Patent Application
20090237088
The present invention relates to a method for inspecting an insulation property of a capacitor.
Recently, electric devices (such as capacitors) of an extremely small size are requested in industry so that the electric devices can be mounted onto a circuit board with a high density. However, there is a problem that a capacitor of an extremely small size sometimes shows poor insulation property. Therefore, it is common that all of capacitors manufactured in industry are inspected in advance of their practical uses.
Until now, there are known two methods, namely the below-described methods (A) and (B), for the inspection of an insulation property of capacitors.
A direct current voltage V0 is applied to a capacitor to be inspected, in which the direct current voltage V0 is predetermined for inspection of an insulation property of the capacitor, to generate an electric current in the capacitor. When an unvaried electric current is observed in the current flowing in the capacitor at a lapse of a time t0, the value of i0 of the unvaried electric current is determined. The value (leakage current value) i0 is then compared with a predetermined upper limit i0H of the leakage current, to evaluate the insulation property of the capacitor. If the value i0 exceeds the upper limit i0H, the capacitor having been inspected is rejected.
It is known, however that there is a problem in the above-mentioned inspection method in that the time t0 when the unvaried electric current in the capacitor occurs is generally from approx. one minute to 5 minutes after the application of the direct current voltage. Accordingly, although the above-mentioned method can evaluate the insulation property of a capacitor with high accuracy, the inspection necessarily requires a long period of time if a great number of capacitors should be inspected.
In the above-mentioned method, the above-mentioned voltage V0 is set at a value of 0.1 to 5 times, preferably 0.1 to 1 time, as much as the rated voltage of the capacitor, and the time t0 is set from one minute to 5 minutes, though the V0 and t0 may be determined depending on the specification of the capacitor to be inspected.
The upper limit i0H of the leakage current is generally determined by the following method. A direct current voltage V0 is applied to a referential capacitor of the same specification as the capacitor to be inspected, and a value of an electric current flowing in the capacitor at a lapse of time t0 after the application of the direct current voltage is measured. The upper limit i0H is set at a value from 1.1 to 2.0 times, preferably 1.1 to 1.5 times, as much as the measured value. If required, the above-mentioned determination method is repeated for a plurality of capacitors (e.g., 5 to 100 capacitors) of the same specification as the capacitor to be inspected, and the measured values are averaged to determine the upper limit i0H.
It is generally accepted that the leakage current of capacitors should be as little as possible. However, it is sometimes desired that a group of capacitors have a leakage characteristic equivalent to each other. In this case, the obtained current value (leakage current value) i0 is generally compared with a predetermined lower limit (i0L) of the leakage current. The comparison of i0 with i0L is also effective to check whether or not the ampere meter works correctly.
The lower limit i0L of the leakage current is generally determined by the following method. A direct current voltage V0 is applied to a referential capacitor of the same specification as the capacitor to be inspected, and a value of an electric current flowing in the capacitor at a lapse of time t0 after the application of the direct current voltage is measured. The lower limit i0L is set at a value from 0.1 to 0.9 time, preferably 0.5 to 0.9 time, as much as the measured value. If required, the above-mentioned determination method is repeated for a plurality of capacitors (e.g., 5 to 100 capacitors) of the same specification as the capacitor to be inspected, and the measured values are averaged to determine the lower limit i0L.
A direct current voltage V0 is applied to a capacitor to be inspected, in which the direct current voltage V0 is predetermined for inspection of an insulation property of the capacitor, to generate an electric current in the capacitor. At a lapse of time ts which is shorter than the aforementioned t0, a value is of a current flowing in the capacitor is determined. The value is is then compared with a predetermined upper limit isH of the leak age current, to evaluate the insulation property of the capacitor. If the value is exceeds the upper limit isH, the capacitor having been inspected is rejected. This judgement is made based on such assumption that an inspected capacitor showing a value is exceeding the upper limit isH at a lapse of time ts will show io exceeding the upper limit i0H when the time t0 lapses.
