US3229161A - Ignitron excitation control circuit - Google Patents

Description

IGNITRON EXCITATION CONTROL CIRCUIT Filed March 25, 1963 "I C 0 F IE. E

E 0 INVENTOR. m ERNEST G. ANGER United States Patent 3,2 29,161 IGNITRON EXCITATIGN CONTROL CIRCUlT Ernest G. Anger, Wauwatosa, Wis., assignor to Square D Company, Paris Ridge, Ill., a corporation of Michigan Filed Mar. 25, 1963, Ser. No. 267,462 14 Claims. (Cl. 315-196) This invention relates to the control of electric discharge devices and is more particularly concerned with a system for controlling the ignition current through the ignition electrode of mercury pool cathode-type electronic discharge devices commonly known as ignitrons.

Ignitron tubes are frequently used to control the magnitude of alternating currents to electrical loads which require frequent switching. While the control which will hereinafter be described may be used in various electrical systems, it is particularly suited for use in a resistance welder apparatus to control the flow of welding current through the primary winding of the welding transformer.

An ignitron has a pool of liquid mercury, as a cathode, and is provided with an ignition electrode known as the ignitor to initiate conduction of the ignitron. The ignitor may consist of a tapered piece of boron carbide that extends down into the mercury pool without being wet by the mercury. \Vhen a sufiicient potential is impressed between the ignitor and cathode of an ignitron, a small are is developed. If the current forming the arc has a suffi cient intensity, electrons are released which ionize the mercury vapor within the ignitron. If the anode of the ignitron has a positive polarity relative to the cathode and sufiicient ionized mercury vapor is present, the ignitron switches from a nonconductive high impedance state to a conductive state presenting practically no impedance between the anode and cathode.

In a welding apparatus the ignitrons are usually connected in inverse parallel pairs between an alternating current supply and the primary winding of a welding transformer to supply alternate half cycles of alternating current to the primary winding. A weld timer determines the conduction interval of the ignitrons by supplying an appropriate signal to a means for initiating conduction of the ignitrons at adjustable instants during the half cycle interval when the anodes of the ignitrons have a positive polarity.

One of the most commonly used circuits for initiating the conduction of ignitrons is known as an anode firing circuit wherein a grid controlled gaseous electrode tube, such as a thyratron, is connected between the anode and control electrode of the ignitron. When this arrangement is employed, current flow at the anode of the ignitron is conducted by the thyratron to the ignition electrode. Another form of control which may be employed to initiate the conduction of the ignitron comprehends an arrangement whereby a separate source is connected through a thyratron across the control electrode and cathode of the ignitron. In this arrangement the separate source provides the exciting current to initiate the conduction of the ignitron. The disadvantage of the separate excitation arrangement is-that relatively large components are required and proper synchronization of the current flow through the ignitor with the anode voltage of the ignitron is difiicult to control. A distinct advantage of the anode firing arrangement, other than the use of less expensive components, is that it inherently causes a continued how of ignition current until the ignitron becomes conductive, at which time the low impedance drop of the ignitron eifectively shorts out the excitation circuit to the ignition electrode. Thus, as soon as the ignitron becomes conductive, the ignition current ceases to flow and will not cause premature failure of the ignition electrodes. It has also 3,229,161 Patented Jan. 11, 1966 been found that anode firing, because of the lower energy requirements necessary for proper ignition of the ignitrons, will cause the ignitrons to operate at a lower temperature than is possible with separate excitation firing. The circuit according to the present invention retains all of the advantages of an anode firing circuit for ignitrons without incurring the disadvantages presented by the thyratrons which have a relatively short life and thus require frequent replacement. The anode firing circuit for ignitrons according to the present invention uses static semiconductor devices, such as silicon controlled rectifiers, which are known to have practically an indefinite life span when properly applied. The circuit is arranged so the silicon controlled rectifiers will not be subjected to the full line voltage which is impressed across the ignitrons. This will permit the silicon controlled rectifiers to be used in a circuit which is normally beyond their rated capacity to provide an ignitron contactor which is low in cost as it can be assembled with standard components and which will provide troublefree operation so as to reduce the maintenance cost and the high down-time costs which heretofore occurredin modern factories when an ignitron contactor is rendered inoperative because of a failure of a thyratron tube in the firing circuit.

It is an object therefore of the present invention to provide a circuit for initiating conduction of a pair of inversely connected-ignitrons which includes a transformer means connected in circuit with the anode and cathode of the ignitrons for supplying anode to cathode current to a pair of unidirectional conducting devices.

