US2454168A - Timing circuit - Google Patents

Description

Nov. 16, 1948.

Filed Aug. 22, 1946 ATTORN EY "UHHJL L 0a d WITNESSES! INVENTOR V 54% I fa warda'fiarfw/g M BY Patented Nov. 16,1948

TIMING CIRCUIT Edward C. Hartwig, Tonawanda, N. Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 22, 1946, Serial No. 692,403

7 Claims.

This invention relates to an electronic timing circuit and it has particular relation to a timing circuit for use wtih an alternating voltage supply.

In resistance welding apparatus, an electronic timing circuit is frequently employed to time certain sequence operations. There are a number of different timing circuits now in use for this and various other purposes, which employ an electric valve which becomes conductive at the end of the timed interval. However, the prior circuits of which I am aware are either somewhat erratic in operation under certain circumstances or involve too many elements to be economically practical in many applications.

The more practical of the prior timing cir=- cuits have a connection between the control circult of the valve and the alternating voltage source. As a result, surge voltages on the supply lines, as may be produced by the operation of other apparatus supplied therefrom, appear in the control circuit of the valve. Consequently, with a surge of the proper polarity, and sufficient magnitude, the valve may be rendered conductive by the surge regardless of the time, and the operation of the system may be seriously af fected.

It is accordingly an object of my invention to provide a new and improved electronic timing circuit.

A further object of my invention is to provide a novel timing circuit of practical design in which variation in the timed interval because of surges in the supply voltage are substantially avoided.

Another object of my invention is to provide a novel timing circuit in which an electric valve is rendered conductive after an accurately measured time interval of a preselected length.

Still another object of my invention is to provide a timing circuit which an electric valve is rendered conductive after an accurately measured time interval of a preselected length in which variation in the time interval because of surges in the supply voltage are substantially avoided.

The features of my invention which are considered novel are set forth with more particularity in the accompanying claims. The invention itself, however, together with the advantages and further objects thereof may be better understood flOi'l'l the following description of a specific embodiment when read in connection with the accompanying drawing, in which the single figure is a schematic circuit diagram of a preferred embodiment of my invention.

As shown in the drawing, a supply transform er 3 has a primary winding 5 energized from a pair of alternating voltage supply lines I and, 9 and a secondary winding II which is provided with a pair of end terminals I3 and I5 and an intermediate terminal IT. The right end terminal I5 of the secondary winding II is connected through the operating coil I9 of a relay ZIand a resistor 23 to the anode 25 of an electric valve 21 which is preferably of the arc-like type, such as a thyratron. The cathode 29 of the valve 21 is connected through a resistor 3i to the same end terminal I5 of the secondary winding II. The cathode 29 is also connected to the inter mediate terminal H of the secondary winding I I irough the normally open initiating switch 33. The initiating switch 33 is shunted by the nor:- mally open contact 35 of a hold-in relay 3I,.the operating coil 39 of which is connected between the cathode 29 and the right end terminal I5 of the secondary winding II through a normally closed stopping switch 4 I A first capacitor 43 and a resistor 45 are connected in series in the order named from the intermediate terminal II of the secondary winding II to the left end terminal I3. The control electrode 41 of the valve 2'! is connected through a grid resistor 49 and a second capacitor 5I to a point 53 intermediate the first capacitor 43 and its associated resistor 45. An adjustable resistor 55, which may be calibrated in units of time within the range of operation for which the circuit is designed, is in shunt across the second capacitor 5I.

Since the relay M is to be energized through the valve 21, a suitable capacitor 5'! may be connected in shunt with the operating coil I9 to maintain the relay in its energized position in each negative half period of the alternating current following a positive half period in which the valve 21 is conductive. By a positive half period, it is meant a half period in which the anode of the valve 27 is positive With respect to the cathode.

A suitable load device 59 is connected between the end terminals I3 and I5 of the secondary winding II through the normally open contac GIoftherelayZI.

To operate the apparatus, the initiating switch 33 may be closed to start the timing interval. Prior to operation of the initiating switch 33, the second capacitor 5| is charged to a predetermined maximum value by the voltage appearing across the portion of the secondary winding I-I between the right end terminal I5 and the intermediate terminal ll plus the voltage appearing across the first capacitor 43. When the voltage on the secondary winding H is such that the left end terminal I3 is positive with respect to the right end terminal I5, current flows from the point 53 intermediate the first capacitor 43 and its associated resistor 45 through the sec ond capacitor the grid resistor 49, the control electrode 41 and cathode 29 of the valve 2'! and, there through the resistor 3| to the right end terminal l5 of the secondary winding. Since the control electrode to cathode path within the valve 21 is in effect a rectifier, substantial current does not flow therethrough when the control electrode is negative with respect to the cathode. Consequently, while an alternating voltage appears across the first capacitor 43, the second capacitor 5| is charged to a preselected maximum value. The voltage across the first capacitor 43, of course, lags behind the voltage between the terminals l3 and H, the amount of lag in a preferred arrangement being of the order of 80 degrees.

