of said condenser, each of the intermediate points in said series circuit having a high impedance with respect to the remote end of said condenser whereby a unilateral condenser charging circuit is provided having substantially no leakage discharge.

2. An electrical signal storage devicevcomprising, a resistance, a thermionic vacuum rectifier, a gas-filled tube of the cold electrode type, and a storage condenser serially connected in the order named to provide a unilateral conducting path across a direct current supply, a normally conducting vacuum tube having an anode connected to the junction of said resistance and said rectifier, and an input terminal for rendering said normally conducting tube non-conducting in response to a signal to be stored.

thermionic vacuum type, a first vacuum tube biased normally conducting and having ananode connected to the junction of said resistance and said second rectifier, a second vacuum tube having a grid coupled to the anode of said first vacuum tube and a cathode capacitively coupled to the junction of said first rectifier and said resistance, and a signal input terminal for said first vacuum tube for rendering said first vacuum tube non-conductive in response to a signal to be stored.

4. An electrical signal storage device comprising, a normally conducting amplifier tube having a signalinput terminal, said amplifier tube being suitably biased to be rendered non-conducting by an input signal, a first thermionic vacuum tube having a cathode and an anode, said anode being connected to the output of said amplifier tube, a condenser, a gas-filled tube of the type having cold electrodes serially connected between said cathode and said condenser to provide a unilateralcharging path for said condenser, a second thermionic vacuum tube biased normally non-conducting and connected in parallel with said condenser, and means for selectably rendering said second thermionic tube conducting to discharge said condenser. V I

5. An electrical signal storage device comprising, a first rectifier, a resistance, a second rectifier, a gas filled tube of the cold electrode type, and a storage condenser,

above elements being serially connected in the order :coupled to the junction of said first rectifier and said resistance, a signal input terminal for said first vacuum tube for rendering said first vacuum tube non-conductive 9 in response to a signal to be stored, means for charging said condenser at substantially the same rate as a charge thereon is dissipated by leakage currents, and means for rapidly discharging said condenser.

6. An electrical signal storage device comprising, a first rectifier, a resistance, a second rectifier, a gas filled tube of the cold electrode type, and a storage condenser, said above elements being serially connected in the order named to provide a unilateral conducting path across a direct current supply, said second rectifier being of the thermionic vacuum type, a first vacuum tube biased normally conducting and having an anode connected to the junction of said resistance and said second rectifier, a second vacuum tube having a grid coupled to the anode of said first vacuum tube and a cathode capacitively coupled to the junction of said first rectifier and said resistance, a signal input terminal for said first vacuum tube for rendering said first vacuum tube non-conductive in response to a signal to be stored, a third vacuum tube having an output terminal at its cathode and an input References Cited in the file of this patent UNITED STATES PATENTS 2,122,464 Golay July 5, 1938 2,467,476 Hallmark Apr. 19, 1949 2,532,534 Bell Dec. 5, 1950 2,708,240 Casey May 10, 1955 2,798,983 Warman July 9, 1957

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