Analog electricity - what is the resistance?

Reprinted from breadboard community ： resistance , This is the whole story ！- Bread board community (eet-china.com)

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Preface

What is resistance ？ Simply speaking , Resistance refers to the resistance encountered by current in the circuit , Or it refers to the ability of the object to block the current . The more resistance , The more resistance the current encounters , So the current is smaller . conversely , The lower the resistance , The less resistance the current encounters , So the greater the current .

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One 、 Introduction to resistance ？

The symbol of resistance is “R”. The unit of resistance is Ohm , It's short for Europe , Use letters “Q” Express .

There is resistance in any object , Conductors are no exception . You may have such an experience , The rice cooker is cooking , The wire will get a little hot , The reason is that the copper used to make wires has resistance , Although the resistance is very small , However, local electric energy will still be consumed under the condition of high current when cooking , Distribute it hot , As shown in the figure .

Is the resistance perfect ？ Of course not. . Because of the existence of resistance , Only then can we control the current . In order to make the current work according to people's wishes , People created resistors . 

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Two 、 resistor

Resistors are elements that limit current , Commonly referred to as resistance , It is one of the most fundamental and commonly used electronic components , It is widely used in all kinds of electronic circuits . Common resistors are shown in the figure below .

Due to different manufacturing materials and structures , There are many kinds of resistors , Common are carbon film resistors 、 Metal film resistor 、 Organic solid core resistor 、 Wire wound resistor 、 Rated tap resistor 、 Variable resistor 、 Sliding wire rheostat and chip resistor .

Carbon film or metal film resistors are commonly used in electronic manufacturing . Carbon film resistor has high stability 、 Good high frequency characteristics 、 The negative temperature coefficient is small 、 Stable pulse load and low cost , It's widely used . The metal film resistor has high stability 、 The temperature coefficient is small 、 Good heat resistance 、 The noise is very low 、 Wide working frequency range and small volume , Application is also very common .

 

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3、 ... and 、 How to identify resistors

The character symbol of the resistor is “R”, The graphic symbols are shown in the figure below . 

The model name of the resistor is 4 Local composition , As shown in the figure .

The first part uses letters "R" Represents the main name of the resistor , In the second part, letters are used to indicate the information that constitutes the resistor , The third part uses numbers or letters to indicate the classification of resistors , The fourth part shows the serial number with numbers . The meaning of resistor model is shown in the following table .

  such as , The model of a resistor is RT11, Indicates that this is an ordinary carbon film resistor . The model of a resistor is RJ71, It means that this is a fine metal film resistor .

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Four 、 What are the characteristics of resistors ？

The characteristic of the resistor is that it favors one over the other for DC and AC , Any current passing through the resistor will encounter certain obstacles and restrictions , And the current must produce a voltage drop on the resistor , As shown in the figure .

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5、 ... and 、 How to understand the parameters of resistors ？

The main parameters of the resistor are Resistance value and Rated power .

(1) Resistance value

Resistance value is abbreviated as resistance value , The fundamental unit is Ohm , It's short for Europe (Ω). There are thousands of euros in common use (KΩ) And megohm (MΩ). The conversion between them is 1MΩ＝loookΩ,1kQ-1000Ω.    There are two ways to mark the resistance value on the resistor .

One is The direct labeling method , Will call pn The value is printed directly on the resistor . for example , stay 5.1Ω The resistor of is printed with “5.1” or “5R1” word , stay 6.8kΩ The resistors of are “6.8k” or “6k8” word , Pictured 2-5 Shown .

Two is Color ring method , That is, print on the resistor 4 Tao or 5 The color ring is used to represent the resistance value, etc , The unit of resistance is Ω.

About 4 Ring resistor , The first l、2 A ring represents two significant digits , The first 3 A ring represents a multiplier , The first 4 The ring indicates the allowable deviation , Pictured 2-6 Shown .    About 5 Ring resistor , The first l、2、3 A ring represents three significant digits , The first 4 A ring represents a multiplier , The first 5 The ring indicates the allowable deviation , Pictured 2-7 Shown .

The color ring is generally black 、 Brown 、 red 、 orange 、 yellow 、 green 、 blue 、 purple 、 ash 、 white 、 gold 、 silver 12 Color , Their meanings are shown in table 2-2. for example , Of a resistor 4 The color rings are “ yellow 、 purple 、 orange 、 silver ”, Then the resistance value is 47kΩ, Error is ±10%. Of a resistor 5 The color rings are “ red 、 yellow 、 black 、 orange 、 gold ”, Then the resistance value is 240kQ, Error is +5%.

In electronic manufacturing , choose 4 Ring or 5 Ring resistance is acceptable . In the same way of frequency selection 、 Bias circuit and other circuits , The resistor with small error should be selected as far as possible , If necessary, use ohmmeter to detect and select .

(2) Rated power

Rated power is another main parameter of resistor , The power of common resistors is 1/8W、1/4W、）/2W、1W、2W、5W etc. , Its symbols are shown in the figure 2-8 Shown , Greater than 5W Directly indicate with numbers .

The resistor with rated power equal to or greater than ten circuit requirements shall be selected in application . What is not marked in the circuit diagram means that the power consumption of the resistor is very small , Don't think . for example , In most amateur electronic manufacturing, there is no request for resistor power , At this time, you can choose l／8W or 1/4W resistor .