Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1998-11-23
2001-06-12
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S551000, C361S056000
Reexamination Certificate
active
06246242
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of screening varistors, particularly those employed as a measure for absorbing surges in electronic circuits and the like.
2. Related Art
There have been proposed various conventional methods for detecting presence/absence of an internal defect in a varistor, which is a voltage-nonlinear resistor (resistor whose resistance varies nonlinearly with applied voltage). In the method disclosed in Japanese Patent Publication (kokoku) No. 6-70658, a pulse is applied to a varistor such that if an internal defect is present, the varistor is destroyed. In the method disclosed in Japanese Patent Publication (kokoku) Nos. 4-66082 and 1-13201, the current-voltage characteristic of a varistor is measured and compared with that of a normal varistor in order to detect presence/absence of an internal defect in the varistor.
The method in which a pulse is applied to a varistor in order to destroy the varistor if an internal defect is present has drawbacks in that if the energy of the pulse is excessively high, the pulse deteriorates non-defective varistors, and if the energy of the pulse is excessively low, detection of defects becomes impossible. Therefore, strict setting and control of pulse energy have been required.
The method in which the current-voltage characteristic of a varistor is measured for the detection of presence/absence of internal defect makes use of a statistical correlation between presence of an internal defect in a varistor and variation in the current-voltage characteristic of the varistor. Thus, in order to reliably reject defective varistors, the conditions for screening non-defective varistors must be made stricter in order to narrow the screening range, leading to the possibility that non-defective products may also be rejected as defectives.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a method of screening varistors in which non-defective varistors are not deteriorated or rejected as defectives and which permits stable detection of presence/absence of internal defects.
In order to achieve the above object, the method of screening varistors according to the present invention is characterized in that dielectric loss of a varistor is measured after a current pulse is applied to the varistor, to thereby screen the varistor according to the measured dielectric loss.
Preferably, the current pulse is a large current pulse, that is, a current pulse having a peak value and pulse width which causes dielectric loss of a varistor to increase if the varistor has an internal defect and to remain substantially unchanged if the varistor does not have an internal defect.
The large-current pulse preferably has a peak value of at least 1 A and 90% or less of an average breakdown current of the varistors and has a pulse width of 100 msec or less. Preferably, the frequency for the measurement of dielectric loss is at least 100 KHz, and the measurement time is 1 sec or less.
When a large-current pulse is applied to a defective varistor having an internal defect, the region having the internal defect loses its nonlinearity and thereby serves as a resistor, which results in an increase in the value of dielectric loss. However, when a large-current pulse is applied to a non-defective varistor having no internal defect, the value of dielectric loss of the non-defective varistor does not increase. Accordingly, when the value of dielectric loss of a varistor measured after application of a large-current pulse does not fall within a predetermined range, the varistor can be judged to be a defective varistor having an internal defect.
According to the present invention, reliable detection of a defective product having an internal defect can be performed through a simple operation of measuring the dielectric loss of a varistor after application of a large-current pulse. Further, non-defective products are neither deteriorated nor rejected as defective products. Moreover, according to the above-described method, varistors which have a deteriorated surge resistance and a deteriorated surge absorbing performance due to existence of internal defects can be screened out within a short period of time. Accordingly, it becomes possible to screen all the varistors, so that the quality of the varistors can be improved. Further, even when varistors to be screened have different surge absorbing characteristics, a value of dielectric loss that serves as a reference value for judgment and that is preset when the varistors are screened is not required to change, if the varistors are formed of the same kind of material. Therefore, the method of the present invention can be applied to a production line for producing a plurality of kinds of varistors.
REFERENCES:
patent: 5142430 (1992-08-01), Anthony
patent: 5225783 (1993-07-01), Suzuki
patent: 5652621 (1997-07-01), Meyer
patent: 1061898 (1959-01-01), None
patent: 60070658 (1992-08-01), None
Kerveros James C
Metjahic Safet
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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