US2497681A - Electronic timing system - Google Patents

Description

Feb. 14, 1950 H, M Y 2,497,681

ELECTRONIC TIMING SYSTEM Filed Dec. 22. 1948 SOURCE o UTILIZATION CIRCUIT Inveh tor? Harry? Mayer, bym 4) 71% His Attorney.

Patented F eb. 14, 1950 ELECTRONIC TIMING SYSTEM Harry F. Mayer, Baldwinsville, N. Y., assignor to General Electric Company, a corporation of New York Application December 22., 1948, Serial No. 66,803

My invention relates to electronic timing systems and more particularly to such systems as may be employed to control the duration of processes. It is a primary object of my invention to provide improved time control means to be used in connection with electrical therapy, photographic processes, X-ray exposures, and similar short-time processes.

My invention is particularly applicable to such processes as are subject to irregular repetition in that only one manual operation is required to initiate the timing cycle. It is an object of my invention to provide a system for timing such processes wherein the cycle is automatically terminated after a pre-determined period of time and thereafter the system is immediately and automatically made ready for another timing cycle.

A particular object of my invention is to provide a time-control system for use in connection with short time processes which energizes a utilization circuit, de-energizes it after a predetermined period of time, and then automatically resets the system in readiness for another similar timing cycle. 1

A particular advantage of my system resides in the use of inexpensive and readily obtainable components. Further-more, a minimum of components is used in order to improve the reliability of the system by reducing the probability of component failure.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which the single figure is a schematic diagram of a system which suitably embodies my invention.

Referring to the drawing, there is shown a. schematic circuit diagram of an electronic timer which may be used to control the energization of a utilization circuit for example, a photographic printing apparatus, X-ray equipment, or similar apparatus (not shown). There is provided a pair of rectifiers I and 2, here shown in the form of a duplex diode 3, and a pair of electron discharge devices 4 and 5. The filaments G, 1, 8 of devices I, 2, 4 and are connected in series 2 Claims. (Cl. 175-320) with a line switch 9 and a dropping resistance In, and are supplied with heating current from a source of operating voltage II, which may be a -vo1t alternating source such as that customarily employed in house circuits. For convenience in establishing a point of reference potential, I have indicated that one side of the line from source II is grounded.

The anode 12 of rectifier l is connected through a conductor 31. a load circuit comprising a parallel combination of a resistance l3 and a capacitance I4, and conductors 38 and 39, to ground. The cathode I5 of rectifier l is connected through a conductor 40 to the anode ll of rectifier 2 and also through a conductor U to the ungrounded side of the utilization circuit not shown. The other side of the utilization circuit is connected to ground through conductor 43. The cathode I1 of rectifier 2 is connected to ground through a load capacitance l8 and conductors 42 and 43.

A timing circuit is provided, comprising a. resistance IS, a variable resistance 2| and capacitance 23. The lower end of resistance I9 is connected to the cathode 20 of device 4 and the upper end is connected to the anode I2 01 rectifier I through conductor 31. Variable resistance 2| is connected between the cathode 20 and the control electrode 22 of device 4, and capacitance 23 is connected between the control electrode 22 of device 4 and the anode l2 of rectifier I through conductors 44 and 31. The anode 24 of device 4 is connected through a conductor 45 to the cathode of I! of rectifier 2. The time constant of resistances l9 and 2| and capacitance 23 determines the duration of power application to the utilization circuit, as will appear more fully later.

I have shown devic 5 as a tetrode tube connected as a triode by virtue of a direct connection between the anode 25 and the screen grid 26. Anode operating potential for device 5 is provided from the ungrounded terminal of source I I through conductor 46 and switch 9. The control grid 21 of device 5 is directly connected to the cathode 20 of device 4 through conductor 41.

