Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
2003-05-05
2004-06-15
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S023000, C338S024000
Reexamination Certificate
active
06750754
ABSTRACT:
BACKGROUND OF THE INVENTION
(A) Field of the Invention
The present invention is related to an over-current protection apparatus, more specifically, to an over-current protection apparatus that can automatically cut off current.
(B) Description of Related Art
Electrical switches include manual switches, breakers, relays, etc. If an over-current occurs at the instance of a switch is being opened, an arcing may be generated at their contacts, i.e., a current exists until the arcing goes off. The arcing would damage the contacts, and the extent of the damage depends on the kind of DC or AC, and the amount of the current and the voltage. Therefore, the limitation of the current and the voltage applied on the contacts to prevent the contacts from being damaged is becoming a crucial point in practice.
The resistance of a positive temperature coefficient (PTC) conductive material is sensitive to temperature variation, which can be kept extremely low at normal operation due to its low sensitivity to temperature variance so that the circuit can operate normally. However, if an over-current or an over-temperature event occurs, the resistance will immediately increase to a high resistance state (e.g. above 10
4
ohm.) Therefore, the over-current will be reversely eliminated and the objective to protect the circuit device can be achieved.
U.S. Pat. No. 5,737,160 and U.S. Pat. No. 5,864,458 both reveal the applications of a PTC element associated with switches. FIG.
1
(
a
) and FIG.
1
(
b
) respectively show the cases of the PTC element and the switches being connected in series and in parallel. Referring to FIG.
1
(
a
), a PTC element
11
is connected with a switch
12
in series. When an over-current occurs, the resistance of the PTC element
11
will increase rapidly, reducing the current flowing in the circuit. Sequentially, the switch
12
is opened to avoid the damage of the PTC element
11
due to high voltage.
In FIG.
1
(
b
), a PTC element
13
is connected with a switch
14
in parallel. The resistance of the PTC element
13
is higher than that of the switch
14
, and thus only minor current flows through the PTC element
13
. As a result, the resistance of the PTC element
13
is still low. When an over-current occurs, the switch
14
is being opened instantly to enforce current flow through the PTC element
13
, so the resistance of the PTC element
13
ramps drastically whereby the current is reduced. Because a possible arcing of the switch
14
has to be taken into account, such kind of apparatus is attributed to apply for low voltage circuitry.
It is necessary to further provide a signal to control the switch
12
or
14
in association with a PTC element of the above over-current protection apparatuses. Basically, a PTC element does not function as a switch, but relies to connect with an extra switch to cut off the current. When the PTC element is tripped, the PTC element has to count on leakage current to keep the PTC element tripped for high resistance sustenance. Under the circumstances of high voltage and leakage current, the PTC element may be aged to lose its protection capability. In addition, if a false signal occurs, an unexpected damage may be induced.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an over-current protection apparatus which can automatically cut off current to protect the protected circuitry, for the high voltage circuit device. Besides, the over-current protection apparatus can be mechanically reset and come back to its normal operation state.
The over-current protection apparatus of the present invention comprises a first electrode plate, a second electrode plate, a third electrode plate, a conductive element and a high resistance material layer. If no over-current occurs, the third electrode plate is electrically conductive to the first electrode plate to form a conducting path. The conductive element is connected to the first electrode plate and the second electrode plate. The high resistance material layer, whose thermal expansion coefficient is smaller than that of the conductive element, is connected to the third electrode plate and the second electrode plate. By virtue of the thermal expansion of the conductive element due to an over-current, the electrical conduction of first electrode plate and the third electrode plate is isolated to enforce the current flows through the high resistance material layer whereby the current is decreased.
The above mentioned over-current protection apparatus may further comprises a thermal conductive and electricity insulating layer to isolate the conductive element and the high resistance material layer, and to be a medium for heat transferring between them. Therefore, the expanded conductive element can be kept to isolate current.
The conductive element may comprise a PTC material, which is capable of thermal expansion.
Another over-current protection apparatus of the present invention comprises an insulating layer having a high thermal expansion coefficient, an upper electrode bar, a lower electrode bar, a first electrode terminal and a second electrode terminal, the upper electrode bar being attached to the insulating layer, the thermal expansion coefficient of the upper electrode bar being smaller than that of the insulating layer, the lower electrode bar being attached to the insulating as well, and the thermal expansion coefficient of the lower electrode bar being smaller than that of the insulating layer. The top of the lower electrode bar may contact the bottom of the upper electrode bar to form a conducting path, and the ends of the first electrode terminal and the second electrode terminal are respectively connected to the upper electrode bar and the lower electrode bar. The insulating layer is expanded by the heat generated from the over-current flowing through the upper electrode bar and the lower electrode bar, and thus the upper electrode bar and the lower electrode bar are dragged by the insulating layer to be separated to cut off current.
The insulating layer having a high thermal expansion coefficient may comprise polyethylene (PE), polypropylene (PP) or other crystallized polymers, and the upper electrode bar and the lower electrode bar may be made of copper, nickel, aluminum or other metals.
REFERENCES:
patent: 3686857 (1972-08-01), Berg
patent: 5737160 (1998-04-01), Duffy
patent: 5864458 (1999-01-01), Duffy et al.
patent: 5907272 (1999-05-01), McGuire
patent: 5945903 (1999-08-01), Reddy et al.
patent: 6392528 (2002-05-01), Myong
patent: 6606023 (2003-08-01), Chiang et al.
patent: 6661633 (2003-12-01), Furuta et al.
Chen Yi-Nuo
Wang David Shau-Chew
Yu Chih-Ming
Easthom Karl D.
Polytronics Technology Corporation
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