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電阻器的特性應用

Applications of resistors by characteristics

Anti Sulfuration Flat Chip Resistors 

Sulfuration is a phenomenon that occurs in the resistor that applies silver for *inner electrode. Typical product where sulfuration is concerned is thick film chip resistor. When a resistor is used under atmosphere including sulfur, sulfur inserts between protective film and outer electrode, which leads to reaction with silver of inner electrode. This reaction is called sulfuration. Silver sulfide produced through this reaction has no conductivity therefore resistor gets disconnected.

Mechanism of Sulfuration

硫化のメカニズム
硫化銀

Sulfur is included as sulfur gas near hot springs or volcanos but also produced through firing of raw oil. Some rubber products apply sulfur to increase the strengths. When you use the above chip resistors under such atmosphere or near the product, the resistor should be prepared e.g. resin-molded. 
KOA's product lineup includes anti-sulfur type resistors that apply non sulfur type inner electrodes and slow-sulfur type flat chip resistors that sulfur is unlikely to insert.

※ "Inner electrode" is the electrode that is not disclosed to outside of resistor, and is applied in order to connect resistive element and outer electrode(which is for soldering on the pattern) electrically.

Anti-Sulfur Type Resistors

・Thick Film Chip Resistors RK73B/H/Z-RT(General/Precision/JumperRK73G-RT(Ultra precision)

・Anti-Surge/Anti-Pulse Resistors SG73-RTSG73S-RT(Anti-surge)/SG73P-RT(Anti-pulse)

・Wide Terminal Chip Resistors WK73-RT

・Low Resistance Resistors   SR73-RT

・High Voltage Resistors HV73-RT

・High Voltage Resistors(Automotive type)   HV73V-RT

・Chip Network Resistors CN-RT(concave type) / CN-KRT(convex type)

[Reference]

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Anti-pulse/Anti-surge resistors

Resistors that tolerate against pulse or surge are required to be used such as; precharge resistors in the circuit where large current is instantaneously applied or, resistors in the circuit where ESD is more likely to be applied. Pulse means the overload of large power and with long duration(large energy) whereas surge means overload like ESD, of high voltage and with short duration. Anti-pulse resistors are unlikely to be damaged even if large power is instantaneously applied. Among SMD type resistors, thick film chip resistors(metal glaze film type) are generally stronger against pulse than metal film chip resistors. Thick film chip resistors with special construction like SG73 or SG73P are strong against pulse or surge. Metal plate chip resistors for current sensing apply metal plate for resistive body. This achieves the strong resistance against pulse or surge. Leaded type resistors, on the other hand, wirewound type resistors that wounds metal resistive wire around the insulator such as CWCW-HRWBGRBWR, are stronger against pulse than film type metal film resistors or carbon film resistors that have metal resistive film formed on the top of insulator. Solid type ceramics resistors that applies ceramics resistive body, are extremely strong against pulse. Anti-surge resistor means a resistor strong against instantaneous high voltage like ESD. SG73S ensures the ESD limiting voltage.

Pulse Resistance Level by resistor type(image)

耐パルス性の違いのイメージ

High Voltage Type Resistors

One of the role of resistors in a circuit is detecting high voltage by voltage dividing. Rated voltage and maximum working voltage are specified for each resistor, which should not be exceeded when it is used. Plural resistors have to be used in serial to the high voltage side of voltage dividing resistor, which requires more numbers of mounting components and wider mounting area. In this case, HV73 or RCR of high max. working resistors can be used to reduce the numbers of mounting components. Especially for HV73, with excellent voltage constant* compared to general-purpose chip resistors, can divide the comparative high voltage at high precision.

※Generally, resistor does not show a certain resistance against voltage. When voltage is high, the resistance will slightly decrease. Voltage coefficient is the indicator showing the decreasing rate of resistance at high voltage against the resistance at low voltage. Measure the resistance at 10% and at 100% of rated voltage or max voltage per element(max. working voltage), whichever is lower, and express on percentage(%/V) or parts per million(ppm/V).

定格電圧比較と高電圧フィードバック回路

Heat Resistance/Moisture Resistance Thin Film Resistors

Thin film resistors have excellent characteristics such as high precision or low current noise whereas they have problem of being used under high temperature and high humidity. RN73H, with the improved heat resistance and moisture resistance than conventional thin film resistors RN73, can be used under severe environment like automotive purposes.

Heat Resistance

Applying excellent heat resistance material to resistive body, all the type of RN73H series except a certain size, achieve higher rated power than RN73. By extending the rated ambient temperature and max. operating temperature(the upper limit of operating temperature range)on the derating curve, the products can be used with higher applicable power at high temperature.

Calculation example of applicable power at ambient temperature 100°C
RN73 1J   Rated Power: 0.063W×0.455(45.5%)=0.029W
RN73H 1J  Rated Power: 0.1W×0.786(78.6%)=0.079W(2.72 times compared to RN73)

負荷軽減曲線

Moisture Resistance

Electric corrosion may be occured in thin film resistors when they are used under high moisture enviroment. RN73H with special protective coating, improved the moisture resistance compared to conventional products and electric corrosion or disconnection are unlikely to be happened. This product shows the excellent characteristics compared to the conventional type in moisture resistance load life test of severe automotive test conditions. 

Comparison of moisture resistance load life test
  Test Method Limit
RN73 40°C±2°C、90% to 95%RH、
1000hrs(1.5hrs ON/0.5 hrs OFF)
±0.5%+0.05Ω
RN73H 85°C±2°C、85%±5%RH、
1000hrs(1.5hrs ON/0.5 hrs OFF)
±0.1%+0.05Ω

Heat Cycle Characteristics

For automotive applications where high heat cycle characteristics are required, solder cracks may be critical for flat chip resistors especially for large size. Solder crack is a phenomenon that crack is caused in solder when the temperature cycle, the gap of linear expansion coefficient between circuit substrate and chip resistor, is given to the solder junction area and the stress focuses on the area. It will lead to bad solder connection.
When it requires large rated power, large size chip resistor is generally used while the large the product size is, the more frequently the solder cracks are caused. To achieve both high power and the countermeasure to solder cracks, plural resistors of the smaller size and the lower rated power are used but it will require the more number of components and the mounting surface area.
In that case, wide electrode type 
WK73, of high rated power and heat cycle or molded type TSLSLSLN is recommended.
Wide terminal surface area offers excellent heat radiation and achieves rated power. Wide terminal type with reversal height and width against standard type, has short distance between electrodes. and higher heat cycle resistance. The rated power is higher because of the excellent heat radiation and wide electrode surface area.

General-Purpose Type
Wide Terminal Type

Molded type has terminal construction as below which enables to ease the stress of solder junction under heat cycle.

モールド封止品の電極構造
 
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