CA1131794A - Method for treating varistors for dc operation - Google Patents
Method for treating varistors for dc operationInfo
- Publication number
- CA1131794A CA1131794A CA330,883A CA330883A CA1131794A CA 1131794 A CA1131794 A CA 1131794A CA 330883 A CA330883 A CA 330883A CA 1131794 A CA1131794 A CA 1131794A
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- Prior art keywords
- varistor
- varistors
- boron
- temperature
- cooling
- Prior art date
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Abstract
ABSTRACT OF THE DISCLOSURE
Zinc oxide varistor devices are rendered stable for long term DC operation by a post sinter thermal treatment process. Heating the varistor devices in air for a period of time in excess of one hour and within a temperature range of from 500°C to 800°C results in stable DC varistor operation.
Zinc oxide varistor devices are rendered stable for long term DC operation by a post sinter thermal treatment process. Heating the varistor devices in air for a period of time in excess of one hour and within a temperature range of from 500°C to 800°C results in stable DC varistor operation.
Description
~ 794 5D 5564 Zinc oxide varistors of the type consisting of sintered discs made primarily of zinc oxide with small additions of other selected metal oxides are found to be electrically unstable when subjected to long term DC
voltages. That is, under a DC voltage stress the varistor leakage current is found to slowly but continuously increase, such that the varistor would eventually fail by thermal runaway if the voltage were maintained. The desired use of zinc oxide varistors for electrical overvoltage protection of DC operating equipment is infeasible due to the above mentioned instability.
The same type of instability phenomena has been observed upon the continuous application of AC voltage (e.g. 60 Hz) and is more fully described in U.S. Patent 4,046,847 issued September 6, 1977 and Canadian application S.N. 324,504 filed March 30, 1979. Both references teach thermal treatment of zinc oxide varistors for the purpose of rendering the varistors relatively stable under subjection to AC voltages. Additionally, Canadian application S.N. 3 3 ~J~ æ ~ filed _r~/~ ~o,/~77 teaches that certain modifications to the basic formulation - such as a reduction in the amount of boron oxide - can alos be effective in rendering the varistors relatively more stable in AC
voltage applications.
Attempts to manufacture zinc oxide varistors for protection purposes during long term operations on DC voltages systems have heretofore been unsuccessful.
Varistors made, for example, from the material described within the aforemtnion U.S. Patent and Canadian Patent application and containing metal oxides which include 0.1 molar percent concentration of BaO, B2O3 and SiO2 when subjected to DC voltages increased in watts loss at a ~ 179~ 5D 5564 much greater rate than when subjected to AC voltages.
In furtherance of the teachings contained within the above U.S. Patent and Canadian Patent applications, the effect of post sintering thermal treatment and formulation modifications on DC voltage stability were investigated.
It was then discovered that thermal treatment and formulation modifications can also cause zine oxide varistors to be effectively stable under DC voltage. Surprisingly, however, te optimum process for the DC application is different than for the AC voltage application.
The purpose of this invention is to describe methods for treating zinc oxide varistors to cause them to operate stably under long term DC voltage applications.
Zinc oxide varistors containing bismuth, barium and boron oxide additives are heated in air after sintering, to a temperature within a range of from 500C to 800C for a period of time in excess of one hour to induce a change within the varistor which causes them to become relatively stable under a continuous DC voltage stress. The optimum temperature is dependent upon the varistor formulation and is approximately 580C for varistors containing 0.01 to 0.03 molar percent boron oxide and approximately 780C for varistors containing 0.10 molar percent boron oxide.
FIGURE 1 is a graphic represnetation of varistor watts loss as a function of time for various post sintering heat treatment temperatures when the voltage stress is an AC voltage;
FIGURE 2 is a graphic representation of varistor DC leakage current as a function of time for various post sintering heat treatment temperatures when the voltage stress is a DC voltage at a level of 30~ of the varistor 10,000 ampere discharge voltage;
~ 79~ 5D 5564 FIGURE 3 iS a graphic representation of varistor DC leakage current as a function of time for a single heat treatment temperature condition for various levels of boron oxide content in the formulation at the same DC
voltage stress as FIGURE 2; and FIGURE 4 is a graphic representation of varistor DC leakage current as a function of time for several heat treatment temperature conditions with the boron oxide content in the formulation reduced from 0.1 to 0.01 molar percent and a DC voltage at a levle of 40% of the varistor 10,000 ampere discharge voltage.
