BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] This invention relates to a discharge tube for voltage control, and more particularly
to a discharge tube which is used for a series gap of an ignition system of an automobile
engine or the like.
2. Description of the Prior Art
[0002] An ignition system of an automobile engine or the like is adapted to supply a high
voltage to an ignition plug to produce a spark. In order to accurately control the
ignition timing, an ignition system with a so-called series gap is employed wherein
a discharge gap is provided in series to an ignition plug. In order to form such a
series gap, a discharge tube is employed wherein inert gas is enclosed in a tube and
a pair of electrodes are provided at the opposite ends of the tube. An exemplary one
of such discharge tubes is shown in FIG. 4. Referring to FIG. 4, the discharge tube
shown includes a glass tube 21 and a pair of metal wire electrodes 22 each covered
with a zirconium nitride layer not shown and mounted in an enclosed relationship at
the opposite end portions of the glass tube 21. Pressurized mixture gas of nitrogen
and hydrogen is filled in the glass tube 21.
[0003] Further, the discharge starting voltage of such a discharge tube must necessarily
be high to some degree. Accordingly, the discharge tube has a problem that, if the
distance between the electrodes is increased to this end, then the discharge maintaining
voltage of the discharge tube is also increased and consequently the loss of energy
to be supplied to an ignition plug is increased. Also there is another problem that
the exhaustion of the electrodes is increased and the life of the discharge tube is
reduced accordingly.
[0004] The inventors have made an investigation for improvement of making opposing faces
of a pair of electrodes less acute or non-acute to raise the discharge voltage and
lower the discharge maintaining voltage. Then, it has been found out that improvement
in accuracy of the discharge voltage can be attained by the provision of a large number
of convex and/or concave portions on a surface at least of the cathode. However, no
solution has been found out yet to the problem that the discharge voltage drops as
the frequency of discharge rises as described above.
SUMMARY OF THE INVENTION
[0005] It is an object of the present invention to provide a discharge tube which has a
stabilized discharge voltage characteristic irrespective of a change of the frequency
of discharge.
[0006] In order to attain the object, according to the present invention, there is provided
a discharge tube which comprises a hollow tubular casing, and a pair of opposing cathode
and anode electrodes secured to the axial opposite ends of the tubular casing to define
in the tubular casing an enclosed hollow spacing in which inert gas is filled such
that a discharge can be started by application of a pulse voltage between the cathode
and anode electrodes, each of the electrodes including an electrode base secured to
the tubular casing and an electrode member securely mounted on the electrode base
in the hollow spacing, the electrode member of the cathode electrode being composed
of a layered metal body including a ground layer and a thin surface layer formed on
a surface of the ground layer opposing to the anode electrode and having a considerably
smaller thickness than the ground layer.
[0007] The electrode member may be of the two-layer structure only including the ground
layer and the thin surface layer or of a three- or more-layer structure including
an additional layer or layers. The thin surface layer can be formed on the ground
layer by plating or the like, and desirably the thickness of the surface layer is
within the range from 5 to 20 µm.
[0008] Preferably, the electrode member of the cathode electrode has another similar thin
surface layer formed on the other surface of the ground layer thereof.
[0009] Also, the electrode member of the anode electrode may be composed of a layered metal
body including a ground layer and a thin surface layer formed on one or each of the
opposite surfaces of the ground layer. The shape of the cathode electrode and/or the
anode electrode can be determined suitably such that it may have an optimum characteristic
in accordance with an application of the discharge tube.
[0010] With the discharge tube of the present invention, even if the frequency of discharge
rises, the discharge voltage does not drop very much. Since the change of the discharge
voltage with respect to the frequency of ignition is improved in this manner, the
discharge tube is advantageous in that it exhibits an excellent effect in maintenance
of a stabilized running condition even if a considerable change takes place in the
number of revolution of an engine of an automobile or the like in which the discharge
tube is incorporated.
[0011] The above and other objects, features and advantages of the present invention will
become apparent from the following description and the appended claims, taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012]
FIG. 1 is a front elevational view, partly in section, showing a discharge tube to
which the present invention is applied:
FIG. 2 is a circuit diagram showing an ignition system with a series gap with which
a discharge tube is to be tested:
FIG. 3 is a graph showing results of a discharge voltage test of discharge tubes conducted
with the ignition system shown in FIG. 2; and
FIG. 4 is a schematic sectional view showing an exemplary one of conventional discharge
tubes.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] Referring first to FIG. 1, there is shown a discharge tube to which the present invention
is applied. The discharge tube shown includes a hollow tubular casing 8 formed from
an alumina ceramics, and a cathode electrode 11 and an anode electrode 12 secured
to the axial opposite ends of the tubular casing 8 and sealed with a suitable means
such as silver solder not shown so as to define a hollow spacing 13 in the casing
8. The cathode electrode 11 includes an electrode base 2 substantially in the form
of a disk secured to the tubular casing 8. and an electrode member 1 having a generally
semi-spherical shape and fitted on and secured to a hub portion 14 of the electrode
base 2 by means of a metal ring 3. The electrode member 1 has a large number of perforations
16 of a suitable size formed at a suitable pitch thereon. Meanwhile. the anode electrode
12 includes an electrode base 6 substantially in the form of a disk secured to the
tubular casing 8, and an electrode member 4 having a substantially flat surface and
fitted on and secured to a hub portion 15 of the electrode base 6 by means of a metal
ring 7. The electrode member 4 also has a large number of perforations 16 of a similar
size formed at a similar suitable pitch thereon. A gas conduit 5 extends axially through
the center of the electrode base 6. Upon production of the discharge tube, suitable
inert gas is introduced into the inner hollow spacing of the discharge tube by way
of the gas conduit 5 so as to replace with the air. After such replacement, an outer
end of the gas conduit 5 is compressed to seal the conduit 5.
