US3008069A

US3008069A - Relaxation oscillators

Info

Publication number: US3008069A
Application number: US735543A
Authority: US
Inventors: Edward J Sheldon; Benjamin R Cole
Original assignee: Raytheon Co
Current assignee: Raytheon Co
Priority date: 1958-05-15
Filing date: 1958-05-15
Publication date: 1961-11-07
Anticipated expiration: 1978-11-07

Description

atent fifice 3,h93,%9 Patented Nov. 7., 1961 3,008,069 RELAXATION ()SClLLATORS Edward J. Sheldon, Lexington, and Benjamin R. Coie, Arlington, Mass, assignors to Raytheon ornpany, a corporation of Delaware Filed May 15, 1958, der. No. 735,543 6 Claims. (6i. 317-446) The invention relates to a relaxation oscillator and more particularly pertains to an oscillator circuit arrangement for use as an electrical timing mechanism to directly control the current through the control winding of an electromagnetic device.

Electrical timing devices are required for many purposes in which the control Winding of a relay is required to be periodically energized for short periods of time. For example, various automatic sampling devices require the rapid performance of sampling operations at regular intervals at a relatively slow repetition rate. By periodically actuating a relay for a short period of time, such sampling can easily be accomplished. Yet another example for the employment of electrical timing devices of the type here disclosed is in automatic calibration circuits where it is desired to periodically calibrate an apparatus during a short time interval but to leave the apparatus available for use at all other times. By periodically energizing a relay to connect the apparatus to a source of calibration signals for a short period of time and maintaining the relay deenergized for a long period of time to isolate the calibration signal source, the objective is readily attained. Hence any generator employed to energize the control winding of such a relay must have a slow repetition rate and develop a high current during the short energization period to cause rapid actuation of the relay. Because of the requirement that a high current be developed during a short period, it has usually been found necessary to provide a buffer current amplifier between the oscillator and the relay winding to prevent loading the oscillator while the winding is drawing a large current. This has resulted in increased complexity and cost of equipment with an attendant increased probability of malfunction due to the increased complexity.

In accordance with the preferred form of the iIIVGIL' tion an improved relaxation oscillator of the phantastron type is employed to periodically generate pulses of current of large magnitude which energize the control winding of a relay. A multi-grid tube is utilized in the oscillator and the space current flowing through the tube is caused to flow alternately to the anode and to the screen grid. The control winding of a relay is directly connected into the screen grid circuit of the improved oscillator and the winding is energized by the large current which periodically flows to the screen grid. The relaxation oscillator develops the slow repetition rate without the necessity of external triggers and the oscillator provides ample current to operate the relay. The circuit, therefore, cornbines the desirable feature of low frequency timing with direct relay operation. The novel circuit makes use of relatively few parts and is inexpensive to construct.

The construction of the invention, and its mode of operation can be better apprehended by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic drawing of the preferred embodiment of the invention,

FIG. 2A indicates the voltage waveform occurring at the plate of the oscillator tube,

FIG. 28 indicates the voltage e at the control grid of the oscillator tube,

FIG. 20 shows the voltage waveform e occurring at the screen grid of the oscillator tube, and

FIG. 2D depicts the voltage waveform e occurring at the suppressor grid of the oscillator tube.

Referring to FIG. 1 which schematically represents the relay control circuit, there is shown an electron tube 1 of the pentode type having three

grids

3, 4, 5 arranged in succession between a cathode 2 and an anode 6. The anode or plate 6 is connected through a resistor 9 to a source of positive voltage indicated by B+ impressed at terminal 18 and the cathode 2 is directly connected to ground. A grid resistor 8 is connected between control grid 3 and a switch 10 whereby the control grid may, depending on the position of the switch, be either connected to ground or to a source of negative voltage B impressed at terminal 17 through a resistor 16. A capacitor 7 is connected between anode 6 and control grid 3. The screen grid 4 is connected through control winding 15 of a relay 19 to the B+ supply whereby the relay will be actuated by an appreciable flow of current to the screen grid 4. The relay is represented by the winding 15, an iron core, a pair of contacts, an armature on which one of the contacts is mounted, and a spring secured to the armature and normally holding the contacts open. A capacitor 13 provides a connection between screen grid 4 and suppressor grid 5. A resistor 12 interconnects the suppressor grid 5 and the 13+ supply and the suppressor grid is returned to ground through the parallel connected resistor 11 and capacitor 14.