FIGURE 1 shows the relationship between AC watts loss and time duration for a plurality of varistors subjected to an AC voltage wherein the varistors were ,.
subjected to various post sintering heat treatment conditions.
This figure is identical to that within the aformentioned Canadian patent application. It is readily apparent from FIGURE 1 that post sintering heat treatment is remarkably beneficial for stabilizing the AC watts loss when the post sintering heat treatment tempexature is in the range of from about 580C to about 780C and when the treatment is performed over a period of time in excess of one hour. The optimum heat treatment temperature was determined to be about 580C. Heat treatment for a time of about four hours was found to be quite satisfactory and it was also found that a slight benefit was obtained by thermal cycling.
That is, discs which were thermally cycled between 400C
and 580C for four cycles held for one hour at 580C during each cycle were somewhat more stable that discs which were cycled only once from 4009C to 580C with the temperature held at 580C for four hours.
Attempts to determined a heat treatment schedule 113179~
for varistors of the same chemical composition as disclosed within the aforementioned Canadian Patent application to improve the stability during operation on DC voltage yielded the unexpected result that the optimum temperature for best stability on DC voltage is some 200C higher than for stability on AC voltage. Additionally, it was found that an even greater improvment in stability can be obtained by reducing the boron content in the varistor. However, the optimum heat treatment temperature for the reduced foron varistors is now reduced to about 580C.
Varistors of the type described earlier wherein the metal oxide additives of SiO2, BaO and B2O3 were each in the amount of 0.1 molar percent were post sinter heat treated by raising the varistor temperature to a desired temperature and holding the varistor at that temperature for one hour before decreasing the varistor temperature to approximately 400C or lower. For most cases the above treatment was repeated for a total of four cycles to the desired tempera-ture since about four cycles are usually sufficient to yield satisfactory varistor stability.
In order to determine the varistor DC stability over a period of time, accelerated tests were made at normal operating voltage levels, but at an elevated temperature in order to obtain meaningfuly data in a reasonable period of time. The DC current was recorded as a function of time for varistors treated at various temperatures in accordance with the method of the instant invention. Since the DC
current reading is a fairly accurate representation of the DC varistor watss loss the lower the current level at any time, and/or the lower the rate of increase of current level at any time, the more desirable (i.e. more stable) the disc.
li3~94 SD 5564 Curve A of FIGURE 2, obtained on a disc which had not been post sintered heat treated at all, shows very poor stability in that the current increased at the rate of a factor of ten in about 24 hours. Curve B of FIGURE 2 was obtained with a disc which had been heated treated by heating to 580C for one hour and this shows a slight improvement over the previous curve A. Curve C was obtained on a disc which had been heat treated by thermally cycling between 400C and 580C four times, holding the disc at 580C
for one hour on each cycle. Curve C shows that the cycled heat treatment is considerably more effective in improving the DC stability than was the single one hour heat treatment over the same temperature excursion. Curves D, E, and F of FIGURE 2 were obtained on discs which were heat treated by thermal cycling between 400C and 680C four times, between 400C and 780C four times, and between 400C and 880C four times respectively. It is to be noted that the most stable disc is obtained at a heat treatment temperature of 780C but that at 880C the result is disastrous. Tests made at a 980C heat treatment temperature gave the same disastrous results as for curve F at 880C. Surprisingly, then, for DC stability the optimum heat treatment temperature was 780C (FIGURE 2) whereas for AC stability on discs of the same composition the optimum temperature was 580C as shown in FIGURE 1.
As mentioned earlier, Canadian patent application A S-N- ~ ~ ~2~ / filed f, ~ 7 ~ teaches that a reduction in the varistor boron oxide content is advantageous for enhnaced stability under AC voltage stress. Further 3 investigations to determine whether the same effect would occur for the DC voltage stressed condition revealed the reults shown in FIGURE 3. For this series of tests the 113~7~ 5D 5564 varistor formulation was varied to provide boron contents of 0.10%, 0.03% and 0.01%, and the heat treatment condition was kept constant at four cycles between 400C and 580C.
