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Points to note on resistor

Frequency Characteristic of Resistor

Resistor have small parasitic inductance(which is regarded to be connected in serial to the resistor in equivalent circuit) and parasitic capacitance(the capacitance between electrodes regarded to be connected in parallel to the resistor in equivalent circuit). Thus, as the frequency increases, parasitic components make the resistors to have reactance components instead of pure resistance. In other words, attention must be paid when using resistors in high frequency region, since its impedance varies with frequency.

The below image shows the frequency characteristics of thick film flat chip resistors in size 0603 (0.06 inch x 0.03 inch). It shows the impedance change of resistor depends on the resistance. When the resistance is low, parasitic inductance becomes dominant in the high frequency range, which increases impedance . On when the resistance is high, the parasitic capacitance becomes dominant in high frequency range, which decreases impedance. When the resistance is extremely low, specifically under milliohm, decrease in parasitic inductance and an increase in resistance of the resistor itself due to the skin effect can occur even at 1 MHz or less.

In addition, leaded type resistors tend to have larger parasitic inductance than chip type resistors. And among the leaded types, wirewound resistors have higher inductance because the resistive wires are wound in the coil form. Use of the smallest possible chip type resistor gives less frequency effect since larger ones tend to have higher stray capacitance and inductance than smaller ones.

 
 

< Equivalent circuit of  resistor >

Electrolytic corrosion

Many people may have experienced water electrolyzation in science experiments during the school days. When a platinum electrode is placed in water with electrolyte and electricity is applied, oxygen is generated from the anode and hydrogen is generated from the cathode. A similar phenomenon occurs inside the resistor.

If a resistor is used with moisture or fluid penetrated inside the coating, the resistive film turns into ion and melt out instead of generating oxygen on the anode side. The resistive film eventually disappears, causing a disconnection. This phenomenon is called electrolytic corrosion because the resistive film seems to be eroded by electricity.

The higher the resistance is, the more electrolytic corrosion is likely to occur. This is because the resistive film with higher resistance has thinner film and narrower pattern, which makes it easy to melt in short time.

Electrolytic corrosion occurs mainly in carbon and metal coatings. To prevent electrolytic corrosion, soldered resistors should be cleaned thoroughly to remove electrolyte components, and resistors should be damp sealed. Possibly, if other required characteristics are to be satisfied, using resistors with metal glaze film, which is hard to ionize, can avoid dealing with the electrolytic corrosion.