Electricity: magnetically operated switches – magnets – and electr – Electromagnetically actuated switches – Vacuum or hermetically sealed type
Reexamination Certificate
2000-06-28
2001-08-07
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Electromagnetically actuated switches
Vacuum or hermetically sealed type
C335S299000
Reexamination Certificate
active
06271740
ABSTRACT:
The present invention pertains to a relay with which a reed switch inserted into an electrostatic shielded pipe is opened and closed by a magnetic coil and a system for measuring very small current on the order of femtoamperes (fA). In particular, it pertains to a relay that is used to control transmission of Joule effect heat of the magnetic coil to the highly insulated holding member that holds the electrostatic shielded pipe and reed switch in order to inhibit the generation of thermally stimulated current (offset current) at this holding member and thereby avoid disturbance of the signal on the signal conductors, or the signal lines, of the relay.
BACKGROUND OF THE INVENTION
FIG. 1
shows the structure of conventional reed relay
100
. Signal conductor terminals, or signal line terminals,
117
a
and
117
b
at both ends of reed switch
101
are held near both ends of electrostatic shielded pipe
103
by insulators
102
a
and
102
b
, which are plates called bushings and have holes in the center through which the terminals pass. This electrostatic shielded pipe
103
is inserted into the cylindrical hollow part of coil bobbin
104
. Coil
105
for excitation is wound and installed in the concave part around the outside of coil bobbin
104
. This concave part is further packed with resin
106
and this is covered by magnetic shield case
107
. Here, electrostatic shielded pipe
103
and coil bobbin
104
are adjacent and contact one another. Moreover, when reed relay
100
is used in the circuit, it is preferred that electrostatic shielded pipe
103
is connected to guard wires that and function as an active guard or passive guard. Please note that unless otherwise specified, the same symbols are used to describe the same structural elements in the figures of the present Specification.
Nevertheless, reed relay
100
poses many problems when used for the measurement of very small currents on the order of fA in terms of the relationship between the latency time or wait time until the measurement values stabilize and offset current reduces, as described below:
FIG. 2
shows the measurement results of the offset current of reed relay
100
. The x-axis shows the time that has passed when 0 seconds serves as the time when excitation current begins to flow to the coil, and the y-axis shows the current flowing through the signal conductors of the relay as detected by an ammeter for very low currents with a guard feature. In this case, the guard terminals of the ammeter are wired so that they are connected to the electrostatic shielded pipe of the relay, with the voltage of the signal conductors being constant at 0V, and a 10 mA rated current flows as excitation current to the coil. Please note the fact that the reed switch is turned on with excitation of the coil in this case. According to
FIG. 2
, excitation current begins to flow to the coil and the negative polarity current gradually increases in approximately 80 seconds to peak at −6 fA. Thereafter the negative polarity current settles down with convergence to approximately 0 fA approximately 300 seconds after starting excitation and stabilization to the steady state.
Thus, conventional reed relays are inappropriate for high-speed and high-precision measurement of very small currents, because an offset current of approximately several fA flows for approximately 100 seconds beginning immediately after the relay has been turned on.
Leakage current or thermo-electromotive force due to a contact potential difference between different types of metals, as described in Japanese Patent Laid-open (Kokai) No. Hei 2(1990)-68,829, and dielectric absorption in an insulator, as described in Japanese Patent Laid-open No. Hei 8(1996)-279,314 were considered to be factors of the above-mentioned offset current in the past, but of course unclear points remain. That is, the explanation of leakage current being transmitted over the surface of an insulator or of current being generated by potential difference between relay terminals due to thermo-electromotive force, which is caused by the difference in the amount of heat conduction towards the both ends of relay terminals, applies only to the steady state current and contradicts the phenomenon of offset current naturally converging at 0 fA. Moreover, the voltage of the signal conductors was constant at 0 V in the measurements shown in
FIG. 2
, and therefore, it does not appear that a potential difference with which dielectric absorption would occur was produced between the signal conductors and the electrostatic shielded pipe.
In any case, in the past very low currents on the order of fA were observed after waiting for approximately 100 seconds until the offset current stabilized, and therefore, high-speed measurement of the very low current was not possible. Very low currents could also be measured by reducing wait time, recognizing that the results would be inaccurate. Nevertheless, the measurement devices have become faster each year and therefore, it is necessary to develop a high-performance reed relay for very low currents with which the wait time can be curtailed and speed can be increased.
The present inventor hypothesized the following based on the fact that the above-mentioned offset current is due to thermally stimulated current that is produced when Joule effect heat generated by the coil propagates or transmits to the contact surface between the metal and the insulator:
That is, by means of the reed relay in
FIG. 1
, heat that has been generated by the coil is transmitted as shown below.
One is coil (
105
)→coil bobbin (
104
)→electrostatic shielded pipe (
103
)→bushing (
102
a
,
102
b
), and the other is coil (
105
)→resin (
106
)→magnetic shield case (
107
)→air.
Plastics and resin materials with high insulating performance generally have thermal conductivity that is two to three orders of magnitude lower than metals, and therefore, heat is conducted via the above-mentioned two routes on the order of materials with
good→poor→good→poor
conductivity. As a result, thermal resistance becomes several 10 K/W and, for instance, when as much as 0.1 W heat is generated by the coil, the temperature of the electrostatic shielded pipe will probably also rise by several K.
The electrons trapped at the surface on the electrostatic shielded pipe
103
side of bushings
102
a
and
102
b
are excited by thermal energy with this rise in temperature and are released to inside electrostatic shielded pipe
103
. In this case, electrons are probably fed from the sides of signal conductor terminals
117
a
and
117
b
to bushings
102
a
and
102
b
, which are insulators, in a form that maintains electric neutrality.
The fact that this hypothesis is correct was demonstrated as follows by experiments. First, as shown in
FIG. 3
, heat-absorbing element
311
, which is called a Peltier device, is fastened directly above coil bobbin
104
of the relay. Coil
105
is not excited. The voltage of the signal conductors is constant at 0 V and only Peltier device
311
is operated. That is, heat passes through coil
105
and is transmitted in the opposite direction from the case for the measurements in above-mentioned
FIG. 2
in the sequence such as:
Peltier element (
311
)←magnetic shield case (
107
)←resin (
106
)←coil (
105
)←coil bobbin (
104
)←electrostatic shielded pipe (
103
)←bushings (
102
a
,
102
b
), and heat is absorbed or cooling takes place.
The offset current waveform in this case is the waveform shown in FIG.
4
A. The inventors discovered that in this case, the polarity of the offset current is positive and opposite from FIG.
2
.
FIG. 4B
shows the results of further flowing a rated current of 10 mA to coil
105
and performing the same operation as with Peltier device
311
under the above-mentioned conditions based on the above-mentioned discovery. It is clear that the current of negative polarity in FIG.
2
and the current of positive polarity in
FIG. 4A
cancel each other out so that the offset current
Agilent Technologie,s Inc.
Barrera Ramon M.
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