When the initiating switch 33 is closed, a circuit is completed through the operating coil 39 of the hold-in relay. A a result, the contact 35 of the relay is closed to provide a holding circuit about the initiating switch. Thus, a circuit from the cathode 29 to the intermediate terminal ll of the secondary winding 1! is completed. When the circuit is thus completed from the cathode 29 to the intermediate terminal IT, further charging of the second capacitor 5| is prevented and cannot occur until the voltage on the second capacitor 5| is less than the voltage on the first capacitor 43 which, as later appears, does not occur until after the timing interval is over.

When the initiating switch 33 is closed, the first and second capacitors 43 and 5| are connected in series between the control electrode 41 and the cathode 29. The valve 2?, of course, has such characteristics that a voltage more positive than a predetermined critical value must be impressed between the control electrode 41 and cathode 29 in a positive half period to render the valve conductive. In a typical and suitable valve such as an RCA-2050 thyratron, the critical volt age is of the order of minus 6 volts.

The voltage then appearing between the control electrode 4! and cathode 29 after the initiat ing switch 33 is closed is essentially the algebraic sum at any instant of the voltage on the second capacitor 5| and the voltage on the first capacitor 43. The charge on the second capacitor 5| is of such polarity as to tend to make the control electrode 4'! highly negative with respect to the oathode 29. The alternating voltage on the first capacitor 43 while lagging behind that between terminals l3 and I7, leads, as applied between the control electrode and cathode, the anode to cathode voltage of the valve by approximately 80 degrees. However, the peak instantaneous value of the first capacitor voltage is considerably less than the charge on the second capacitor 5| when switch 33 is closed so the valve 21 is maintained non-conductive.

As the second capacitor 43 is no longer being charged after the initiating switch 33 is closed, it begins to discharge through the adjustable resistor 55 in shunt therewith at a rate preselected by the adjustment of the resistor. After a predetermined time interval as determined by the setting of resistor 55, the voltage on the second capacitor 5| is reduced to such an extent that the algebraic sum of the voltages of the first and second capacitors become more positive than the critical value to render the valve 21 conductive. I prefer the amount of lead of the first capacitor voltage with respect to the anode to cathode voltage of the valve to be of the order of degrees since the first capacitor voltage then reaches its peak value in a positive direction at approximately the same time in a period of the source voltage that the anode to cathode voltage of the valve becomes positive to permit the valve to be rendered conductive. Consequently, if the valve 21 is to be rendered conductive in a halfperiod, such action will occur at or near the beginning of the half-period.

When the valve 21 becomes conductive, current flows from the right end terminal l5 of the secondary winding I through the operating coil IQ of the relay 2|, the resistor 23 the anode 25 and cathode 29 of the valve 2'1, the closed contact 35 of the hold-in relay 31 to the intermediate terminal H. The relay 2| is thus energized to complete the circuit through the load 59 and since the valve 21 becomes conductive at or near the beginning of a half-period definite operation of the relay is assured.

Thereafter, the stopping switch 4| may be opened at will to deenergize the hold-in relay 3! and thereby cause the valve 21 to become nonconductive. As a result, the relay 2| is deenergized and the load circuit is opened. Reoperation of the timing circuit may be initiated by reclosing the initiating switch 33 after a short interval in which the second capacitor 5| is recharged.

It is apparent that the particular instant in a period of the first capacitor voltage at which the valve 21 becomes conductive depends upon the amount of phase displacement of the first capacitor voltage with respect to the anode to cathode voltage. Moreover, for any given phase displacement the valve always becomes conductive in successive operations at or near the same relative instant in a period of the first capacitor voltage, even with different time intervals within a practical range. Now the optimum condition for capacitor discharge timing requires that the end of the timing interval occurs when the voltage remaining across the discharging capacitor, in this case the second capacitor 5|, is approximately 37% of its original value. The optimum condition may be shown in the following equations in which EC is the capacitor voltage, 15 represents time, R represents the resistance of the discharge circuit, C is the capacity of the capacitor, and E0 is the initial voltage on the capacitor. The voltage relations for a discharging capacitor may be expressed as t EFEOE RC 1 Then i an, 1 VFfi RC dE LL MR To find the value of RC which causes dill to be maximum we place Equation 3 equal to zero, and solve for RC. The solution is:

t=RC (4) Substituting Equation 4 in Equation 1, it appears that To obtain this optimum condition, it is then necessary that the instantaneous voltage on the first capacitor 43 at the instant in a period at which the valve 21 is to be rendered conductive, be approximately 37% of the original maximum charge on the second capacitor 5|.