Another object of the present invention is to provide an anode firing circuit for a pair of inversely connected ignitrons, which firing circuit is subjected to a substantially lower voltage than the load circuit of the ignitrons.

A further object of the present invention is to provide an anode firing circuit for an ignitron which includes a semiconductor and a transformer means which is connected in a circuit parallel with the ignitron and in series with the semiconductor.

Another object of the present invention is to provide an anode firing circuit for initiating the conduction of an ignitron which includes a silicon controlled rectifier and a transformer means which is shorted by the ignitron when the ignitron is conductive and which is arranged to supply the rectifier with current at a voltage substantially lower than the voltage of the supply for the ignitron.

A still further object of the present invention is to provide an anode firing circuit for initiating conduction of a pair of ignitrons connected in inverse parallel between an alternatng current supply and a primary winding of a welding transformer in a resistance welder apparatus which circuit, in response to a signal from a weld timer of the welder apparatus, causes a pair of silicon controlled rectifiers to become conductive at adjust-able instants during-the half cycles after voltage zero of the voltage wave of the supply and wherein the silicon controlled re ct-ifiers are connected in series with the ignitor .electrode of the ignitrons and with a transformer means which is connected in a circuit with the anodes of the .fir-ing circuit for a pair of ignitrons incorporating the features of the present invention.

FIG. 2 illustrates a series of curves of one form of 3 the firing pulse which may be provided to cause conduction of the ignitrons in FIG. 1.

In the drawings, a firing circuit for an ignitron type contactor is illustrated which may be use-d to control the flow of current from an alternating current source to an alternating current load. While the alternating current load shown is a resistance welding transformer in FIG. 1, it is understood that laternating current fiow to other type loads may be controlled by the ignitron contactor employing the firing scheme according to my invention.

In FIG. 1, an alternating current source, not shown, is connected to supply two sets of input leads L1 and L2. A weld timer indicated by the numeral is provided to supply a suitable signal to initiate conduction of a pair of ignitrons 12 and 14 in a manner which will be hereinafter set forth. The weld timer shown will provide a suitable signal at the beginning and during the weld interval to govern the flow of energy to a pair of Welding electrodes 16 and 18 and may be any suitable well known type, such as the timer shown in Patent 3,015,067, issued December 26, 1961, to C. W. Porter and assigned to the assignee of the present invention, or a timer as shown in application Serial No. 129,828, filed August 7, 1961, which has been assigned to the assignee of the present invention by the inventors C. F. Meyer and]. I. Eckl.

The timer shown in the Porter patent includes gaseous electronic tubes and the timer shown in the Meyer and Eckl application utilizes static switching devices, such as transistors, and is arranged to supply signals as shown on curve C in FIG 2 of the drawings.

The weld timer 10 is energized from the source through the leads L1 and L2 and is connected through a pulsing transformer 20 to a signal pulser 22. In FIG. 2, a curve A illustrates the voltage wave of the source which is connected to the leads L1 and L2. Because of the inductive characteristics of the welding transformer, the voltage of curve A will cause a lagging current flow in the welding transformer circuit as shown in curve B. The timer 10 is arranged at the beg-inning and during the weld interval to supply a signal as shown by curve C in FIG. 2; that is, a series of voltage pulses which occur as shown during the respective half cycles of current shown by the curve B. It will be seen from curve C that at the beginning of the weld interval during periods of L1 polarity, the output of the weld timer 10 will provide a negative polarity voltage which exists throughout the current half cycle L1. At the beginning of the L2 half cycle, the output signal from the timer 10 changes from a negative polarity voltage to a zero voltage. This signal continues throughout part of the L2 half cycle. The pulsing transformer 20 is arranged so that if the voltage through its primary winding changes it will provide an output signal consisting of sharp voltage spikes, as shown in curve D. The voltage output pulses of transformer 20 are impressed as inputs to the signal pulser 22 which in response thereto provides an output as shown on curve E which consists of a series of sharp voltage spikes. The output of the signal pulser 22 is impressed across a primary win-ding of a transformer 24.

If desired, the weld timer may be also provided with an arrangement for phase shifting the occurrence of the signals in FIG. 2. This arrangement will provide heat control in the resistance welder and is achieved by shifting the occurrence of the signals of curve C in timed relation to the source voltage as indicated by a broken line in curve C and a corresponding shift of the pulses in curves D and E which will cause the conduction of the ignitrons to be initiated at adjustable instants after voltage zero of the source voltage as shown by the broken line curve of curve B.