With switch 10 in the OFF or left position, control grid 3 is biased sufliciently negative to hold tube 1 at cut off by a voltage impressed through resistor 16. It is assumed that while the switch is in the OFF position the

capacitors

7, 13, and 14 are fully charged so that the screen grid 4 is at B+ potential and the suppressor grid 5 is positive with respect to ground. When the switch it) is placed in the ON or right position the potentials are as shown in FIG. 2 at time t that is, the voltage e on the grid 3 is sufliciently negative to hold the tube at cutolf and since no current is flowing in the tube the plate voltage e is at B+, the voltage e on the screen grid 4 is at B+, and the voltage e on the suppressor grid 5 is at a positive value. Capacitor 7 commences to discharge through resistors 8, 11, 12, and 9 when switch 10 is moved to the ON position until at time t the voltage at control grid 3 rises sufliciently to cause current to flow in the tube. The suppressor grid 5 at time t is highly positive and hence nearly all the initial current in tube 1 flows to the plate 6. Upon the inception of current flow in the tube at time t the voltage at the plate 6 drops abruptly causing a negative going signal to be coupled through capacitor 7 to control grid 3 tending to reduce current in the tube and holding the control grid 3 near cut-oil. The phenomenon which now occurs is known as the Miller action and an exposition of this process is set forth on pages et seq. of vol. 19 entitled, Waveforms, of the Radiation Laboratory Series, first edition, published by McGraw Hill. Because of the Miller action, the potential e at the plate 6 drifts down at a rate determined by resistor 9, capacitor 7 and the gain of the tube 1 while the grid voltage e rises very gradually to permit an increasing current to flow. The values of resistor 9 and capacitor 7 may be selected so that the plate drifts down at a very slow rate, and a rate in the neighborhood of 1 or 2 seconds is feasible without requiring inordinately large values of capacitance for capacitor 7. When the plate voltage e bottoms at time 1 that is, when the plate cannot become further negative, the control grid voltage e which had been held below ground by the feedback through capacitor 7, now jumps to ground potential because the plate potential is immobilized and the feedback through capacitor 7 ceases. The jump in control grid voltage e at time t increases current flow in tube 1, but all of the increased current flows 3: to the screen grid 4 since the plate is immobilized and cannot accept any increase in current. The current flowing to the screen passes via conductor 29 into relay winding 15 thereby causing a large drop in screen potential because of the windings inductive reactance and causing the relay 19 to be actuated. The drop in screen potential c at time Z is coupled through capacitor 13 to the suppressor grid causing the potential e to fall and reducing the current to the plate 6. A reduction in plate current results in a rise in plate voltage e which is coupled through capacitor '7 to control grid 3 causing the control grid to become positive and draw grid current. The reduction in plate current causes an increase in the space current flowing to the screen grid, causing a further drop in screen potential, which is coupled through capacitor 13 to further decrease the potential e of the sup pressor grid 5 and thereby further decreasing the current to the plate 6. The action is regenerative and in a very short time nearly all the space current flows to the screen grid and flow to the plate 6 is very nearly cut-off. Though current to the plate is rapidly reduced, the plate potential e cannot immediately climb to 13+ because capacitor 7 inhibits any rapid change in potential. However, as capacitor 7 becomes charged by a current largely drawn through control grid 3, the plate potential e will rise exponentially towards 3+ as shown by the waveform between times 1 and t While the plate is rising, capacitor l3 discharges through lead 26, screen grid 4', cathode l, and resistor 11 so that the suppressor grid 5 which had been driven heavily negative now begins to rise as indicated in FIG. 2D by the waveform e immediately after time t The capacitor 14 tends to oppose the discharge of capacitor 13 and increases the discharge time of capacitor 13 so that the voltage at the plate 6 has an increased time in which to climb toward B When the suppressor grid potential rises sufiiciently as at time t space current in tube It is permitted to again flow to the plate 6 causing the plate voltage e to drop by a step of about five volts. This drop is coupled through capacitor 7 to control grid 3 and causes that grid to abruptly drop to a negative voltage and sharply reduces current flow in the tube. A reduction in space current in the tube causes a reduction in current to the screen grid 4 thereby causing a sharp rise in e as shown by the waveform of FIG. 2C at time t the rise in screen grid potential being in turn coupled through capacitor 13 to the suppressor grid 5 causing that grid to become more positive whereby more of the space current in the tube flows to the plate 6. Thus a regenerative action occurs and the space current rapidly switches to the plate with none or an inappreciable amount of the space current flowing to the screen grid. When the space current switches to the plate at time t the Miller action again occurs and the plate voltage will rift down between time t and L; to commence another cycle of operation.

The cyclical operation of the relay is controlled entirely by the intermittent flow of current to the screen grid of oscillator tube 1. During the time (t to t that the plate voltage drifts down, the flow of space current to the screen grid is negligible and the winding 15 of the relay is not energized. The period t to t for plate voltage run down can be made quite long without requiring large values of capacitance so that the oscillator is admirably suited to develop a slow repetition rate. When the space current switches to the screen grid at time t a large current, in the order of ten to fifteen milliamperes can be caused to flow in the winding of the relay so that actuation of the relay is assured. By selection of circuit components the time, between 1 and t during which current flows to the screen grid is made short. Hence, the relay winding is periodically quickly energized by pulses of current developed at a slow repetition rate in the screen grid of the oscillator tube.

