Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Patent
1982-09-20
1984-11-20
Griffin, Donald A.
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
252520, H01G 410, H01L 2912
Patent
active
044842511
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a current restriction element responsive to applied voltage, in particular, relates to a polycrystalline ferro-electric ceramics having a rectangular hysterisis characteristics, in which an impedance of said ceramics depends upon voltage applied to the same.
BACKGROUND OF THE INVENTION
The present element has the main component B.sub.a T.sub.i O.sub.3 and/or other additives S.sub.r, Z.sub.r, S.sub.n, and/or P.sub.b which replace partially said component.
The present invention provides a low loss impedance element for an alternate current, by using the change of an apparent dielectric constant of ceramics of B.sub.a T.sub.i O.sub.3 series.
Generally, a power source for industry and home consumer is 50 or 60 Hz, 100 volts or 200 volts of single phase or three phase power source. And, a passive element for adjusting current for a low impedance load coupled with said power source is, for instance, an inductance (L), a capacitance (C), and a resistance (R).
Among them, a resistance element is the cheapest, but it has the disadvantage that the heat generation is large, and further, the generated heat is not only the undesired power loss, but also the generated heat decreases the reliability of the whole system by raising the temperature of the same. Therefore, a resistance element is not used at present except for restricting very small current.
An inductance element (L) is mostly utilized at present, for instance as a choke coil (a ballast in a fluorescent lamp circuit is one of them).
An inductance element has the advantages that loss is small even in a large current situation, heat generation is small, and therefore, the reliability of the system itself does not decrease by the use of an inductance element. Further, when the current is large, a small inductance element is enough, and therefore, the number of turns of a coil may be small. However, when the current is large, a thick conductor must be used for a coil, and therefore, the size and/or the weight of the element is large, thus, it might be difficult to install an inductance element for current restriction in a miniaturized small apparatus.
When a capacitance element (C) is used, the capacity must be large in case of low frequency like 50 or 60 Hz. In particular, when large current flows, the reactance must be small and therefore, large capacitance must be utilized. Further, an electrolytic capacitor with aluminium film which is a typical large capacity capacitor at present is not sufficiently reliable at high temperature. Accordingly, an impedance element with a small size, high capacity, high withstand voltage, high thermal stability, and low cost has been desired.
A prior art for satisfying the above requirements is a ceramic capacitor having the dielectric material B.sub.a T.sub.i O.sub.3.
The reason for that is the dielectric constant of B.sub.a T.sub.i O.sub.3 is higher than 20,000 while the dielectric constant (specific inductive capacity) of other materials like mica, paper, or plastics film is in the range between 3 and 15. Therefore, a capacitor with B.sub.a T.sub.i O.sub.3 is small in size as compared with the capacity of the same.
Further, since ceramics are sintered with the temperature 1300.degree.-1400.degree. C., the thermal stability is excellent, and the operational reliability is also excellent.
However, a capacitor with B.sub.a T.sub.i O.sub.3 has the disadvantage that the capacity for a unit volume is not so large, since the B.sub.a T.sub.i O.sub.3 series dielectric body is ceramics and therefore, it can not be folded.
A capacity C of a capacitor with a pair of parallel electrodes is given by the equation below. ##EQU1## where C is capacity (Farad), .epsilon..sub.0 is the dielectric constant of the vacuum, .epsilon..sub.s is the specific inductive capacity of the dielectric body (ratio of the dielectric constant to .epsilon..sub.0), S is the area (m.sup.2) of the electrodes, t is the thickness (m) of the dielectric body. Accordingly, in order to obtain large capacity, the
REFERENCES:
patent: 2899611 (1959-08-01), Bradley et al.
patent: 4226735 (1980-10-01), Sakabe
patent: 4241378 (1980-12-01), Dorrian
patent: 4403236 (1983-09-01), Mandai et al.
Japanese Laid-open Publication No. 56621/80 cited with mark Y in the International Search Report.
Abe Hisao
Iwaya Shoichi
Masujima Sho
Masumura Hitoshi
Sakamoto Norimasa
Griffin Donald A.
TDK Electronics Co. Ltd.
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