[0014] Each of the electrode members 1 and 4 is formed from a layered body in the form of
a plate which at least includes, though not shown, a ground layer which may be, for
example, a copper plate having a thickness of about 0.5 mm, and a thin surface layer
formed on each of the opposite surfaces of the ground layer and having a significantly
small thickness of 5 to 20 µm, preferably of 10 µm, comparing with the ground layer.
The surface layer may be formed, for example, by electroplating of nickel.
[0015] Two different discharge tubes were produced actually in accordance with the present
invention, and a performance test of them was conducted.
Example 1
[0016] A copper plate of a thickness of 0.5 mm having a large number of perforations of
about 0.5 mm formed at a pitch of about 1 mm therein was prepared. The perforated
copper plate was shaped by press work so as to be rounded at peripheral edge portions
thereof, and electroplating of nickel was performed to a thickness of about 10 µm
on the opposite inner and outer surfaces of the perforated copper plate. An electrode
member 1 obtained in this manner was fitted on and secured to an electrode base 2
by means of a metal ring 3 to form a cathode electrode 11 as shown in FIG. 1.
[0017] Another electrode 4 having a substantially flat surface was produced in a similar
manner and fitted on and secured to an electrode base 6, which had a gas conduit 5
extending therethrough, by means of another metal ring 7 to form an anode electrode
12.
[0018] Then, the cathode electrode 11 and the anode electrode 12 produced in this manner
were assembled in an opposing relationship to each other to the opposite ends of a
tubular casing 8 of an alumina ceramics and then sealed with silver solder. Then,
inert gas was introduced for replacement with air into the internal hollow spacing
13 of the discharge tube by way of the gas conduit 5, and then an outer end of the
conduit 5 was compressed for enclosure, thereby obtaining a discharge tube I according
to the present invention.
Example 2
[0019] Instead of forming the electrodes 1 and 4 from perforated copper plates. they were
formed by shaping similar perforated stainless steel plates by press work and then
performing electroplating of gold to a thickness of about 10 µm on the opposite inner
and outer surfaces of the perforated stainless steel plates. Then, a discharge tube
II of the present invention was produced by assembling the electrode plates 1 and
4 to a tubular casing 8 in a similar manner as the example 1 described hereinabove.
Performance Test
[0020] A performance test was conducted with such an ignition system with a series gap as
shown in FIG. 2. Referring to FIG. 2. the ignition system shown includes an ignition
coil T, a pair of discharge gaps G₁ and G₂ provided by a discharge tube and an ignition
plug, respectively, and a floating capacitance C provided by a high tension cable,
a distributor cap and so forth. An ignition signal is transmitted to the ignition
coil T by way of a transistor Tr.
[0021] Using the device, discharge voltage characteristics of the discharge tubes I and
II of the present invention and a further discharge tube III assembled using electrodes
of the same shapes formed from stainless steel plates were tested. Results of such
test are shown in FIG. 3. From FIG. 3, it can be seen that the discharge tubes I and
II of the present invention present considerably small drops of the discharge voltage
with respect an increase of the ignition frequency comparing with the discharge tube
III.
[0022] Having now fully described the invention, it will be apparent to one of ordinary
skill in the art that many changes and modifications can be made thereto without departing
from the spirit and scope of the invention as set forth herein.
1. A discharge tube, comprising a hollow tubular casing, and a pair of opposing cathode
and anode electrodes secured to the axial opposite ends of said tubular casing to
define in said tubular casing an enclosed hollow spacing in which inert gas is filled
such that a discharge can be started by application of a pulse voltage between said
cathode and anode electrodes, each of said electrodes including an electrode base
secured to said tubular casing and an electrode member securely mounted on said electrode
base in said hollow spacing, said electrode member of said cathode electrode being
composed of a layered metal body including a ground layer and a thin surface layer
formed on a surface of said ground layer opposing to said anode electrode and having
a considerably smaller thickness than said ground layer.
2. A discharge tube as claimed in claim 1, wherein the thickness of said thin surface
layer of said cathode electrode is within the range from 5 to 20 µm.
3. A discharge tube as claimed in claim 1, wherein said electrode member of said cathode
electrode has another similar thin surface layer formed on the other surface of said
ground layer thereof.
4. A discharge tube as claimed in claim 1, wherein said electrode member of said anode
electrode is composed of a layered metal body including a ground layer and a thin
surface layer formed on one or each of the opposite surfaces of said ground layer.