Definition OF Circuit Board Resistance - Popular Science

This paper aims to briefly analyze the definition of resistance in circuit board. Circuit board is an important part of electronic equipment, where the resistance plays a key role. Resistance is a passive element used to limit the flow of current, its main function is to regulate the current and voltage in the circuit by blocking the flow of electrons. The basic unit of resistance is ohm (Ω), which indicates the amount of current passing through the resistance at a unit voltage.

The Definition OF Resistivity

In the case of a certain temperature, there is the formula R=ρl/s where ρ is the resistivity, l is the length of the material, S is the area. It can be seen that the resistance of the material is proportional to the length of the material and inversely proportional to its cross-sectional area. resistivity is a physical quantity used to express the electrical resistance of various substances. In the case of a certain temperature, there is the formula R=ρL/s where ρ is the resistivity, L is the length of the material, S is the area. It can be seen that the resistance of the material is proportional to the length of the material, that is, when the material and cross-sectional area are unchanged, the longer the length, the greater the resistance of the material: and the inverse ratio with the cross-sectional area of the material, that is, when the material and length are unchanged, the larger the cross-sectional area, the smaller the resistance. From the above equation, we can see the definition of resistivity: ρ=RS/L derivation formula: R=ρV/S2 R=ρL2/V substances with low resistivity are called conductors, common conductors are mainly metals, and the electrical conductivity in nature is silver, followed by semiconductors, silicon germanium. When there is an external electric field, the free electrons of the metal constantly collide with the positive ions on the lattice nodes as thermal oscillators in the movement, so that the movement of the electrons is hindered, and thus it has a certain resistance. Other substances that are not easy to conduct electricity, such as glass, rubber, etc., have high resistivity and are generally called insulators. A substance between a conductor and an insulator, such as silicon, is called a semiconductor. The scientific symbol for resistivity is ρ(Rho). The resistance of the known object can be calculated by the resistivity ρ, length l and section area A: ρ=RA/I, in this formula, the resistance R unit is ohms, the length l unit is meters, the section area A unit is square meters, the resistivity ρ unit is ohms · meters resistivity calculation formula is: ρ=RS/Lρ is the resistivity -- the common unit Ω·mS is the cross-sectional area -- the common unit ㎡R is the resistance value -- the common unit ΩL is the length of the wire -- the common unit m is another calculation formula for the resistivity: ρ=E/Jρ is the resistivity -- common unit Ω·mm2/m[1] E is the electric field intensity -- common unit N/CJ is the current density -- common unit A/㎡ (E, J can be a vector)

The Relationship Between Resistivity And The Medium

1. The resistivity ρ is not only related to the material of the conductor, but also to the temperature of the conductor. In the range of little temperature change: the resistivity of almost all metals changes linearly with temperature, that is, ρ=ρo(1+at). Where t is the temperature in Celsius, ρo is the resistivity at O℃, and a is the temperature coefficient of resistivity.

2. Since the resistivity changes with temperature, for the resistance of some electrical appliances, the physical state in which they are located must be explained. For example, the resistance of a 220 V-100 W lamp filament is 484 ohms when energized, and only about 40 ohms when not energized.

3. Resistivity and resistance are two different concepts. Resistivity is a property that reflects the obstruction effect of a substance on the current, and resistance is a property that reflects the ability of an object to hinder the current.

4. The DC resistivity of superconductors suddenly disappears at a certain low temperature, which is called zero resistance effect. The conductor has no resistance.

Resistivity Between Different Materials

(1) Silver 1.65 ×10-8

(2) Copper 1.75 ×10-8

(3) Gold 2.40×10-8

(4) Aluminum 2.83 ×10-8

(5) Tungsten 5.48 ×10-8

(6) Iron 9.78 ×10-8

(7) Platinum 2.22 ×10-7

(8) Manganese copper 4.4 ×10-7

(9) Mercury 9.6 ×10-7

(10) Constantan 5.0 ×10-7

(11) Nichrome alloy 1.0 ×10-6

(12) Fe-Cr aluminum alloy 1.4 ×10-6

(13) Aluminum nickel-iron alloy 1.6 ×10-6

Classification OF Common Resistance Gases

By material:

1, wire wound resistor is made of resistance wire wound into resistor made of high resistance alloy wire wound on the insulation skeleton, coated with heat-resistant glaze insulation layer or insulating paint. Winding resistance has low temperature coefficient, high precision resistance, good stability, heat resistance and corrosion resistance, mainly used for precision high-power resistance, the disadvantage is poor high-frequency performance and large time constant.

2. Carbon synthesis resistor is made of carbon and synthetic plastic.

3, the carbon film resistor is plated with a layer of carbon on the porcelain tube, and the crystalline carbon is deposited on the ceramic rod skeleton. Carbon film resistor is the most widely used resistor with low cost, stable performance, wide range of resistance value, low temperature coefficient and voltage coefficient.

4, the metal film resistor is plated with a layer of metal on the porcelain tube, and the alloy material is steamed on the surface of the ceramic rod skeleton by vacuum evaporation. The metal film resistor has higher precision, better stability, less noise and temperature coefficient than the carbon film resistor. It is widely used in instruments and communication equipment.

5, the metal oxide film resistor is plated with a layer of tin oxide on the porcelain tube, and a layer of metal oxide is deposited on the insulation rod. Because it is itself an oxide, it is stable at high temperature, heat-resistant shock, strong load capacity according to the purpose, there are general, precision, high frequency, high voltage, high resistance, high power and resistance network.