Circuit For Starting And Maintaining A Discharge Through A Gas Discharge Tube

Abstract

A capacitance, which is charged by the output of a DC power supply, is placed in series aiding relationship with the output of the DC power supply during the start period of a gas discharge tube, i.e., until ignition is achieved and the capacitance discharges through the tube. This permits the required "fill in" or "boost" voltage to be obtained during starting without drawing excessive peak currents from the power supply when ignition is started.

Description

This invention relates to apparatus for starting and maintaining a discharge plasma in a gas discharge tube and, more particularly, to an improved start circuit which is particularly suitable for use with lasers and arc lamps.

It is the practice to utilize a current regulated DC power supply to produce a gas discharge plasma in a laser or arc lamp. It is more difficult to initiate a discharge through a gas discharge device than it is to maintain the discharge once it has been started. Therefore, in order to aid in starting the discharge, a pulse of AC voltage is applied through an igniter coil in cooperative relationship with the gas discharge tube and at the same time a higher than normal "fill in" or "boost" voltage is applied for a short period between the cathode and anode electrodes of the gas tube.

In the prior art, the current regulated power supply includes means for increasing the power supply voltage temporarily during the start period to obtain the required "fill in" or "boost" voltage. It is not economical to increase the capacity of the power supply utilized from that needed to maintain steady state continuous discharge through the gas discharge device merely for the purpose of accommodating the increased power supply voltage temporarily required for only a very short time interval to start discharge through the gas discharge device. Thus, the relatively low capacity of the power supply used is such that excessive peak currents occur when ignition is started.

It is an object of the present invention to provide the required "fill in" voltage without causing these undesired high peak currents during starting.

Selectively operable means are effective when unoperated and in the absence of any discharge through the gas discharge device in permitting a capacitance to be charged by the output of the DC power supply. Operation of the selectively operable means is effective both in energizing the igniter coil of the gas discharge device, as is conventional, and in applying the voltage across the charged capacitance in series-aiding relationship with the output of the DC power supply between the anode and cathode electrodes of the gas discharge device. Upon ignition of the gas discharge device the capacitance is discharged therethrough. Thus, the required "fill in" voltage is supplied during the start period by the previously charged capacitance, rather than by any increase in voltage in the power supply output itself.

These and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken together with the accompanying drawing in which the sole FIGURE illustrates a preferred embodiment of the present invention.

Referring to the FIGURE, DC power supply 10, which is preferably a current-regulated power supply, has its negative terminal connected directly to cathode electrode 12 of laser tube 14. The positive terminal of DC power supply 10 is connected to anode electrode 16 of laser tube 14 through unidirectional conducting device 18, which is poled as shown.

A voltage divider is connected between cathode electrode 12 and anode electrode 16 of laser tube 14. This voltage divider is composed of first resistance 20, having one end thereof connected to cathode electrode 12 of laser tube 14, second resistance 22, having one end thereof connected to the other end of resistance 20, and third resistance 24 having one end thereof connected to the other end of resistance 22 and the other end of resistance 24 connected to anode electrode 16 of laser tube 14. Resistance 20 has a relatively high value, such as 10,000 ohms, for instance. Resistance 22 has an even higher value, such as 75,000 ohms, for instance. Third resistance 24 has a very low value, such as 10 ohms, for instance. Capacitance 26 has one end thereof connected to the junction of first resistance 20 and second resistance 22 and the other end thereof connected to the junction of second resistance 22 and third resistance 24, as shown. Capacitance 26 may have a value of from 2 to 3 microfarads for instance.

Contactor 28 includes normally closed contacts 30, which are connected as shown between the positive terminal of DC power supply 10 and anode electrode 16 of laser tube 14, normally open contacts 32, which are connected as shown between the positive terminal of DC power supply 10 and that end of capacitance 26 which is connected to the junction between first resistance 20 and second resistance 22, and contact-actuating coil 34.

Surrounding laser tube 14 and in cooperative relationship therewith is igniter coil 36. Igniter coil 36 and switch-actuating coil 34 of contactor 28 are connected in parallel and are simultaneously energized by AC voltage 38 in response to the closure of manually operated switch 40, which may be a pushbutton switch.

Considering now the operation of the circuit shown in the sole FIGURE, in the absence of any discharge between cathode electrode 12 and anode electrode 16 of laser tube 14 and with manually operated switch 40 unoperated, current will flow from the positive terminal of DC power supply 10 through normally closed contact 30 of contactor 28, very low third resistance 24 and relatively high first resistance 20 to the negative terminal of power supply 10 to thereby charge capacitance 26. Capacitance 26 will be charged with the left end thereof, connected to the junction between first resistance 20 and second resistance 22, relatively negative with respect to the right end thereof, connected to the junction between second resistance 22 and third resistance 24. Relatively high first resistance 20 will control the rate of charging capacitance 26. After capacitor 26 is fully charged, current will continue to flow from DC power supply 10 through the voltage divider composed of first resistance 20, second resistance 22 and third resistance 24.

In response to the closure of manually operated switch 40, both switch-actuating coil 34 of contactor 28 and ignitor coil 36 are energized by AC voltage 38. This results in the opening of normally closed contacts 30 and the closure or normally open contacts 32 of contactor 28. In response thereto the positive terminal of DC power supply 10 is connected through contacts 32 to the relatively negative end of capacitance 26 and the relatively positive end of capacitance 26 is connected through very low third resistance 24, which is merely a current-limiting resistance, to anode electrode 16 of laser tube 14.