Curve X of EIGURE 3 is the varistor current for 0.1% boron content and is identical to curve C of FIGURE 2. Curve Y of FIGURE 3 is the varistor current for 0.03% boron and curve Z is the varistor current for varistors containing 0.01% boron. It is readily apparent therefore that reducing the varistor boron content does effect the varistor DC
stability.
In order to determine whether the optimum heat treat temperature for best stability under DC voltage stress is the same for varistors having reduced boron formulation as for the usualy boron concentration of 0.1%, tests were made on varistor discs containing 0.01% boron heat treated at four cycles between 400C to 580C, 400C to 680C, and 400C to 780C. For this series of tests the DC test voltage was increased from the previous 30% to 40% of the discharge voltage of the varistor when discharging a 10,000 ampere impulse which accounts for the higher ~urrent levels indicated in FIGURE 4 as compared to FIGURES 2 and 3. Curve L of FIGURE 4 represents the varistor leakage current for varistors heat treated at four cycles between 400C and 580C, Curve M represents the varistor leakage current for varistors heat treated at four cycles between 400C
and 680C and Curve N represents the varistor leakage current for varistors heat treated at four cycles between 400C and 780C. For varistors containing the reduced boron content of 0.01%, the optimum heat treatment temper-ature for best DC stability was found to 580C instead of 780C as was the case with 0.1% boron in the varistor. The reason for the different boron concentrations is not at 1~3~7~4 5D 5564 this well understood.
What has been discovered therefore is that zinc oxide varistors having the same composition of metal additives when subjected to a DC voltage stress for extended periods of time require a different post sinter thermal treatment temperature for stability than varistors subjected to an AC voltage stress. Furthermore, the optimum heat treatment temperature for stability varies as the compositions of the varistor is varied, a higher heat treatment temperature being required for higher boron containing varistors and a lower heat treatment temperature being required for lower boron containing varistors. The best results for DC stability to date is obtained when the boron content is in the order of 0.03% or less and with the post sinter thermal treatment consisting of about four cycles between 400C and 580C, holding at 580C for about one hour during each cycle.
The optimum boron content for the most stable DC
operation appears to be in the order of 0.01 to 0.03 molar percent. As shown in FIGURE 3, for a thermal treatment temperature of 580C, the 0.03 mole percent B2O3 composi-tion gives a lower initial leakage current than the 0.01 percent B2O3 composition but the rate of increase of leakage current with time was slightly higher for the 0.03 percent B2O3 composition. Thus it is not precisely clear which is the more desirable composition from an overall standpoint and, as a practical matter, either one can be considered quite satisfactory depending upon the specific application.
Similarly, it is not entirely clear that 580C is the lowest desirable thermal treatment temperature. The results shown in FIGURE 4 for varistors having a boron ~ 113~794 5D 5564 content of 0.01 mole percent show that 580C is better than 680C but it is not known at this time exactly what the optimum temperature should be. From a practical point of view a thermal treatment temperature of 580C for a 0.01 molar percent B2O3 composition yields a very satisfactory result.
Although the method of treating zinc oxide varistors for stable DC applications is directed to over-voltage protective devices, this is by way of example only. The methods of this invention find application wherever stable DC varistors may be required.
voltages. That is, under a DC voltage stress the varistor leakage current is found to slowly but continuously increase, such that the varistor would eventually fail by thermal runaway if the voltage were maintained. The desired use of zinc oxide varistors for electrical overvoltage protection of DC operating equipment is infeasible due to the above mentioned instability.
The same type of instability phenomena has been observed upon the continuous application of AC voltage (e.g. 60 Hz) and is more fully described in U.S. Patent 4,046,847 issued September 6, 1977 and Canadian application S.N. 324,504 filed March 30, 1979. Both references teach thermal treatment of zinc oxide varistors for the purpose of rendering the varistors relatively stable under subjection to AC voltages. Additionally, Canadian application S.N. 3 3 ~J~ æ ~ filed _r~/~ ~o,/~77 teaches that certain modifications to the basic formulation - such as a reduction in the amount of boron oxide - can alos be effective in rendering the varistors relatively more stable in AC
voltage applications.
Attempts to manufacture zinc oxide varistors for protection purposes during long term operations on DC voltages systems have heretofore been unsuccessful.