The above-mentioned method (B) is advantageous in that the inspection of an insulation property of capacitors can be completed within a period of time shorter than the period of time required for performing the aforementioned method (A). However, since the current value is is determined prior to the time t0 when the electric current flowing in the capacitor stabilizes, that is, the determination is performed under such conditions that the value of the electric current flowing in the capacitor still varies or decreases as a lapse of time, it is sometimes difficult to obtain a correct current value is if the time for determining the current value is is not set very precisely. Moreover, if the time for determining the current value is is set to be shorter so as to shorten the period required for the inspection of a capacitor, a value of the electric current flowing in the capacitor varies greatly at a lapse of time and the determined value may fluctuate in an increased wide range. This means that the accuracy of inspection lowers.
In the method (B), the above-mentioned time ts is generally set to a time within 0.01 to 90% per the time t0, depending on the specification of the capacitor to be inspected. However, if the inspection with high accuracy is desired, the time ts should not be so short. The current voltage V0 to be applied to a capacitor to be inspected is predetermined in the same manner as in the aforementioned method (A).
The upper limit isH of the leakage current is generally determined by the following method. A direct current voltage V0 is applied to a referential capacitor of the same specification as the capacitor to be inspected, and a value of an electric current flowing in the capacitor at a lapse of time ts after the application of the direct current voltage is measured. The upper limit isH is set at a value from 1.1 to 2.0 times, preferably 1.1 to 1.5 times, as much as the measured value. If required, the above-mentioned determination method is repeated for a plurality of capacitors (e.g., 5 to 100 capacitors) of the same specification as the capacitor to be inspected, and the measured values are averaged to determine the upper limit isH.
If the determined current value is should be compared with a predetermined lower limit isL as is in the method (A), the lower limit isL of the leakage current is generally determined by the following method. A direct current voltage V0 is applied to a referential capacitor of the same specification as the capacitor to be inspected, and a value of an electric current flowing in the capacitor at a lapse of time ts after the application of the direct current voltage is measured. The lower limit isL is set at a value from 0.1 to 0.9 time, preferably 0.5 to 0.9 time, as much as the measured value. If required, the above-mentioned determination method is repeated for a plurality of capacitors (e.g., 5 to 100 capacitors) of the same specification as the capacitor to be inspected, and the measured values are averaged to determine the lower limit isL.
Japanese Patent Provisional Publication 11-211777 (JP 11-211777A) discloses a method for inspecting an insulation property (method for judging whether capacitors are acceptable or failed) which is based on a gradient of a waveform of an electric current flowing in a capacitor (charging current characteristics) which is obtained prior to t0 at which the stable current is observed.
In the above-mentioned method, the insulation property of the capacitor is inspected, for instance, by obtaining a gradient of the waveform of the current at two points of time and examining whether or not the difference between the two gradients is a value between the predetermined upper limit and lower limit.
The patent publication indicates that the disclosed inspection method enables to inspect the insulation property of capacitors with high accuracy in a shortened period of time.
However, there probably is a problem in that the accuracy of the inspection likely lowers due to possible errors occurring in the determination of the gradients of the current waveform unless the time for measuring the gradients is precisely set, because the gradients are measured prior to the time t0 when the electric current flowing in the capacitor stabilizes. The accuracy further lowers if the period of time for measuring the gradients of the current waveform is shortened for shortening the period required for the inspection. This is because the electric current greatly varies and hence the gradients measured under the condition greatly varies.
In addition, the disclosed method requires complicated mathematic processing such as differential processing for obtaining the gradient of the current waveform and subtraction between the measured gradients.
It is an object of the invention to provide a simple method for inspecting an insulation property of capacitors with high accuracy in a shortened period of time.
The present inventors have discovered that an electric current (charging current) flowing in a capacitor by application of a direct current voltage becomes a continuously unvaried electric current when a discharge current produced in the capacitor by decreasing the direct current voltage is decreased and that the inspection of a capacitor for determination of its insulation property can be made with high accuracy in a shortened period of time.
