Gas discharge tube with a metal oxide varistor housing

Abstract

 An electrical over-voltage suppression device integrating crowbar protection with clamping protection in a single component form, a gas volume defined by a gas discharge tube with spaced-apart electrodes acting as arcing targets providing crowbar protection, and a varistor integral to the gas discharge tube providing clamping protection.

Description

[0001] The present invention relates to electrical voltage surge suppression devices. Suppression devices are widely used for protection of electrical equipment from overvoltage conditions caused by electrical surges, lightning, high voltage line contact and the like.

[0002] Known surge protection devices are clasified into two classes: crowbar type devices and clamping type devices. Crowbar devices include plasma surge arrestors, spark gap devices, gas discharge tubes (GDTs), and silicon controlled rectifiers (SCRs). These devices limit the current flow into the protected circuit by abruptly switching from a high impedance state, e.g., an open circuit, to a low impedance state, e.g., a short circuit, responsive to the circuit voltage exceeding a set impedance switching threshold level.

[0003] When in a high impedance state, no current flows through the crowbar device and no power is consumed. Once switched into the low impedance state, the voltage across the crowbar device is at a relatively low level and the device does not dissipate a significant portion of the power delivered during the overvoltage event but rather passes the power from line to ground. For example, once the voltage across a GDT exceeds a characteristic gas breakdown voltage (the GDT threshold voltage), the GDT switches to a low impedance state where the voltage across the GDT is less than 15 volts. Thus crowbar devices are suited for operating in high current conduction modes for relatively long periods of time, allowing for the protection of circuits during relatively long surges. The crowbar devices, however, do not provide any surge protection below the switching threshold voltage for a particular transient waveform.

[0004] Clamping devices include zener (avalanche) diodes and metal oxide varistors (MOVs). In contrast to crowbar devices, clamping devices limit the voltage transient to a specified voltage level by varying the clamping device's intemal resistance responsive to the applied voltage. Clamping devices advantageously provide rapid response and protect against fast rising surge conditions. In the case of a zener diode device, the zener diode presents an open to the circuit until a voltage surge in excess of the diode breakdown voltage is present in the circuit. With the zener diode conducting, the voltage across the circuit is limited to the zener breakdown voltage. Zener diodes are typically used in low voltage applications and, due to capacitive effects, are generally not suitable to RF applications.

[0005] With a MOV device, the clamping device's internal resistance changes with the applied voltage so that the MOV acts as a voltage sensitive, nonlinear resistive element in parallel with the circuit being protected. As a clamping device must absorb the surge energy at the clamped voltage, clamping devices cannot withstand the same high level of surge current as a gas discharge tube. MOVs suffer from high current surges causing cumulative degradation and performance changes. Thus, while suitable for higher voltage applications than zener diode devices, fuses may be required to protect the MOV from damage by exposure to excessive voltage levels.

[0006] It is known that both crowbar and clamping devices are each best suited for particular applications. An object of the invention is to provide protection against both fast rising voltage levels and protection at relatively high voltages for an extended period, crowbar and clamping devices have been used in combination.

[0007] For example, discharge tubes (both interior gas or exterior gap) have been used in combination with varistor as overvoltage protective devices. The specification of U.S. Patent No.4,262,317 to Baumbach discloses discrete MOVs wired in parallel with a GDT; U.S. Patent No.s 5,383,085 to Boy, et al. and 5,559,663 to Tanaka, et al. disclose mounting a MOV adjacent a GDT packaged within a single housing. Disadvantageously, each of these combinations are relatively complex, may be susceptible to damage from shock or vibration, and are relatively expensive to manufacture.

[0008] The present invention improves on the prior art with a simpler, less complex to manufacture, electrical over-voltage suppression device integrating crowbar protection with clamping protection in a single component form.

[0009] An object of the present invention is to provide a gas discharge tube suppressor with both crowbar and clamping protection without the use of a varistor separate from the gas discharge tube, and to provide a voltage arrestor using a gas discharge tube for crowbar protection and an integral varistor for clamping protection.

[0010] Another object is to provide a voltage suppression device providing both crowbar protection and clamping protection in single component form.

[0011] Another object is to provide a voltage surge suppression device having a gas discharge tube and varistor formed as a single component, and also to provide a gas surge arrestor using varistor ceramic as component of a sealed gas housing.

