First, the definition of resistance

When current flows through a certain material, the material current has a certain hindering effect, and this hindering effect is the resistance of the material. It is represented by the symbol R. The unit of resistance is ohm [m], which is represented by the symbol Ω. In addition to ohms, the commonly used units are kΩ (kiloohm) and MΩ (megaohm). The relationship between them is:

Conversion between potentials

Figure (a) shows several common resistor objects, and (b) shows the graphic symbols:

Resistors and Resistor Symbols

The law of resistance

The resistance of the conductor is proportional to the length of the conductor and inversely proportional to the cross-sectional area of ​​the conductor when the material is determined, which can be expressed as:

resistance law

In the formula, ρ is the resistivity of the conductor, in Ω·m; L is the length of the conductor, in m; S is the cross-sectional area of ​​the conductor, in m2.

3. High-frequency equivalent model of resistance

high frequency equivalent model

Effects of Ls:

(1) For the power line coupling high-frequency interference generated by the digital circuit through the digital power supply, the impedance of the parasitic inductance Ls is much larger than 1Ω, which can play the role of isolating the digital interference.

(2) In the switch circuit, when the switch is turned off, the parasitic inductance Ls of the main circuit will generate a high voltage on the switch in order to prevent the current from changing, causing the switch to break down.

Fourth, the use of important resistors explained

1. 0 ohm resistance

Debug and Compatible Design

when jumper

Connect between ground and ground, power supply and IC

2. The function of pull-up and pull-down resistors

Prevent the input from floating

Reduce the interference of external current on the chip

Increase or decrease drive current

The potential of the changed level, commonly used in TTL-CMOS matching

Supply current to OC gate

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