Varistors made, for example, from the material described within the aforemtnion U.S. Patent and Canadian Patent application and containing metal oxides which include 0.1 molar percent concentration of BaO, B2O3 and SiO2 when subjected to DC voltages increased in watts loss at a ~ 179~ 5D 5564 much greater rate than when subjected to AC voltages.
In furtherance of the teachings contained within the above U.S. Patent and Canadian Patent applications, the effect of post sintering thermal treatment and formulation modifications on DC voltage stability were investigated.
It was then discovered that thermal treatment and formulation modifications can also cause zine oxide varistors to be effectively stable under DC voltage. Surprisingly, however, te optimum process for the DC application is different than for the AC voltage application.
The purpose of this invention is to describe methods for treating zinc oxide varistors to cause them to operate stably under long term DC voltage applications.
Zinc oxide varistors containing bismuth, barium and boron oxide additives are heated in air after sintering, to a temperature within a range of from 500C to 800C for a period of time in excess of one hour to induce a change within the varistor which causes them to become relatively stable under a continuous DC voltage stress. The optimum temperature is dependent upon the varistor formulation and is approximately 580C for varistors containing 0.01 to 0.03 molar percent boron oxide and approximately 780C for varistors containing 0.10 molar percent boron oxide.
FIGURE 1 is a graphic represnetation of varistor watts loss as a function of time for various post sintering heat treatment temperatures when the voltage stress is an AC voltage;
FIGURE 2 is a graphic representation of varistor DC leakage current as a function of time for various post sintering heat treatment temperatures when the voltage stress is a DC voltage at a level of 30~ of the varistor 10,000 ampere discharge voltage;
~ 79~ 5D 5564 FIGURE 3 iS a graphic representation of varistor DC leakage current as a function of time for a single heat treatment temperature condition for various levels of boron oxide content in the formulation at the same DC
voltage stress as FIGURE 2; and FIGURE 4 is a graphic representation of varistor DC leakage current as a function of time for several heat treatment temperature conditions with the boron oxide content in the formulation reduced from 0.1 to 0.01 molar percent and a DC voltage at a levle of 40% of the varistor 10,000 ampere discharge voltage.
FIGURE 1 shows the relationship between AC watts loss and time duration for a plurality of varistors subjected to an AC voltage wherein the varistors were ,.
subjected to various post sintering heat treatment conditions.
This figure is identical to that within the aformentioned Canadian patent application. It is readily apparent from FIGURE 1 that post sintering heat treatment is remarkably beneficial for stabilizing the AC watts loss when the post sintering heat treatment tempexature is in the range of from about 580C to about 780C and when the treatment is performed over a period of time in excess of one hour. The optimum heat treatment temperature was determined to be about 580C. Heat treatment for a time of about four hours was found to be quite satisfactory and it was also found that a slight benefit was obtained by thermal cycling.
That is, discs which were thermally cycled between 400C
and 580C for four cycles held for one hour at 580C during each cycle were somewhat more stable that discs which were cycled only once from 4009C to 580C with the temperature held at 580C for four hours.
Attempts to determined a heat treatment schedule 113179~
for varistors of the same chemical composition as disclosed within the aforementioned Canadian Patent application to improve the stability during operation on DC voltage yielded the unexpected result that the optimum temperature for best stability on DC voltage is some 200C higher than for stability on AC voltage. Additionally, it was found that an even greater improvment in stability can be obtained by reducing the boron content in the varistor. However, the optimum heat treatment temperature for the reduced foron varistors is now reduced to about 580C.
Varistors of the type described earlier wherein the metal oxide additives of SiO2, BaO and B2O3 were each in the amount of 0.1 molar percent were post sinter heat treated by raising the varistor temperature to a desired temperature and holding the varistor at that temperature for one hour before decreasing the varistor temperature to approximately 400C or lower. For most cases the above treatment was repeated for a total of four cycles to the desired tempera-ture since about four cycles are usually sufficient to yield satisfactory varistor stability.