Since the negative terminal of DC power supply 10 is still connected to cathode electrode 12 of laser tube 14, the total voltage applied between cathode electrode 12 and anode electrode 16 of laser tube 14 will be the sum of the output of DC power supply 10 and the charged voltage of capacitance 26, because with contact 32 closed they are connected in series-aiding relationship. This results in the potential at anode electrode 16 of laser tube 14 exceeding that at the positive terminal of DC power supply 10, causing unidirectional conducting device 18 to be back biased.

The application of this increased voltage between cathode electrode 12 and anode electrode 16 of laser tube 14, together with the simultaneously energization of igniter coil 36 by AC voltage 38, causes breakdown of the gas in laser tube 14 and a discharge plasma to be initiated therein. This results in a low resistance discharge path for capacitance 26 through low, current-limiting, third resistance 24 and the discharge plasma of laser tube 14.

As capacitance 26 continues to be discharged through the discharge plasma, the potential at anode electrode 16 of laser tube 14 finally drops to the point where it is less than the potential at the positive terminal of DC power supply 10. At this point, unidirectional conducting device 18 is no longer back biased, but is forward biased. When unidirectional conducting device 18 becomes forward biased, the potential at anode electrode 16 of laser tube 14 is clamped to the potential at the positive terminal of DC power supply 10. Therefore, at this time the voltage between cathode electrode 12 and anode electrode 16 is just equal to the output of DC power supply 10. It might be pointed out that the discharge of capacitance 26 usually takes place very quickly, before switch 40 is released. When switch 40 is released, normally closed contact 30 of contactor 28 are reclosed and normally open contacts 32 of contactor 28 are reopened. In this case, the voltage between cathode electrode 12 and anode electrode 16 of laser tube 14 is still solely the output of DC power supply 10.

Claims

I claim:

1. In a circuit for starting and maintaining a discharge through a gas discharge tube having a cathode electrode and anode electrode, said circuit comprising a positive terminal and a negative terminal connectable to a DC power supply, coupling means for coupling said positive terminal to said anode electrode and for coupling said negative terminal to said cathode electrode, an igniter adapted to be coupled in cooperative relationship with said tube for starting a discharge therethrough in response to energization thereof by an ignition voltage when said igniter is coupled to said tube, and selectively operable means coupled to said igniter for supplying said ignition voltage only when operated; the combination therewith of a capacitance, and means including said coupling means effective when said coupling means are coupled to said electrodes of said tube for charging said capacitance with said power supply to a given voltage in response to said selectively operable means being unoperated and in the absence of a discharge through said tube, for serially coupling said power supply terminals and said capacitance across said cathode and anode electrodes of said tube to apply the output of said power supply and said given voltage thereto in series-aiding relationship in response to said selectively operable means being operated, for discharging said capacitance through said tube upon the start of a discharge therethrough, and for coupling solely said power supply terminals across said cathode and anode electrodes of said tube to apply thereto only said power supply output upon the completion of the discharge of said capacitance.

2. The combination defined in claim 1, wherein said coupling means includes a unidirectional conducting device connected between one terminal of said power supply and the one of said electrodes of said tube coupled to that terminal, said unidirectional conducting device being poled to be normally conductive, and wherein said means including said coupling means comprises a capacitance charging resistance connected between the other of said electrodes of said tube and one end of said capacitance, means excluding said unidirectional conducting device connecting the other end of said capacitance to said one electrode, and a pair of switch contacts under the control of said selectively operable means for connecting said one end of said capacitance to said one terminal solely when said selectively operable means is operated.

3. The combination defined in claim 2, wherein said means connecting the other end of said capacitance to said one electrode includes a current-limiting resistance.

4. The combination defined in claim 2, wherein said means including said coupling means further comprises a shunt resistance shunting said capacitance.

5. The combination defined in claim 2, wherein said coupling means further comprises second pair of switch contacts under the control of said selectively operable means for connecting said one terminal directly to said one electrode solely when said selectively operable means is unoperated.

6. The combination defined in claim 1, wherein said gas discharge tube is a gas laser tube.

7. The combination defined in claim 1, wherein said power supply is a current-regulated power supply.

8. The combination defined in claim 1, wherein the output of said power supply is insufficient in and as of itself to effect starting of discharge through said tube, but is sufficient to maintain discharge therethrough once started, and the sum of the output of said power supply and said given voltage is sufficient to effect starting of discharge through said tube.

9. A circuit for starting and maintaining a discharge through a gas laser tube having a cathode electrode and an anode electrode; said circuit comprising a DC power supply having a negative terminal connected directly to said cathode electrode; a unidirectional conducting device connecting a positive terminal of said power supply to said anode electrode, said unidirectional conducting device being poled to be normally conducting; a voltage divider comprising a first relatively high resistance having one end thereof connected to said cathode electrode, a second relatively high resistance having one end thereof connected to the other end of said first resistance, and a third relatively low resistance having one end thereof connected to the other end of said second resistance and the other end of said third resistance connected to said anode electrode; a capacitance having one end thereof connected to the junction of said first and second resistances and the other end thereof connected to the junction of said second and third resistances; a contactor having a normally closed pair of switch contacts, a normally open pair of switch contacts, and a contact-actuating coil; means for connecting said normally closed pair of contacts between said positive terminal and said anode electrode; means for connecting said normally open pair of contacts between said positive terminal and said one end of said capacitance; an igniter coil in cooperative relationship with said gas laser tube, said igniter coil being connected in parallel with said contact-actuating coil; and a control circuit for said parallel connected igniter and contact-actuating coils including a normally open manually operated switch for applying an AC voltage to said parallel-connected ignitor and contact-actuating coils in response to the closure of said manually operated switch.