[0012] The present invention includes an electrical suppression device integrating crowbar protection with clamping protection in a single component, comprising first and second spaced-apart electrodes, a gas medium positioned between the electrodes and having both a relatively low current conducting state and a relatively high current conducting state, said high conducting state corresponding to application of a characteristic breakdown voltage between the first and second electrode, a voltage variable resistor connected between the first and second electrodes, in which the resistor is a body extending from the first electrode to the second electrode and shaped to form in combination with the electrodes a sealed housing to retain the gas medium.

[0013] The invention also includes a circuit over-voltage protective device with voltage-dependent current paths comprising, an envelope containing a volume of gas, said envelope having varistor ceramic portions with terminations, arcing targets attached to said envelope and exposed to said gas, device contacts for connecting said envelope into a circuit, attached to said envelope and operatively connected to apply a circuit voltage to said arcing targets and said terminations, whereby upon applying circuit voltage to said contacts the protective device provides a voltage sensitive, nonlinear current path through said varistor ceramic portions until said applied circuit voltage ionizes said gas between said arcing targets and provides a current path between the arcing targets.

[0014] The invention will now be described, by way of example, with reference to the accompanying drawingsin which:

[0015] Figure 1 illustrates a prior art GDT surge arrestor.

[0016] Figure 2 is an equivalent circuit of the Figure 1 GDT arrestor.

[0017] Figure 3 is an illustration of one embodiment of the present invention.

[0018] Figure 4 is the equivalent circuit of the Figure 3 device.

[0019] Figure 5 illustrates another embodiment of the present invention.

[0020] Figure 6 is an equivalent circuit of the Figure 5 embodiment.

[0021] Figure 1 shows prior art GDT surge arrester 10 and consists of housing 20, electrodes 30, device leads 40, and fill gas 50. Housing 20 may be metal, glass, or ceramic and holds electrodes 30 at a proper spacing distance. The volume defined by housing 20 and electrodes 30 may contain ionizable fill gas 50. Electrodes 30 act as arcing targets and may be shaped to optimize current arcing at fill gas 50 breakdown voltage. Electrodes 30 may include metal end caps 35 sealing against housing 20 to contain and prevent leakage of fill gas 50.

[0022] Figure 2 provides an equivalent circuit of prior art GDT surge arrester 10. Prior art GDT 10 functions as a crowbar device, triggered at fill gas 50 characteristic breakdown voltage value, e.g., 400-600 volts, shorting line to ground across gap 80.

[0023] Figure 3 refers to an embodiment for surge suppressor 60 includes electrodes 30, device leads 40, fill gas 50, and varistor housing 70. Varistor housing 70 in combination with electrodes 30 define a gas discharge tube containing fill gas 50. Varistor housing 70 may serve to hold electrodes 30 spaced apart at a optimized distance for current arcing. Electrodes 30 may include end caps 35 forming part of the gas discharge tube and providing a mounting surface for device leads 40.

[0024] Figure 4 provides an equivalent circuit diagram Surge suppressor 60 functions as varistor 90 in parallel with a arc gap 80. Advantageously, varistor 90, serves as a transient suppressor providing clamping protection up to a specified voltage, after which crowbar protection is provided by arc gap 80.

[0025] Arc gap 80 trigger voltage is pre-set at a value to complement the operation of varistor 90, yet sufficiently low to avoid damaging varistor 90. When arc gap 80 trigger voltage is exceeded, fill gas 50 ionizes, changing from a nonconducting state to a conducting state, and current arcs across gap 80, both shorting line to ground to provide crowbar protection and shorting out varistor 90 to avoid subjecting varistor 90 to a voltage, and the resulting high current, in excess of a level that would cause damage to varistor 90. This arrangement thus provides both crowbar and clamping protection in a single component device and further obviates the need for a fuse to protect the varistor.

[0026] Varistor housing 70 comprises a zinc oxide ceramic and acts as a metal oxide varistor. In one zinc oxide embodiment, surge suppressor 60 is in the form of a cylinder having an exterior length of 0.196 inches and diameter of 0.316 inches, an interior volume of 7.45 x 10^-3 cubic inches filled with a gas of consisting principally of Argon results in a GDT having a breakdown voltage of 600 volts. With a typical zinc oxide ceramic, this embodirnent would exhibit a clamping voltage of 400 volts at 10 amps.