In order to determine the varistor DC stability over a period of time, accelerated tests were made at normal operating voltage levels, but at an elevated temperature in order to obtain meaningfuly data in a reasonable period of time. The DC current was recorded as a function of time for varistors treated at various temperatures in accordance with the method of the instant invention. Since the DC
current reading is a fairly accurate representation of the DC varistor watss loss the lower the current level at any time, and/or the lower the rate of increase of current level at any time, the more desirable (i.e. more stable) the disc.
li3~94 SD 5564 Curve A of FIGURE 2, obtained on a disc which had not been post sintered heat treated at all, shows very poor stability in that the current increased at the rate of a factor of ten in about 24 hours. Curve B of FIGURE 2 was obtained with a disc which had been heated treated by heating to 580C for one hour and this shows a slight improvement over the previous curve A. Curve C was obtained on a disc which had been heat treated by thermally cycling between 400C and 580C four times, holding the disc at 580C
for one hour on each cycle. Curve C shows that the cycled heat treatment is considerably more effective in improving the DC stability than was the single one hour heat treatment over the same temperature excursion. Curves D, E, and F of FIGURE 2 were obtained on discs which were heat treated by thermal cycling between 400C and 680C four times, between 400C and 780C four times, and between 400C and 880C four times respectively. It is to be noted that the most stable disc is obtained at a heat treatment temperature of 780C but that at 880C the result is disastrous. Tests made at a 980C heat treatment temperature gave the same disastrous results as for curve F at 880C. Surprisingly, then, for DC stability the optimum heat treatment temperature was 780C (FIGURE 2) whereas for AC stability on discs of the same composition the optimum temperature was 580C as shown in FIGURE 1.
As mentioned earlier, Canadian patent application A S-N- ~ ~ ~2~ / filed f, ~ 7 ~ teaches that a reduction in the varistor boron oxide content is advantageous for enhnaced stability under AC voltage stress. Further 3 investigations to determine whether the same effect would occur for the DC voltage stressed condition revealed the reults shown in FIGURE 3. For this series of tests the 113~7~ 5D 5564 varistor formulation was varied to provide boron contents of 0.10%, 0.03% and 0.01%, and the heat treatment condition was kept constant at four cycles between 400C and 580C.
Curve X of EIGURE 3 is the varistor current for 0.1% boron content and is identical to curve C of FIGURE 2. Curve Y of FIGURE 3 is the varistor current for 0.03% boron and curve Z is the varistor current for varistors containing 0.01% boron. It is readily apparent therefore that reducing the varistor boron content does effect the varistor DC
stability.
In order to determine whether the optimum heat treat temperature for best stability under DC voltage stress is the same for varistors having reduced boron formulation as for the usualy boron concentration of 0.1%, tests were made on varistor discs containing 0.01% boron heat treated at four cycles between 400C to 580C, 400C to 680C, and 400C to 780C. For this series of tests the DC test voltage was increased from the previous 30% to 40% of the discharge voltage of the varistor when discharging a 10,000 ampere impulse which accounts for the higher ~urrent levels indicated in FIGURE 4 as compared to FIGURES 2 and 3. Curve L of FIGURE 4 represents the varistor leakage current for varistors heat treated at four cycles between 400C and 580C, Curve M represents the varistor leakage current for varistors heat treated at four cycles between 400C
and 680C and Curve N represents the varistor leakage current for varistors heat treated at four cycles between 400C and 780C. For varistors containing the reduced boron content of 0.01%, the optimum heat treatment temper-ature for best DC stability was found to 580C instead of 780C as was the case with 0.1% boron in the varistor. The reason for the different boron concentrations is not at 1~3~7~4 5D 5564 this well understood.
What has been discovered therefore is that zinc oxide varistors having the same composition of metal additives when subjected to a DC voltage stress for extended periods of time require a different post sinter thermal treatment temperature for stability than varistors subjected to an AC voltage stress. Furthermore, the optimum heat treatment temperature for stability varies as the compositions of the varistor is varied, a higher heat treatment temperature being required for higher boron containing varistors and a lower heat treatment temperature being required for lower boron containing varistors. The best results for DC stability to date is obtained when the boron content is in the order of 0.03% or less and with the post sinter thermal treatment consisting of about four cycles between 400C and 580C, holding at 580C for about one hour during each cycle.
The optimum boron content for the most stable DC
operation appears to be in the order of 0.01 to 0.03 molar percent. As shown in FIGURE 3, for a thermal treatment temperature of 580C, the 0.03 mole percent B2O3 composi-tion gives a lower initial leakage current than the 0.01 percent B2O3 composition but the rate of increase of leakage current with time was slightly higher for the 0.03 percent B2O3 composition. Thus it is not precisely clear which is the more desirable composition from an overall standpoint and, as a practical matter, either one can be considered quite satisfactory depending upon the specific application.