As shown in FIG. 1, the lead L1 is connected through a junction 25 to an anode 26 of ignitron 14 and through a junction 28 to a cathode 30 of ignitron 12. The lead L2 is connected through a primary winding 32 of a welding transformer to a junction 34 which is connected to an anode 36 of the ignitron 12 and through a junction 38 to a cathode 40 of ignitron 14. The primary winding 32 of the welding transformer supples energy to a secondary winding 42 which is connected to the welding electrodes 16 and 18 to cause welding current to flow through a pair of parts 44 which are to be welded together.

The ignitron 12 has an initiating elect-rode known as an ignitor 46 and the ignitron 14 has an initiating electrode known as an ignitor 48. The ignitor 46 is connected through a junction 50 to a cathode electrode 52 of a unidirectional conducting device, such as a silicon controlled rectifier 54, which has an anode 56 and a control electrode known as a gate 58. The ignitor 48 of the ignitron 14 is connected through a junction 60 to a cathode 62 of another unidirectional conducting device, such as a silicon controlled rectifier 64, which has an anode 66 and control electrode, known as a gate 68.

A transformer winding 70 is connected between the junction 28 and 38. In the embodiment shown, the transformer winding 70 is provided with a pair of intermediate taps 72 and 74. The tap 72 is connected to the anode 56 of the rectifier 54. The tap 74 is connected to the anode 66 of the rectifier 64. Back-to-back rectifiers and 82 are connected as surge suppressors between the anodes and the cathodes of the rectifiers 54 and 64. These 'back-toback rectifiers 80 and 82 act as transient voltage suppressors in the circuits associated therewith to protect the rectifiers 54 and 64 from damage due to transient voltages.

The transformer 24 has a pair of secondary windings 84 and 86. One of the output terminals of the winding 84 is connected through a suitable diode 88 and a junction 90 to the gate 58 of the rectifier 54. The other output terminal of the winding 84 is connected through a junction 92 to the cathode 52 of the rectifier 54. A bias resistor 94 is connected between junction-s 90 and 92. Similarly, one of the output terminals of the winding 86 is connected through a diode 96 and a junction 98 to the gate 68 of the rectifier 64. The other output terminal of the winding 86 is connected through a junction 100 to the cathode 62 of the rectifier 64. A resistor 102 is connected between junctions 98 and 100.

The operation of the circuit above described is as follows: if it be assumed that the ignitrons 12 and 14 are both nonconducting, current will flow through the winding 70 and the winding 32. The winding 70 is shown as connected as an auto-transformer and during intervals when the secondary circuits connected to taps 72 and 74 are open, will provide a high impedance path for current flow through primary winding 32. During periods when lead L1 is positive, the portion of the winding 70 between terminal 74 and junction 38 will cause the anode 66 of the rectifier 64 to have a positive polarity relative to its cathode 62. Similarly, when lead L2 is positive, the portion of the winding 70 between terminal 72 and junction 28 will cause the anode 56 of the rectifier 54 to have a positive polarity relative to its cathode 52. As previously explained, when the ignitrons are nonconducting, they present a high impedance which changes to a low impedance as they change to their conductive states. Thus, when either of the ignitrons 12 or 14 becomes conductive, a short circuit path is formed across winding 70 to remove the supply for the anode to cathode voltage of the rectifiers 54 and 64.

At the beginning and during the weld interval the transformer 24 causes the windings 84 and 86 to be provided with a series of voltage pulses which occur as shown by curve B of FIG. 2. These voltage pulses are conducted by the diodes 88 and 96 to the junctions 90 and 98 where they appear between the gates 58 and 68 and cathodes 52 and 62 of the rectifiers 54 and 64 to cause the rectifiers to become conductive. When the rectifier 54 conducts, current flows from the junction 72 through the rectifier 54 to the ignitor 46 to cause the ignitron 12 to conduct as previously explained. Similarly, when the rectifier 64 is rendered conductive, current flows from the terminal 72 through'th'e rectifier 64 to the ignitor 46 to cause the ignitron14 to be rendered conductive.

The winding 70 is preferably wound of a wire of suitahlesize to provide low resistance and the coil windings thereof are suitably interleaved to provide low leakage reactance so the winding 70 provides a very low short circuit impedance at the terminals 72 and 74. This arrangement will provide the system with a rapid response. The winding 70 as shown is connected in parallel with the ignitrons 12' and 14 so that as the ignitrons 12 and 14 conduct, theyeffectively provide a short circuit across the winding 70. While the winding 70 is shown as an autotransformer, other transformer arrangements may be used to redum the anode to cathode voltages of the rectifiers 54 and 64. The arrangement shown makes it possible to use silicon controlledrectifiers having a lower voltage rating than would be required if the anodes of the rectifiers were directly connected to the anodes of the ignitrons as conventional ignitron anode firing circuits. For example: if the source causes the leads L1 and L2 to have a potential of 440 volts asis' conventional in resistance welding practice, then the rectifiers because of the use of the suitable taps of the winding 70, will provide an anode to cathode voltage across the rectifiers of 110 volts which is sufiicient to initiate the conduction of the ignitrons without damaging the rectifiers or requiring special costly devices.