To avoid undesirable effects as a result of surges in the supply voltage, the relation between the first capacitor 43 and the resistor 45 is arranged, in accordance with my invention, to form, in efiect, a filter for the surge voltages usually encountered. Thus, a surge in the supply voltage does not appear with substantial magnitude in the alternating voltage across the capacitor 43 so that undesirable effects on the operation of the valve 21 are substantially avoided. At the same time, the arrangement should be such that the voltage on the first capacitor 43 at an instant in a period thereof corresponding to the instant at which the valve is to become conductive is approximately 37% of the maximum charge on the second capacitor 5|. To obtain these conditions, the resistor 45 and the capacitor are selected to form the filter with respect to the voltage surges. The position of the intermediate terminal I l is selected to provide the desired voltage relation.

It is to be noted that the first capacitor 43 serves not only as a part of a filter for surge voltages but also provides an alternating voltage Ec=.37E0, approximately in the control circuit of the valve to insure positive operation and in addition enables the establishment of the optimum condition for capacitor discharge timing in operation of the valve throughout the entire range of time settings.

While I have shown and described a preferred embodiment of my invention, I am aware that many modifications thereof may be made without departing from the spirit of the invention. I do not intend, therefore, to limit my invention to the specific circuit disclosed.

I claim as my invention:

1. A timing circuit for use with a source of alternating voltage comprising an electric valve having an anode, a cathode and a control electrode, circuit means including an initiating device and effective upon operation of said initiating device for connecting said anode and cathode in a circuit with said source, a filter connected in a circuit energized from said source for filtering surges in said source voltage so that such surges do not appear in substantial magnitude in the alternating voltage across said filter, a capacitor connected in series with said filter from said control electrode to said cathode after operation of said initiating device, means effective only prior to operation of said initiating device to cause said capacitor to be charged to a predetermned maximum value, said valve having such characteristics as to become conductive after operation of said initiating device only when the control electrode to cathode voltage is more positive than a critical value which normally requires a charge on said capacitor no greater than a predetermined value less than said maximum value, and a discharge circuit including a second resistor in shunt with said capacitor to discharge it at a preselected rate following operation of said initiating device.

2. A timing circuit for use with a source of alternating voltage comprising an electric valve having an anode, a cathode and a control electrode, circuit means including an initiating device and effective upon operation of said initiating device for connecting said anode and cathode in a circuit with said source, a filter connected in a circuit energized from said source for filtering surges in said source voltage so that such surges do not appear in substantial magnitude in the alternating voltage across said filter, a capacitor connected in series with said filter from said control electrode to said cathode after operation of said initiating device, means efie-ctive only prior to operation of said initiating device to cause said capacitor to be charged to a predetermined maximum value, said valve having such characteristics as to become conductive after operation of said ini iating device only when the control electrode to cathode voltage is more positive than a critical value which normally re quires a charge on said capacitor no greater than a predetermined value less than said maximum value, and a discharge circuit including a second resistor in shunt with said capacitor to discharge it at a preselected rate following operation of said initiating device, whereby said valve becomes conductive after a preselected time interval at a predetermined instant in a period of the voltage on said element, said plurality of elements being arranged to provide a voltage on said one element displaced in phase relative to said source voltage and having an instantaneous value at said instant which is approximately 37% of said maximum value.

3. A timing circuit for use with a source of alternating voltage comprising an electric valve having an anode, a cathode and a control electrode, circuit means including an initiating device and efi'ective upon operation of said initiating device for connecting said anode and cathode in a circuit with said source, a firs-t capacitor and a first resistor connected in a circuit energizcd from said source and forming in effect a filter for surges in said source voltage so that such surges do not appear in substantial magnitude in the alternating voltage across said first capacitor, a second capacitor connected in series with said first capacitor from said control electrode to said cathode after operation of said initiating device, means effective only prior .to operation of said initiating device to cause said second capacitor to be charged to a predetermined maximum value, said valve havin such characteristics as to become conductive after operation of said initiating device when the control electrode to cathode voltage is more positive than a critical value which ordinarily requires a charge on said second capacitor no greater than a predetermined value less than said maximum value, and a discharge circuit including a second resistor in shun-t with said second capacitor to discharge it at a preselected rate following operation of said initiating device.