By way of example, a circuit constructed according to FIG. 1 with the following values of circuit components has performed satisfactorily, causing the relay to he actuated for a period of about twenty milliseconds at a repetition rate of one actuation per second.

Tube 1 Type 6AS6. Resistor 8 1 megohm. Resistor 9 200 K ohms. Resistor 11 510 K ohms. Resistor 12 l megohm. Capacitor 7 .1 uf. Capacitor 13 .1 uf. Capacitor 14 .022 uf.

B+ voltage volts.

The waveforms occurring at various locations in the circuit are depicted in idealized form in FIG. 2 but it should be understood that the values of circuit components which are employed will alter the idealized forms without in any other way departing from the operation which has been described above.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A circuit for directly controlling the periodic actuation of an electromagnetic relay comprising a vacuum tube having a cathode, control grid, screen grid, suppressor grid, and anode, said cathode being connected to a common junction, impedance means connecting said anode to a source of potential, a first capacitor connecting said anode to said control grid, means to discharge said first capacitor through a path including said common junction, a second capacitor interconnecting said screen and suppressor grids, the control winding of said relay connecting said screen grid to said source of potential, means to charge said second capacitor from said source, and means to discharge said second capacitor through a path including said screen grid and said cathode.

2. A circuit for directly controlling the periodic actuation of a relay having a control winding comprising a' vacuum tube having a cathode, control grid, screen grid, suppressor grid, and anode, said cathode being connected to a common junction, a source of potential having its positive terminal connected through an impedance to said anode, a first capacitor interconnecting said anode and said control grid, means to discharge said first capacitor through a path including said common junction, the control winding of said relay connecting said screen grid to said positive terminal, a second capacitor interconnecting said screen and suppressor grids, means to charge said second capacitor from said source, and a parallel resistance-capacitance network connected between said suppressor grid and said cathode.

3. An oscillator circuit for directly controlling the periodic actuation of a relay having a control winding comprising a vacuum tube having a cathode, control grid, screen grid, suppressor grid, and anode, said cathode being connected to a common junction, a source of potential having its positive terminal connected through an impedance to said anode, a first capacitor connected between said anode and said control grid, means to discharge said first capacitor through a path including said common junction, a voltage dividing resistive network connected to said potential source, said suppressor grid being connected to an intermediate point of said network, a second capacitor connected between said intermediate point and said screen grid, the control winding of said relay connecting said screen grid to said potential source, and a third capacitor connected in parallel with a portion of said resistive network and between said intermediate point and said cathode.

4. A relaxation oscillator circuit for directly controlling the periodic actuation of an electromagnetic relay comprising a vacuum tube having a cathode, control grid,

screen grid, suppressor grid, and anode, an impedance connecting said anode to the positive terminal of a potential source, said oathode being connected to a common junction in circuit with the negative terminal of said source, a first capacitor connecting said anode to said control grid, 2. switch connected to said control grid whereby in one position of said switch the control grid is biased to cut ofi conduction in said tube and in another position of said switch said control grid is connected in circuit with said common junction, the control winding of said relay connecting said screen grid to said positive terminal, a second capacitor interconnecting said screen and suppressor grids, means to charge said second capacitor from said source, and means to complete a discharge path through said screen grid and said cathode for said second capacitor.

5. A relaxation oscillator circuit for directly controlling the periodic actuation of an electromagnetic relay com,- prising a vacuum tube having a cathode, control grid, screen grid, suppressor grid, and anode, an impedance connecting said anode to the positive terminal of a potential source, said cathode being connected to a common junction in circuit with the negative terminal of said source, a first capacitor interconnecting said anode and said control grid, a switch having one station connected to a source of negative potential and a second station connected to said cathode, means connecting said control grid to said switch whereby said control grid may be connected to either station, said relay having a control winding connected between said screen grid and the positive terminal of said source, a second capacitor interconnecting said screen and suppressor grids, a resistor connected between said suppressor grid and said positive terminal, and a parallel resistance-capacitance network connected between said suppressor grid and said cathode.

6. A circuit for directly energizing a current responsive device comprising a vacuum tube having a cathode, con trol grid, screen grid, suppressor grid, and anode, said cathode being connected to a common junction, impedance means connecting said anode to a source of po tential, a first capacitor connecting said anode to said ioontrol grid, means to discharge said first capacitor through a path including said common junction, a second capacitor interconnecting said screen and suppressor grids, said current responsive device having a winding connecting said screen grid to said source of potential, means to charge said second capacitor from said source, and means to discharge said second capacitor through a path which includes said screen grid and said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,226,561 Herold Dec. 31, 1940 2,275,016 Koch Mar. 3, 1942 2,431,237 Freeman Nov. 18, 1947 2,755,385 Parsons July 17, 1956