NTC thermistors

Details NTC thermistors NTC thermistors

1999-05-04

2001-06-05

Easthom, Karl D. (Department: 2832)

Electrical resistors

Resistance value responsive to a condition

Current and/or voltage

C338S221000, C338S232000, C338S276000

Reexamination Certificate

active

06242998

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to negative temperature characteristic (NTC) thermistors for suppressing rush currents.
NTC thermistors are characterized as having lower resistance at elevated temperatures than at the normal temperature. Because of this unique characteristic, NTC thermistors are frequently utilized as a circuit element incorporated in a power source circuit of a device for suppressing the rush current which may flow into the source circuit at the instant when the power switch for the device is turned on.
As shown in
FIG. 6
, a prior art NTC thermistor
1
of a type enclosed inside a case and used for suppressing rush currents is generally structured so as to have elongated power-supply terminals
5
and
6
connected to electrodes
3
and
4
formed on two mutually opposite main surfaces of a circular disk-shaped thermistor element
2
, both the thermistor element
2
and the power-supply terminals
5
and
6
being enclosed inside a heat-resistant resin case
7
. The thermistor element
2
is supported by and sandwiched between the tips of the terminals
5
and
6
inside the hollow internal space of the resin case
7
, while the other ends of the terminals
5
and
6
penetrate the body of the resin case
7
, extending to its exterior.
One of the methods of improving the effect of such an NTC thermistor
1
to suppress a rush current has been to increase the volume of the NTC thermistor element
2
so as to increase its heat capacity such that the rise of its temperature due to the heat emitted by itself will be limited and the lowering of its resistance can be reduced. This method is not a practical one, however, because the cost of the NTC thermistor element takes up a large portion of the total cost of the product and the cost of the NTC thermistor element will increase if its volume, or its size, is increased.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide NTC thermistors of the type enclosed in a case which can suppress rush currents effectively without using an NTC thermistor element with a large volume.
An NTC thermistor embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising power-supply terminals connected to electrodes individually formed on a pair of mutually opposite side surfaces of a planar NTC thermistor element and having a case which encloses the NTC thermistor element and the terminals, but also wherein at least one of the main surfaces of the planar NTC thermistor element makes a surface-to-surface contact with an inner wall of the case. The NTC thermistor element may be quadrangular, or polygonal more generally, and comprise an oxide of a rare earth transition element such as LaCo oxide. The power-supply terminals may comprise a metallic material such as Cu or a Cu—Ti alloy. The case may comprise a ceramic material.
An NTC thermistor embodying this invention may be conveniently inserted in series between a power source and a heat-emitting element for a heater, say, of an electronic copier, serving to fix carbon on a sheet of copy paper. If a copier is thus structured, not only can rush currents be more effectively suppressed but the rated current value can be increased.


REFERENCES:
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patent: 3925748 (1975-12-01), Slocum
patent: 4395623 (1983-07-01), Shimada et al.
patent: 4698614 (1987-10-01), Welch et al.
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patent: 5142265 (1992-08-01), Motoyoshi et al.
patent: 5504371 (1996-04-01), Niimi et al.
patent: 5703000 (1997-12-01), Nakayama et al.
patent: 5714924 (1998-02-01), Takeuchi et al.
patent: 5760336 (1998-06-01), Wang
patent: 5764470 (1998-06-01), Mihara et al.

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