Accordingly, the present invention resides in a method for inspecting insulation property of a capacitor, which comprises the steps of:
applying a direct current voltage V1 higher than a voltage V0 to a capacitor to be inspected, said voltage V0 being predetermined for inspection of insulation property of the capacitor, whereby generating an electric current in the capacitor;
lowering the direct current voltage V1 to the voltage V0 at a lapse of a time t1, whereby converting the electric current in the capacitor into a continuously unvaried electric current;
determining a value i0 of the continuously unvaried electric current; and
comparing the value i0 with an upper limit of a leakage current which is predetermined for the capacitor to be inspected,
provided that if the continuously unvaried electric current is observed twice or more after the lowering of the direct current voltage, a value is of the first continuously unvaried electric current is compared with an upper limit of a leakage current-relating value which is predetermined for the capacitor to be inspected.
The inspection method of the invention may further comprises the step of comparing the value i0 with a lower limit of a leakage current which is predetermined for the capacitor to be inspected,
provided that if the continuously unvaried electric current is observed twice or more after the lowering of the direct current voltage, a value is of the first continuously unvaried electric current is compared with a lower limit of a leakage current-relating value which is predetermined for the capacitor to be inspected.
In the inspection method of the invention, the direct current voltage V1 and the time t1 may be determined by a method comprising the steps of:
applying an optionally determined direct current voltage V2 higher than the voltage V0 to a referential capacitor, whereby generating an electric current in the referential capacitor;
lowering the direct current voltage V2 to the voltage V0 at a lapse of an optionally determined time t2, and detecting a pattern of an electric current flowing in the referential capacitor;
repeating a combination of the above-mentioned steps to find a combination of a direct current Voltage V2 and a time t2 to give a pattern having a continuously unvaried electric current; and
allotting the found combination of a direct current voltage V2 and a time t2 to a combination of the direct current voltage V1 and the time t1.
In the inspection method of the invention, the comparison between the determined current value i0 (or is) and the predetermined upper limit i0H (or isH) can be performed using relating values, for instance, by comparing a resistance value converted from the current value i0 (or is) and a resistance value converted from the predetermined upper limit i0H (or isH) using the value of the direct current voltage applied to the capacitor.
In the same way, the comparison between the determined current value i0 (or is) and the predetermined lower limit i0L (or isL) can be performed using relating values, for instance, by comparing a resistance value converted from the current value i0 (or is) and a resistance value converted from the predetermined lower limit i0L (or tsL) using the value of the direct current voltage applied to the capacitor.
Therefore, the present invention provides a simple method for inspecting an insulation property of capacitors in a shortened period of time, which can be performed by applying a direct current voltage to a capacitor to be inspected; subsequently decreasing the direct current voltage; and determining the value of a continuously unvaried current flowing in the capacitor which is produced by the decrease of the direct current voltage.
Further, the inspection method of the invention enables to inspect an insulation property of the capacitor with high accuracy, because the value of an electric current is determined on the continuously unvaried current and the value of the continuously unvaried current does not greatly vary when the time of the determination of the current value slightly deviates.
The inspection method of the invention is further described referring to the figures illustrated in the attached drawings.
As shown in
By the comparison, if the capacitor shows a current value i0 exceeding the upper limit i0H of the leakage current, the capacitor is rejected.
In other words, according to the inspection method of the invention, a charged electric current (which is varied as a lapse of time) flowing in the capacitor which is produced by the application of a direct current V1 to a capacitor to be inspected is converted into a continuously unvaried current by adding a discharge current produced by lowering (or decreasing) the direct current voltage from V1 to V0, and the continuously unvaried current value i0 is utilized for the inspection of an insulation property of the capacitor.
Therefore, according to the inspection method of the invention, the insulation property of a capacitor can be inspected in a shortened period of time (which is shorter than the aforementioned time t0) by simple procedures comprising the steps of applying a direct current voltage V1 to a capacitor to be inspected; lowering the direct current voltage V1 to the voltage V0 at a lapse of time t1 to convert the electric current flowing in the capacitor into a continuously unvaried current; and determining the value i0 of the continuously unvaried current.
For the reasons described above, according to the inspection method, a reliably stable value i0 of the continuously unvaried current can be detected even when the time for the detection of the current value slightly deviates. Therefore, the inspection method of the invention enables to inspect an insulation property of a capacitor with high accuracy. The accuracy of the inspection can be further increased if the value of the continuously unvaried current is measured plural times and the measured values are averaged to obtain the current value i0.