A relay 28 is also provided, having a pair of normally closed contacts 29 and 30, and adapted upon energization thereof to interrupt the alternating voltage appearing across rectlfiers l and through conductors 48 and 43 to ground. A condenser 33 is connected in parallel with the energizing coil 3| to filter the rectified anode current through device and to prevent relay 28 irom chattering. Contact 23 oi relay 28 is connected tothe ungrounded terminal of source II and to one contact 34 of suitable switching means, here shown as a normally-open pushbutton switch 35. Contact 30 is connected to the other contact 33 oi pushbutton switch 35 and to the ungrounded side of the utilization circuit through conductors 43, 40 and 4|. I

In operation, the filaments 3, I, 3 of devices 3, 4, 5 are energized by closing switch 3. The heating circuit may be traced from the ungroundedside of source II, through switch 3, resistance III, filaments 6, 8 and I in series, to

ground. At the same time, alternating line voltage is impressed across rectifiers I and 2. One

rectifier circuit may be traced from source II through switch 9, normally-closed contacts 29 and 30, conductor 49, cathode I5 and anode I2 oi rectifier I, conductor 31, resistance I3 and capacitance I4 in parallel, and through conductors 38 and 39 to ground. The other'rectifier circuit may be traced from source through switch 9, contacts 29 and 30, conductors 49 and 40, anode l6 and cathode I! of rectifier 2, capacitance I8, and through conductors 42 and 43 to ground. When the filaments are heated, rectifier I charges capacitance H to a negative unidirectional voltage with respect to ground. The positive unidirectional voltage across capacitance I8 is impressed as anode operating potential on device 4. The full negative potential across capacitance I4 ,is immediately impressed on grid 22 through conductor 44 and capacitance 23. The potential of the cathode 20 with respect to the potential of grid 22 is determined by the potential drop across resistance l9. This potential drop will initially be slightly less than the cut-off value of grid-cathode potential; that is, it will be just equal to the value of negative grid bias which allows a small fiow of anode current sufiicient to produce that drop across resistance I3. It will be apparent that this potential drop cannot be greater than the cut-oil value of gridcathode potential, for if it were, then no anode current would flow and there would be no grid bias.

The capacitance 2 3 concurrently begins to charge, through resistance 2|, from the potential across the cathode resistance I9. This means in efiect that some of the anode-cathode current may be regarded as flowing through the resistance 2 I and capacitance 23, but this diversion of a portion of the anode-cathode current does not affect the operation of the circuit. As the charge builds up across capacitance 23 through variable resistance 2| and cathode resistance I9, control grid 22 becomes less negative and the anode current of device 4 increases. As the anode current of device 4 rises, the voltage drop across the cathode resistance I9 capacitance 23. As a result of this cumulative action, capacitance 23 charges as if the time constant of the circuit comprising resistance 2| and capacitance 23 were multiplied by approximately the amplification factor of device 4. Thus, comparatively small values of capacitance and resistance result in a relatively large charging time constant. In some cases, it may be desired increases and further charges to tap capacitor 23 and resistance 2| across only a portion of resistance I3.

Since initially the potential drop across cathode resistance I9 cannot exceed the cut-ofl voltage of device 4, the control grid 2'! of device 5 will be at a substantial negative potential with respect to ground. For example, if the potential across resistance I3 and capacitance I4 is 100 volts negative with respect to ground, the potential at the cathode 20 of device 4 and control grid 27 of device 5 may be volts negative with respect to ground. Thus, initially the device 5 is completely cut off. As the charge across capacitance 23 increases, the voltage drop across resistance 2| decreases. As pointed out above, the increasing current flowing through device 4 increases the voltage drop across resistor I8.

'Since this voltage opposes that across the capacitor I4, the potential at cathode 20 and control electrode 21 becomes progressively less negative with respect to ground. The potential of cathode 20 rises at a slightly lower rate than the potential on grid 22, since the action is simply that of a cathode-follower with gain slightly less than unity.