Similarly, it is not entirely clear that 580C is the lowest desirable thermal treatment temperature. The results shown in FIGURE 4 for varistors having a boron ~ 113~794 5D 5564 content of 0.01 mole percent show that 580C is better than 680C but it is not known at this time exactly what the optimum temperature should be. From a practical point of view a thermal treatment temperature of 580C for a 0.01 molar percent B2O3 composition yields a very satisfactory result.
Although the method of treating zinc oxide varistors for stable DC applications is directed to over-voltage protective devices, this is by way of example only. The methods of this invention find application wherever stable DC varistors may be required.
Claims (11)
1. A method for treating a zinc oxide varistor of the type containing the oxides of barium and boron as additivies to improve DC stability comprising the steps of:
sintering the varistor at a temperature sufficient to form a solid unitary body;
cooling the varistor to a temperature of at least 400°C; and heating the varistor to at least 480°C for a period of at least one hour.
sintering the varistor at a temperature sufficient to form a solid unitary body;
cooling the varistor to a temperature of at least 400°C; and heating the varistor to at least 480°C for a period of at least one hour.
2. The method of claim 1, including the further steps of:
cooling the varistor to a temperature of at least 400°C; and reheating the varistor to a temperature of at least 480°C for a period of at least one hour.
cooling the varistor to a temperature of at least 400°C; and reheating the varistor to a temperature of at least 480°C for a period of at least one hour.
3. The method of claim 1 wherein the varistor is heated to a temperature range of from 480°C to 780°C.
4. The method of claim 1 wherein the varistor comprises equimolar concentrations of the oxides of barium and boron.
5. The method of claim 2 wherein the varistors are cooled and reheated for up to four cycles.
6. The method of claim 1 wherein the barium oxide comprises at least 0.10 mole percent and the boron oxide comprises from 0.01 mole percent to 0.10 mole percent.
7. The method of claim 6 wherein the boron oxide comprises 0.03 mole percent.
8. The method of claim 6 including the steps of cooling the varistors to at least 400°C and reheating the varistors to at least 480°C.
9. The method of claim 8 including the steps of cooling the varistors to at least 400°C and reheating the varistors to at least 480°C for at least one additional cycle.
10. A method for treating zine oxide varistors of the type consisting of equimolar concentrations of barium and boron oxides of 0.10 percent for stable DC
operation comprising the steps of:
heating the varistors after sintering to a temperature to at least 580°C for one hour;
cooling the varistors to at least 400°C;
recooling the varistors to at least 400°C
and reheating the varistors to at least 580°C for at least one additional cycle.
11. The method for manufacturing zine oxide varistors of the type containing the oxides of boron barium for stable DC operation comprising the steps of:
providing at least 0.10 mole percent of the barium oxide and from 0.01 to 0.10 mole percent of the boron oxide;
sintering the varistor at a temperature sufficient to form a solid unitary body;
operation comprising the steps of:
heating the varistors after sintering to a temperature to at least 580°C for one hour;
cooling the varistors to at least 400°C;
recooling the varistors to at least 400°C
and reheating the varistors to at least 580°C for at least one additional cycle.
11. The method for manufacturing zine oxide varistors of the type containing the oxides of boron barium for stable DC operation comprising the steps of:
providing at least 0.10 mole percent of the barium oxide and from 0.01 to 0.10 mole percent of the boron oxide;
sintering the varistor at a temperature sufficient to form a solid unitary body;
Claim 11 continued:
cooling the varistor body to at least 400°C;
and reheating the varistor body to at least 480°C for at least one hour.
cooling the varistor body to at least 400°C;
and reheating the varistor body to at least 480°C for at least one hour.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA330,883A CA1131794A (en) | 1979-06-29 | 1979-06-29 | Method for treating varistors for dc operation |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA330,883A CA1131794A (en) | 1979-06-29 | 1979-06-29 | Method for treating varistors for dc operation |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1131794A true CA1131794A (en) | 1982-09-14 |
Family
ID=4114582
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA330,883A Expired CA1131794A (en) | 1979-06-29 | 1979-06-29 | Method for treating varistors for dc operation |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1131794A (en) |
-
1979
- 1979-06-29 CA CA330,883A patent/CA1131794A/en not_active Expired
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