The inspection method of the invention preferably comprises a step of comparing the value i0 of the continuously unvaried current with a lower limit i0L of a leakage current which is predetermined for the capacitor to be inspected. As is described hereinbefore, it is generally accepted that the determined leakage current value i0 of a capacitor should be as little as possible. However, it is sometimes desired that a group of capacitors have a leakage characteristic equivalent to each other. In this case, the obtained current value (leakage current value) i0 can be compared with a predetermined lower limit (i0L) of leakage current. The comparison of i0 with i0L is also effective to check whether or not the ampere meter works correctly.
In the inspection method of the invention, the value V0 of the direct current to be applied to the capacitor can be the same as the predetermined voltage V0 which is adopted in the aforementioned conventional method (A). The upper limit i0H and lower limit i0L of the leakage current to be compared with the value of the continuously unvaried current can be the same as the predetermined upper limit i0H and lower limit i0L which are adopted in the conventional method (A).
In other words, the inspection method of the present invention can be performed employing the direct current voltage V0, upper limit i0H and lower limit i0L which are adopted in the inspection procedures according to the conventional method (A). If the conventional inspection method (A) has been not performed on the same type of the capacitor to be inspected, the direct current voltage V0, upper limit i0H and lower limit i0L can be newly determined in the conventional method (A).
The voltage V1 is set at a value generally within 1.001 to 2 times, preferably 1.001 to 1.5 times, as much as the voltage V0.
The time t1 is generally set at a time shorter than the time t0 by 4 milliseconds or more. In other words, the time t1 can be set at a time generally within 3 milliseconds to 10 seconds, preferably within 3 milliseconds to 1 second, more preferably within 3 to 500 milliseconds. The inspection of an insulation property can be performed within a shorter period of time than the period of time required for the inspection according to the method (A), if the time t1 is set at a time shorter than the time t0 by 4 milliseconds or more.
As described hereinbefore, the inspection method of the invention comprises a procedure of lowering the value of the direct current voltage applied to the capacitor from V1 to V0. In this procedure, the lowered voltage V0 can be a voltage within V0±0.3×(V1−V0), preferably within V0±0.2×(V1−V0).
The value V1 of the direct current voltage shown in
In
The value V1 of the direct current voltage shown in
In
As is understood from
In the above-described inspection procedures, the upper limit isH and lower limit isL to be compared with the value is of the continuously unvaried current can be predetermined in the same manner as described in the aforementioned method (B). Accordingly, the inspection method of the present invention can be performed employing the direct current voltage V0, upper limit isH and lower limit isL which are adopted in the inspection procedures according to the conventional method (B). If the conventional inspection method (B) has been not performed on the same type of the capacitor to be inspected, the direct current voltage V0, upper limit isH and lower limit isL can be newly determined in the conventional method (B).
In the inspection method of the invention described above, the voltage V1 and time t1 are preferably determined by the procedure comprising the below-mentioned steps 1) and 2).
Step 1) for applying an optionally determined direct current voltage V2 higher than the voltage V0 to a referential capacitor, whereby generating an electric current in the referential capacitor, and then lowering the direct current voltage V2 to the voltage V0 at a lapse of an optionally determined time t2, and detecting a pattern of an electric current flowing in the referential capacitor.
The referential capacitor is an optionally selected capacitor from a group of practically acceptable capacitors prepared under the same specification.
The voltage V2 is set at a value generally within 1.001 to 2 times, preferably 1.001 to 1.5 times, as much as the voltage V0.
The time t2 is generally set at a time shorter than the time t0 by 4 milliseconds or more. In other words, the time t2 can be set at a time generally within 3 milliseconds to 10 seconds, preferably within 3 milliseconds to 1 second, more preferably within 3 to 500 milliseconds.
Step 2) for repeating a combination of the above-mentioned steps to find a combination of a direct current voltage V2 and a time t2 to give a pattern having a continuously unvaried electric current, and allotting the found combination of a direct current voltage V2 and a time t2 to a combination of the direct current voltage V1 and the time t1.