While certain preferred embodiments of the invention have been specifically disclosed, it is understood that the invention is not limited thereto, as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.

What is claimed is:

1. In combination with a source of alternating current having a pair of terminals, a first ignitron having an anode, a cathode and an ignition electrode, a second ignitron having an anode, a cathode and an ignition electrode, means connecting the anode of the first ignitron and the cathode of the second ignitron to one terminal of the source, means connecting the cathode of the first ignitron and the anode of the second ignitron to the other terminal of the source, an autotransformer having a winding interconnecting the anodes of both ignitrons, said transformer having at least one tap dividing said winding into two portions, a first unidirectional conducting device having an anode, a cathode and a control electrode, a second unidirectional conducting device having an anode, a cathode and a control electrode, means connecting in series across one of said winding portions the anode and the cathode of the first device and the ignition electrode and the cathode of the first ignitron, means connecting in series across the other of said winding portions the anode and the cathode of the second device and the ignition electrode and the cathode of the second ignitron, and circuit means connected between the control electrodes and the cathodes of each of said devices for initiating conduction of said devices.

2. For use in supplying current from a source of alternating current to a load, the combination comprising: a pair of ignitrons interposed in a circuit between the source and the load in such a manner so as to conduct alternate half cycles of current from the source to the load, each of said ignitrons having an anode, a cathode and an ignition electrode arranged for switching the ignitrons from a nonconductive to a conductive state, transformer means connected in a circuit parallel with said ignitron circuit, said transformer having a pair of outputs, a first unidirectional conducting device connected in a series with the ignition electrode and the cathode of one of said ignitrons across one of said outputs and a second unidirectional conducting device connected in a series circuit with the 6 ignition electrode and the cathode of the other ignitron across the other of said outputs.

3. For use in supplying current from a source of alternating current to a load, the combination comprising; a pair of ignitrons interposed in a first circuit between the source and the load in such a manner so as to conduct alternate half cycles of current from the source to the load, each of said ignitrons having an anode, a cathode and an ignition electrode arranged for switching the ignitrons from a nonconductive to a conductive state and thereby changing the impedance of said circuit from a high value to a negligible value, a pair of unidirectional conducting devices each having an anode, a cathode and a control electrode arranged for initiating the conduction of the device, transformer means interposed in a circuit between the source and the load in a circuit parallel with said first circuit, said transformer means having an impedance substantially less than the high impedance value of said first circuit and greater than the negligible value of the impedance of said first circuit when the ignitrons are conducting whereby the ignitrons substantially provide a short circuit path for the transformer means, said transformer means having a pair of output portions, a second circuit means connecting the anode and the cathode of one of said devices in a series circuit with one of said output portions and the ignition electrode and the cathode of the first ignitron and a third circuit means connecting the anode and the cathode of the other of said pair of devices in a series circuit with the other output portion and the ignition electrode and the cathode of the second ignitron for initiating conduction of the ignitrons in response to current flow through the transformer means when the ignitrons are nonconductive.

4. In combination; a pair of ignitrons connected in inverse parallel between an alternating current source and a load, each of said ignitrons having an anode, a cathode and an ignition electrode, a pair of unidirectional conducting devices each having an anode, a cathode and a control electrode for initiating conduction of said devices, circuit means connecting the anode and the cathode of one of said unidirectional conducting devices in a series circuit with the ignition electrode and cathode of one of said ignitrons, circuit means connecting the anode and the cathode of the other of said unidirectional conducting devices in a series circuit with the ignition electrode and cathode of the other of said ignitrons, and transformer means connected in a parallel circuit with both of said ignitrons between said source and load and said circuit means including a portion of the transformer means in each of said series circuits for initiating conduction of said ignitrons in response to the conduction of said unidirectional conducting devices.

5. The combination as set forth in claim 4 wherein each of the unidirectional conducting devices is a silicon controlled rectifier.

6. The combination as set forth in claim 4 wherein the transformer means comprises an autotransformer.

7. The combination as set forth in claim 4 wherein each of the unidirectional conducting devices is a silicon controlled rectifier and the transformer means comprises an autotransformer.

