Characteristics of Resistors
Basic Knowledge of Resistor
Frequency Characteristic of Resistor
Resistors 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 the higher the frequency becomes, the more the parasitic components are affected, which leads the resistors have reactance components. You have to pay attention for using resistors in high frequency range because impedance of resistors are affected depending on frequency. The below image shows the frequency characteristics of thick film flat chip resistors in size 0603(0.6 inch x 0.3 inch). It shows the impedance change of resistor depends on resistance. When the resistance is low, parasitic inductance becomes dominant in the high frequency range, which makes higher impedance . When the resistance is high, the parasitic capacitance becomes dominant in high frequency range, which makes smaller impedance. When the resistance is extremely small e.g. under milliohm, surface effect enables to lead the phenomenon that parasitic inductance drops and resistance of resistive body itself increases.
Temperature Rise and Heating of Resistor
Resistor is an element that converts electric energy into heat energy. It always generates heat when the electric power is consumed and the temperature rises according to the consumed power. To control the temperature rise of a resistor, the generated heat needs to be dissipated efficiently. For chip resistors, the most of the generated heat conducted from the electrode to Cu foil pattern on the P.W.B, and finally dissipated to the air or chassis. As the image below, expanding the pad pattern of the resistor or expanding Cu foil pattern where the resistor is connected, will lead to good heat radiation and control the temperature rise.
Temperature rise of the resistor is also controlled by improving thermal conductivity of PWB; by thick Cu foil pattern, forming solid pattern on the back side of PWB or solid pattern inside the layer if it is a multilayer substrate. Our wide terminal type flat chip resistors WK73 have excellent heat radiation characteristic and perform under high power. Examples of the wide terminal flat chip resistors are shown below. Forming the electrodes on long sides of this type of resistor, shortens the distance between the heat generating point and the electrode, enables to conduct much volumes of heat to PWB by large electrodes, which leads to improve heat radiation of resistor. The rated power is much more improved than that of standard resistor in same size.
Many people may experience water electrolyzation in science emperic during the school days. Insert platinum electrode into water where electrolyte is included and turn electricity so oxgen evolves from the anode and hydrogen from the cathode. Similar phenomenon happens inside of a resistor. If you continue to use resistor of which the coating inside has been infiltrated by air containing moisture or water, resistive body turns into ion and melt out instead of generating oxygen at the anode side. Resistive body disappears finally leading to disconnection. This phenomenon is called electric corrosion because the resistive body seems to be erroded by electricity whereas the disconnection caused by electric corrosion is called disconnection by electric corrosion. The higher the resistance is, the more electric corrosion is likely to happen. This is because the resistive body with high resistance has thin film and narrow pattern, which is easier for the resistive body to be melted in short time. Eletric corrosion occurs mainly for carbon film or metal film type resistors. To avoid this, wash the resistor after soldering to remove the electrolyte as well as seal the resistor excluding moisture. If there should be no characteristic problem, you can replace it by metal glaze film resistors which can hardly turn into ion.
- Image of Electric Corrosion