In the step 2), the “a continuously unvaried electric current” means that the current value is continuously within 0.75 to 1.25 times, preferably 0.80 to 1.20 times, more preferably 0.90 to 1.10 times, as much as a current value observed at the time of start of the unvaried electric current, within a period of time of 4 milliseconds or longer.
The appropriate combination of the voltage V2 and time t2 can be found out by repeating the above-mentioned step 1) varying the voltage V2 but keeping the time t2, or varying the time t2 but keeping the voltage V2, or varying both of voltage V2 and time t2.
For instance, the pattern of a continuously unvaried current flowing in the capacitor can be detected by setting the voltage V2 at a voltage higher than the voltage V0 by a small voltage value (such as a value of 0.01 to 0.5 V) and observing a pattern of an electric current produced by lowering the voltage V2 to the voltage V0. If a pattern of thus produced electric current is not a pattern of the continuously unvaried current, a slightly high voltage V2 is set and the voltage V2 is lowered to observe a pattern of an electric current. These procedures are repeated until a pattern of the continuously unvaried electric current is observed at a lapse of time t2. Thus, the appropriate combination of the voltage V2 and time t2 is detected
If the appropriate combination of the voltage V1 and time t1 is once detected, there is no need of repeatedly detecting an appropriate combination of the voltage V1 and time t1 to perform the inspection of an insulation property of capacitors having the same specification as that of the reference capacitor.
The present invention is further described by the following non-limiting examples.
An commercially available ceramic capacitor (or condenser) was inspected with reference to its insulation property by the below-mentioned procedures according to the conventional inspection method (B) under the following conditions:
Direct current voltage (V0) predetermined for inspection of an insulation property of the ceramic capacitor: 30 V
Time (ts) predetermined for determination of a value (is) of an electric current flowing in the capacitor: 50 milliseconds
Upper limit (isH) to be compared with the current value (is): 25 μA
Lower limit (isL) to be compared with the current value (is): 15 μA.
The direct current voltage (30 V) was applied to the ceramic capacitor. At a lapse of 50 milliseconds, an electric current flowing in the capacitor was detected. The detected current value was 19 μA, which was between the upper limit 25 μA and the lower limit 15 μA. Therefore, it was confirmed that the inspection of the ceramic capacitor was successfully performed for 50 milliseconds.
A ceramic capacitor prepared under the same specification as that of the above-mentioned commercially available ceramic capacitor was obtained.
First a direct current voltage of 30.1 V was applied to the capacitor. At a lapse of 20 milliseconds (which was tentatively predetermined), the voltage was lowered to 30 V, and the electric current flowing in the capacitor after the lowering of the voltage was observed to examine if a continuously unvaried current was produced. Thereafter, a direct current voltage of 30.2 V was applied to the capacitor. At a lapse of 20 milliseconds, the same observation was performed. The same procedures except that the applied direct current voltage was increased by 0.1 V were repeated until the production of a continuously unvaried current was confirmed. As a result, it was confirmed that the continuously unvaried current of approx. 19 μA was produced during 30 to 50 milliseconds when a direct current voltage of 30.5 V was applied and the voltage was lowered to 30 V at a lapse of the predetermined 20 milliseconds.
Thereafter, 100 ceramic capacitors prepared under the same specification as that of the above-mentioned commercially available ceramic capacitor were collected.
To each of the ceramic capacitors was applied a direct current voltage of 30.5 V (V1). At a lapse of 20 milliseconds (t1), the voltage was lowered to 30 V, and the electric current flowing in the capacitor at a time (t3) of 10 milliseconds after the lowering of the voltage was observed to confirm that a continuously unvaried current was produced. It was confirmed that all of the inspected ceramic capacitors gave values of the continuously unvaried current between 16 μA and 22 μA at the time t3.
Therefore, it was confirmed the inspection of the ceramic capacitor was complete for a period of 30 milliseconds, which is 60% of the period of time (50 milliseconds) required for performing the conventional inspection method (B) described in the above-mentioned Comparison Example.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-071595 | Mar 2008 | JP | national |