8. The combination as set forth in claim 4 wherein each of the unidirectional conducting devices is a silicon controlled rectifier which has a firing circuit connected between the control electrode and the cathode of the rectifier for initiating conduction of the rectifier and wherein the firing circuit includes means for providing a series of pulses between the control electrode and cathode of the devices.

9. The combination as set forth in claim 8 wherein the ring circuit is coupled to a weld timer for controlling the initiation of conduction of the devices and thereby the initiation of conduction of the ignitrons.

10. In combination; a pair of ignitrons connected in inverse parallel between an alternating current source and a load for supplying alternate half cycles of alternating current to the load, each of said ignitrons having an anode, a cathode and an ignition electrode, a pair of unidirectional conducting devices each having an anode, a cathode and a control electrode for initiating conduction of said devices, circuit means connecting the anode and the cathode of one of said unidirectional conducting devices in a series circuit with the ignition electrode and the cathode of one of said ignitrons, circuit means connecting the anode and the cathode of the other of said unidirectional conducting devices in a series circuit with the ignition electrode and the cathode of the other of said ignitrons, transformer means having a portion connected in a parallel circuit with both of said ignitrons between said source and load and a portion of said transformer means in each of said series circuits for initiating conduction of said ignitrons in response to the conduction of said devices, and firing means connecting between the control electrode and cathode of each of said devices for adjustably initiating the conduction of the devices at variable instants during the half cycles of the supply.

11. The combination as set forth in claim 10 wherein the firing means is arranged to supply the devices with a series of current pulses after the instant of initiation of conduction of the devices.

12. In combination: an ignitron having a pair of main electrodes connected between an alternating current supply and a load, and an anode firing circuit for the ignitron including a semiconductor and an autotransformer winding, said winding having a main portion connected to the main electrodes of the ignitron to be in a circuit parallel to the ignitron and said winding having a portion of said main portion connected in a series circuit with pair of main electrodes of the semiconductor.

13. The combination as recited in claim 12 wherein the semiconductor is a silicon controlled rectifier.

14. The combination as recited in claim 12lwherein the semi-conductor has a control electrode which is arranged for initiating conduction of the semi-conductor connected in a circuit with a phase shift circuit means for initiating conduction of the semi-conductor at vari able instants of a half cycle current pulseof the alternating current supply.

References Cited by the Examiner UNITED STATES PATENTS 2,175,921 10/1939 Schnarz 315-196 X 3,089,984 5/1963 Randolph 31S199 X 3,112,437 11/ 1963 Rockafellow 323-24 JOHN W. HUCKERT, Primary Examiner. DAVID J. GALVIN, Examiner.

Claims (1)

1. IN COMBINATION WITH A SOURCE OF ALTERNATING CURRENT HAVING A PAIR OF TERMINALS, A FIRST IGNITRON HAVING AN ANODE, A CATHODE AND AN IGNITION ELECTRODE, A SECOND IGNITRON HAVING AN ANODE, A CATHODE AND AN IGNITRON AND THE MEANS CONNECTING THE ANODE OF THE FIRST IGNITRON AND THE CATHODE OF THE SECOND IGNITRON TO ONE TERMINAL OF THE SOURCE, MEANS CONNECTING THE CATHODE OF THE FIRST IGNITRON AND THE ANODE OF THE SECOND IGNITRON TO THE OTHER TERMINAL OF THE SOURCE, AN AUTOTRANSFORMER HAVING A WINDING INTERCONNECTING THE ANODES OF BOTH IGNITRONS, SAID TRANSFORMER HAVING AT LEAST ONE TAP DIVIDING SAID WINDING INTO TWO PORTIONS, A FIRST UNIDIRECTIONAL CONDUCTING DEVICE HAVING AN ANODE, A CATHODE AND A CONTROL ELECTRODE, A SECOND UNIDIRECTIONAL CONDUCTING DEVICE HAVING AN ANODE, A CATHODE AND A CONTROL ELECTRODE, MEANS CONNECTING IN SERIES ACROSS ONE OF SAID WINDING PORTIONS THE ANODE AND THE CATHODE OF THE FIRST DEVICE AND THE IGNITION ELECTRODE AND THE CATHODE OF THE FIRST IGNITRON, MEANS CONNECTING IN SERIES ACROSS THE OTHER OF SAID WINDING PORTIONS THE ANODE AND THE CATHODE OF THE SECOND DEVICE AND THE IGNITION ELECTRODE AND THE CATHODE OF THE SECOND IGNITRON, AND CIRCUIT MEANS CONNECTED BETWEEN THE CONTROL ELECTRODES AND THE CATHODES OF EACH OF SAID DEVICES FOR INITIATING CONDUCTION OF SAID DEVICES.

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