Practical Resistors

Introduction

电阻是电路中最常用的组件。由于它们是如此普遍，因此它们具有多种样式和制造技术。在本节中，我们将不尝试对所有可用电阻进行全面讨论。取而代之的是，我们将为通常用于在面包板上构建原型电路的电阻类型提供一个简单的指南。

Resistors are manufactured in a variety of ways. Most commonly available commercial resistors are carbon composition or wire-wound; however, resistors on integrated circuits are generally made of semiconductor materials.

Resistors can have either a fixed or variable resistance. Identification and use of these two types of resistors is (at least superficially) fairly different, so we will discuss them separately in the subsections below.


Breadboards

Variable Resistors

Variable resistors, sometimes called rheostats or potentiometers, have resistance which may be varied by a mechanical control (usually a knob or a screw) or an electronic one. Potentiometers have three pins while rheostats have two pins. Potentiometers are more common since they can be used as rheostats1). The Digilent analog parts kit contains a variety of potentiometers. A potentiometer from the parts kit is shown in Fig. 1.

Figure 1. Potentiometer from the Digilent Analog Parts Kit.

The pins of a typical potentiometer are shown in Fig. 2. The control sets the resistance between terminals 1 & 2 (R13)andbetween terminals 2 & 3 (R23). In an ideal potentiometer R13 + R23 is a constant. For the potentiometers in the Digilent analog parts kit, the total resistance R13 + R23 is encoded in exponential notation on the top of the potentiometer as three numbers. The first two numbers are the mantissa of the number; the third number is the exponent. For the potentiometer shown in Fig. 1, the encoded number is 502. This corresponds to a resistance of 50×102, or 5000Ω. This means that both R13 and R23 can be adjusted to between 0Ω and 5000 Ω.

Figure 2. Pin descriptions for potentiometer.

Fixed Resistors

固定电阻提供一个指定的电阻值，并具有两个端子。固定电阻器的电路符号如图3所示。数字模拟零件套件提供了两种类型的固定电阻：大电阻器和各种较小的低功率电阻器。两种电阻的示例如图4所示。

Figure 3. Fixed resistor circuit symbol.Figure 4 (a). High-power resistor.Figure 4 (b). High-power resistor.

Resistors that are physically large enough will usually have their resistance value printed directly on them, as in the resistor of Fig. 4(b). Smaller resistors, like those shown in Fig. 4(a), generally use a color code to identify their resistance value. The color code consists of a series of three to six colored bands. The color coding scheme is provided in Fig. 5. All resistors in the Digilent analog parts kit have four bands—for these resistors, the first three bands provide a nominal resistance value2)in exponential notation. Like the potentiometer, these colors correspond to a number in exponential notation; the first two colors correspond to numbers which give the mantissa of the number, and the third band provides the exponent. The fourth color band provides a tolerance between the actual and expected resistance. An example of calculating a nominal resistance from a color code is provided below.

Example

A resistor has the following color bands below. Determine the resistance value and tolerance.

  • First band: Red
  • 第二乐队：黑色
  • 指数：橙色
  • Tolerance: Gold

Resistance = (20+0)x103± 5% = 20kΩ ± 1k Ω

Figure 5. Resistor color codes.

Important Point

电阻可以固定或可变。数字模拟零件套件中最常见的电阻器是小的固定电阻。这些电阻的电阻值在一系列有色带中编码在电阻器上。颜色代码在上面的图5中提供。您应该努力了解如何从电阻器上的颜色代码确定电阻值。

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1) But not the other way around.
2) 名义上的意思是“名义上”。在在gineering-speak, it generally means that the given value should be the actual value, but probably isn't. It is often used to denote a desired or expected value. By the way, engineers often use the term nominal synonymously with normal. Usually, this exchange (though incorrect) makes sense the way engineers use the terms.