When switch 9 is closed, an anode circuit for device 5 may be traced from source II through switch 9, conductor 46, anode 25 and cathode 32, relay coil 3| and through conductors 48 and 43 to ground. Thus device 5 has positive operating potential applied during the positive halicycles of voltage from alternating source II. However, initially device 5 does not conduct because its control grid 21 is quickly biased by the f large negative voltage with respect to ground appearing at cathode 20 of device 4. As the cathode potential of device 4 becomes less negative, device 5 gradually begins to conduct. Its anode current increases until coil -3| is sufilciently energized by the rectified anode current to open contacts 29 and 30, removing power from the rectifiers and from'utilization circuit. Since power is removed from therectifiers, uni-directional potential is no longer supplied to device 4, and all the electrodes return to zero potential. Consequently, device 5 continues to conduct and contacts 29 and 30 of relay 28 remain open.

In order to insure that device 5 is not momentarily cut on after operation of relay 28, bleeder resistance I3 is connected across capacitance I4 but no bleeder resistance is connected across capacitance I8. In this manner the negative direct voltage appearing across capacitance I4 is made to discharge more rapidly than the positive direct voltage appearing across the capacitance I8.

A stable condition of equilibrium now exists, since the device 5 remains conducting and relay 28 remains energized. The utilization circuit may be re-energized at any desired time by closing the normally open pushbutton switch 35. When the pushbutton switch 35 is pressed, contacts 34 and 36 are closed, power is applied to the rectifiers I and 2, and the timing tube 4 repeats the charging cycle, as heretofore described. The large negative unidirectional voltage appearing across capacitance I4 cuts ofi the control tube 5 and deenergizes relay 28 in a fraction of a second, so that the pushbutton 35 may then be released. At the end of the charging cycle, relay 28 again picks up, deenergizing the utilization circuit and removing power from the rectifiers. The length of time during which the utilization circuit is energized is set to the desired value by adjustment of resistance 2|.

This process timer has been built and tested and found to operate as hereinabove described. Merely by way of illustration and in no sense by way of limitation, I have found that the following circuit constants may be employed for the elements of the circuit:

The time during which the utilization circuit was energized could be set anywhere within a 20 range of from about above circuit constants.

While I have shown and described a preferred embodiment of my invention, other embodiments to 20 seconds, using the and modifications will occur to those skilled in 25 the art, and I intend to cover in the appended claims all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electronic timing system comprising an electron discharge device having an anode, cathode and control grid, a first source of positive operating potential connected to said anode, a

second source-of negative potential connected to 35 said cathode through a common cathode resistor, a timing capacitor connected between said grid and a point on said resistor, means for charging said capacitor in response to voltage across said resistor comprising a second resistor connected between said grid and a second point on said cathode resistor, whereby said device draws increasing current through said cathode resistor as said capacitor charges, a second electron dis charge device having a grid circuit and an anode circuit, said anode circuit including a third source of operating potential and a current-responsive relay, means for biasing said grid circuit in recompleting a circuit spouse to the voltage across said second sourceand said cathode resistor in series, the anode current in said second device being insuflicient to actuate said relay until the voltage across said cathode resistor rises to a predetermined value, a utilization circuit normally energized when said relay is in unactuated position, and means responsive to actuation of said relay simultaneously to disconnect both said first and second sources from said first device and to deenergize said utilization circuit.

2. An electronic timing system for energizing a utilization circuit for a predetermined period of time comprising, in combination, a source of alternating voltage, separate means for rectifying opposite peaks of said voltage to produce a positive unidirectional potential and a negative unidirectional potential, a pair of electron discharge devices each having a cathode, control grid and anode, for impressing said positive potential on the anode of the first of said devices, means for impressing said negative potential on the cathode of said first device through a first resistance, a connection from the cathode of said first device to the control electrode of the other of said devices, means connecting the anode of said other device to a terminal of said source, a capacitance connected between the control grid of said first device and a point on said first resistance, a charging circuit for said capacitance comprising a second resistance connected between said grid and the cathode of said first device, a normally closed relay having an energizing coil, said relay between said source and utilization circuit in its closed position, said coil being connected in series relation between the cathode of said second device and the other terminal of said source, said relay being arranged upon actuation thereof simul aneously to disconnect both said rectifying means from said source and to open said utilization circuit, and switching means for simultaneously reconnecting both said rectifying means to said source and completing said utilization circuit.

. HARRY P. MAYER.

No references cited.

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