4. A timing circuit for use with a source of alternating voltage comprising an electric valve having an anode, a cathode and a control electrode, circuit means including an initiating device and effective upon operation of said initiating device for connecting said anode and cathode in a circuit with said source, a first capacitor and a first resistor connected in a circuit energized from said source and forming in effect a filter for surges in said source voltage so that such surges do not appear in substantial magnitude in the alternating voltage across said first capacitor, a second capacitor connected in series with said first capacitor from said control electrode to said cathode after operation of said initiating device, means effective only prior to operation of said initiating device to cause said second capacitor to be charged to a predetermined maximum value, said valve having such characteristics as to become conductive after operation of said initiating device when the control electrode to cathode voltage is more positive than a critical value which ordinarily requires a charge on said second capacitor no greater than a predetermined. value less than said maximum value, and a discharge circuit including a. second resistor in shunt with said second capacitor to discharge it at a preselected rate following operation of said initiating device, whereby said valve becomes conductive after a preselected time interval at a predetermined instant in a period of the voltage across said first capacitor, said first capacitor and first resistor being arranged .to provide a voltage on said first capacitor having an in.- stantaneous value at said instant which is approximately 37% of said maximum value.

5. A timing circuit comprising impedance means adapted to function as a source of alternating voltage and having a pair of end terminals and an intermediate terminal, an electric valve having anode, a cathode and a control electrode, circuit means connecting said anode to one of said end terminals, a first resistor connected in a circuit from said cathode to said one end terminal, a first capacitor and a second resistor connected in series in the order named from said intermediate terminal to the other of said end terminals and having a time constant such that surges in said source voltage do not appear in substantial magnitude in the alternating voltage across said first capacitor, a second capacitor connected in circuit from said control electrode to a point intermediate said first capacitor and second resistor whereby said second capacitor charged to a preselected maximum value, an iitiating device operable to complete a circuit from said cathode to said intermediate terminal whereby further charging of said second capacitor is prevented and said first and second capacitors are connected in series from said con-- trol electrode to said cathode, said valve having such characteristics as to become conductive when the charge on said second capacitor is less than a predetermined critical value which is less than said maximum value, and a discharge cir- 8 cuit including a third resistor in shunt with said second capacitor to discharge it at a preselected rate following operation of said initiating device.

6. A timing circuit for use with a source of alternating voltage comprising an electric valve having an anode, a cathode and a control electrode, circuit means including an initiating de vice and effective upon operation of said initiating device for connecting said anode and cathode in a circuit with said source, a filter connected in a circuit energized from said source for filtering surges in said source voltage so that such surges do not appear in. substantial magnitude in the alternating voltage across said filter, a capacitor connected in series with said filter from said control electrode to said cathode after operation of said initiating device, means effective only prior to operation of said initiating device to cause said capacitor to be charged to a predetermined maximum value, said valve having such characteristics as to become conductive after operation of said initiating device only when the control electrode to cathode voltage is more positive than a critical value which normally requires a charge on said capacitor no greater than a predetermined value less than said maximum value, and a discharge circuit including a variable resistor in shunt with said capacitor to discharge it at a preselected rate following operation of said initiating device.

'7. A timing circuit for use with a source of alternating voltage comprising an electric valve having an anode, a cathode and a control electrode, circuit means including an initiating device and effective upon operation of said initiating device for connecting said anode and cathode in a circuit with said source, a first capacitor and a first resistor connected in a circuit energized from said source and forming in efiect a filter for surges in said source voltage so that such surges do not appear in substantial magnitude in the alternating voltage across said first capacitor, a second capacitor connected in series with said first capacitor from said control electrode to said cathode after operation of said initiating device, means effective only prior to operation of said initiating device to cause said second capacitor to be charged to a predetermined maximum value, said valve having such characteristics as to be come conductive after operation of said initiating device when the control electrode to cathode voltage is more positive than a critical value which ordinarily requires a charge on said second capacitor no greater than a predetermined value less than said maximum, value, and a discharge circuit including a variable resistor in shunt with said second capacitor to discharge it at a preselected rate following operation of said initiating device.

EDWARD C. HARTWIG.

No references cited.

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