[0027] Figures 5 and 6 depict a three terminal, 2 GDT surge arrestor embodiment of the present invention is depicted. Three terminal arrestor 100 includes three spaced apart electrodes 30 and varistor housing 70 defining 2 gas discharge tubes.

[0028] An electrical over-voltage suppression device integrating crowbar protection with clamping protection in a single component form, a gas volume defined by a gas discharge tube with spaced-apart electrodes acting as arcing targets providing crowbar protection, and a varistor integral to the gas discharge tube providing clamping protection.

Claims

1. An electrical suppression device integrating crowbar protection with clamping protection in a single component, comprising first and second spaced-apart electrodes, a gas medium positioned between the electrodes and having both a relatively low current conducting state and a relatively high current conducting state, said high conducting state corresponding to application of a characteristic breakdown voltage between the first and second electrode, a voltage variable resistor connected between the first and second electrodes, in which the resistor is a body extending from the first electrode to the second electrode and shaped to form in combination with the electrodes a sealed housing to retain the gas medium.

2. A device as claimed in claim 1, wherein the electrodes each include a metal plate portion and the resistor body is a gas discharge tube and a varistor formed as a single component.

3. A device as claimed in claims 1 or 2, including third electrode spaced-apart from the second electrode, a gas medium positioned between the second and third electrodes and having both a relatively low current conducting state and a relatively high current conducting state, said high conducting state corresponding to application of a characteristic breakdown voltage between the second and third electrodes, and a voltage variable resistor connected between the second and third electrodes.

4. A single component electrical suppression device having a cavity with opposing electrodes therein, the cavity being filled with a gas having a relatively low current conducting state when a voltage applied to the opposing electrodes is below a characteristic breakdown value and having a high current conducting state when the voltage applied to the opposing electrodes is above the characteristic breakdown value, at least a portion of the device comprises a voltage sensitive, nonlinear resistive element providing voltage control below the characteristic breakdown value, and the resistive element preferably surrounds at least a portion of the cavity.

5. A device as claimed in claim 4, wherein the cavity is defined by a gas discharge tube and a varistor formed as a single component.

6. A surge suppressor device having a gas discharge member comprising a gas enclosure containing a volume, two arcing electrodes mounted therein for shunting current by forming an arc within the volume between the two arcing electrodes at a breakdown voltage, and a varistor member for shunting current at a clamping voltage through the varistor member, in which the varistor is integral with the gas enclosure.

7. A crowbar surge suppressor having first and second spaced apart electrodes with a voltage applied thereto presenting an open circuit between the two electrodes while the applied voltage remains below a threshold voltage and presenting a short circuit between the two electrodes when the applied voltage is above the threshold voltage, in which the inclusion of a clamping surge suppressor integral with the crowbar surge suppressor structure providing a voltage variable resistance below the threshold voltage, with the clamping surge suppressor being a varistor.

8. A circuit over-voltage protective device with voltage-dependent current paths comprising, an envelope containing a volume of gas, said envelope having varistor ceramic portions with terminations, arcing targets attached to said envelope and exposed to said gas, device contacts for connecting said envelope into a circuit, attached to said envelope and operatively connected to apply a circuit voltage to said arcing targets and said terminations, whereby upon applying circuit voltage to said contacts the protective device provides a voltage sensitive, nonlinear current path through said varistor ceramic portions until said applied circuit voltage ionizes said gas between said arcing targets and provides a current path between the arcing targets.

9. A surge suppressor device for shunting current both at a clamping voltage and at a breakdown voltage comprising a varistor in the form of an open ended, hollow cylinder providing voltage sensitive, nonlinear resistant upon application of a differential voltage across the varistor ends and shunting current at a characteristic damping voltage, opposing electrodes operatively positioned at opposing varistor cylinder openings serving as arcing targets and for defining a discharge volume therebetween that upon application of a differential voltage to the opposing electrodes exceeding a characteristic breakdown voltage, a current arc forms between the electrodes shunting current therebetween, electrical contacts for connecting each varistor cylinder end with an electrode providing paralleling current paths through the suppressor, and in which the opposing electrodes and varistor cylinder in combination form a sealed volume.

10. A suppressor device as claimed in claim 9, wherein the sealed volume contains a gas having a breakdown voltage of 600 volts with a 100 volt/microsecond impulse, and the varistor is a metal oxide varistor having a clamping voltage of 400 volts at 10 amps.

Drawing