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MODUS electrical component is a resistive device with nonlinear voltage-current characteristics, mainly used for voltage clamping when the circuit is subjected to overvoltage, absorbing excess current to protect sensitive devices. The resistive body material of the varistor is a semiconductor, so it is a type of semiconductor resistor. The currently widely used "zinc oxide" varistor has a main body composed of divalent element zinc and hexavalent element oxygen. Therefore, from a material perspective, the zinc oxide varistor is a "II-VI group oxide semiconductor". In Taiwan, China, varistors are called "surge absorbers", and are sometimes also referred to as "electric shock (surge) suppressors (absorbers)". A varistor is a voltage-limiting protection device. Utilizing the nonlinear characteristics of the varistor, when an overvoltage appears across the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed value, thereby achieving protection for the downstream circuit. The main parameters of a varistor include: varistor voltage, surge current capacity, junction capacitance, response time, etc. The response time of a varistor is at the ns level, faster than a gas discharge tube, but slightly slower than a TVS tube. Generally, its response speed can meet the requirements for overvoltage protection in electronic circuits. The junction capacitance of a varistor is generally in the range of hundreds to thousands of Pf, and in many cases it is not suitable for direct application in the protection of high-frequency signal lines. When applied in AC circuit protection, its larger junction capacitance will increase leakage current, which needs to be fully considered in the design of protection circuits. The surge current capacity of a varistor is large, but smaller than that of a gas discharge tube. The varistor is abbreviated as VDR, and is a voltage-sensitive nonlinear overvoltage protection semiconductor component.

MODUS electrical component classification

For most conductors, at a certain temperature, their resistance remains almost constant at a fixed value; this type of resistance is called linear resistance. The resistance of some materials changes significantly with current (or voltage), and their voltage-current characteristic is a curve; this type of resistance is called nonlinear resistance. For a nonlinear resistor under a given voltage (or current), the ratio of voltage to current is the static resistance at that operating point, and the slope on the voltage-current characteristic curve is the dynamic resistance. The methods for expressing nonlinear resistance characteristics are complex, but these nonlinear relationships are widely applied in electronic circuits.

MODUS electrical component principle

1. Emitter emits electrons to the base region: The power supply Ub is applied to the emitter junction through the resistor Rb, forward-biasing the emitter junction. The majority carriers (free electrons) in the emitter region continuously cross the emitter junction into the base region, forming the emitter current Ie. At the same time, the majority carriers in the base region also diffuse towards the emitter region, but since the concentration of majority carriers is much lower than that in the emitter region, this current can be ignored. Therefore, it can be considered that the emitter junction is primarily an electron flow. 2. Diffusion and recombination of electrons in the base region: After entering the base region, electrons first accumulate near the emitter junction, gradually forming an electron concentration gradient. Under the action of this concentration gradient, the electron flow diffuses in the base region towards the collector junction and is pulled into the collector region by the electric field of the collector junction, forming the collector current Ic. A very small portion of electrons (because the base region is very thin) recombine with holes in the base region. The ratio of the diffused electron flow to the recombined electron flow determines the amplification capability of the transistor. 13. Collector region collects electrons: Due to the large reverse voltage applied externally to the collector junction, the electric field force generated by this reverse voltage prevents electrons in the collector region from diffusing into the base region, while simultaneously pulling electrons that have diffused near the collector junction into the collector region, thereby forming the main collector current Icn. In addition, the minority carriers (holes) in the collector region also undergo drift motion, flowing towards the base region to form a reverse saturation current, represented by Icbo. Its value is very small, but it is extremely sensitive to temperature.

MODUS electrical component characteristics

The main parameters characterizing resistance characteristics include nominal resistance and its allowable tolerance, rated power, load characteristics, and temperature coefficient of resistance. Load characteristics: When the ambient working temperature is below tR, the resistor must not exceed its rated power. When exceeding tR, the load power must be derated. Each type of resistor has specified load characteristics. In addition, the allowable load can be correspondingly reduced under low atmospheric pressure. Under pulse load, the average pulse power is far lower than the rated power, and there are generally separate regulations. Within the specified ambient temperature range, the average relative change in resistance for every 4 °C change in temperature is expressed in ppm/°C. In addition to the above parameters, there are other technical indicators such as nonlinearity (the degree to which the current and applied voltage characteristic deviates from a linear relationship), voltage coefficient (the relative rate of change in resistance per unit change in applied voltage), current noise (the ratio of the effective value of the noise potential generated by current flow within the resistive body to the test voltage, expressed by the current noise index), high-frequency characteristics (the curve showing the decrease in resistance as the operating frequency increases due to the influence of distributed capacitance and inductance within the resistive body), and long-term stability (the process of irreversible change in resistance value due to environmental conditions during long-term use or storage).

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