万用表规范操作

1. 表笔插孔定义与接线规则

4 插孔万用表包含 C O M \boldsymbol{\mathrm{COM}} COM、 V Ω \boldsymbol{\mathrm{V\Omega}} VΩ、 m A \boldsymbol{\mathrm{mA}} mA、 10 A \boldsymbol{10\mathrm{A}} 10A 四个端子。

C O M \mathrm{COM} COM 孔：为公共端（Common），黑色表笔固定插入此插孔，所有测量工况下，黑表笔位置保持不变。其余插孔由测量类型决定红表笔接入位置：
V Ω \mathrm{V\Omega} VΩ 孔：直流电压、交流电压、电阻、二极管、通断测量；该端子输入阻抗达到数兆欧至十兆欧，电压测量时对被测回路的取流水平很低。
m A \mathrm{mA} mA 孔：小电流测量，适配上限通常为 200 m A 200\ \mathrm{mA} 200 mA，内部串联精密采样电阻与保险丝。
10 A 10\mathrm{A} 10A 孔：大电流测量，内部配置低阻值分流器，不适合长时间通电。

1.1 极性惯例与表型差异

电工通用颜色编码约定：黑色对应参考地或负电位，红色对应正电位。

数字万用表：红表笔对应内部测试电源正极。
指针式万用表：红表笔对应内部测试电源负极，黑表笔对应内部测试电源正极；该特性会改变二极管正向导通测试的表笔电位关系。

C O M \mathrm{COM} COM 端子本身不恒等于负极，电位属性随测量模式变化：

电压测量： C O M \mathrm{COM} COM 接参考电位点，作为负参考端。
电阻测量： C O M \mathrm{COM} COM 构成仪表内部测试电池的返回回路。
电流测量： C O M \mathrm{COM} COM 作为电流进出仪表的公共通路。

1.2 接线错误对应的后果

红表笔接入电流插孔后，直接并联至电压回路，等同于将仪表内部短路，会熔断保险丝、烧毁分流电阻或主电路板。

直流测量极性颠倒时，数字表显示负读数；指针表发生指针反向偏转，多次反向冲击可造成表头机械损伤。

1.3 电流测量附加约束

电流测量必须将仪表串联接入回路； 10 A 10\mathrm{A} 10A 档位单次测量时长控制在 10 ∼ 15 s 10\sim15\ \mathrm{s} 10∼15 s，两次测量间隔不小于 15 m i n 15\ \mathrm{min} 15 min，防止分流器过热、阻值漂移。

2. 量程选择通用规范

待测参数数值未知时，档位旋钮置于最大量程，读取示数后逐级下调至适配档位。

表笔保持带电连接状态时，禁止旋转档位旋钮，触点打火会击穿电路板。

禁止使用低于被测电压的量程开展测量，例如以 200 m V 200\ \mathrm{mV} 200 mV 档位测量 220 V 220\ \mathrm{V} 220 V 市电，会造成仪表击穿，同时存在触电风险。

3. 直流电压测量

表笔配置：红表笔接入 V Ω \mathrm{V\Omega} VΩ 孔，黑表笔接入 C O M \mathrm{COM} COM 孔，表笔并联至被测两点。

档位切换至直流电压档（ D C V \mathrm{DCV} DCV），红表笔接高电位端，黑表笔接低电位端，稳定静置 2 s 2\ \mathrm{s} 2 s 读取示数。

屏幕出现负数值，代表表笔极性颠倒，互换两支表笔即可获得正向读数。

弱电板卡测量（发射机控制板等）选用 20 V 20\ \mathrm{V} 20 V 量程，手部不触碰表笔金属探针，避免静电击穿精密半导体器件。

4. 交流电压测量

表笔配置：红表笔接入 V Ω \mathrm{V\Omega} VΩ 孔，黑表笔接入 C O M \mathrm{COM} COM 孔，表笔并联至被测两点。

档位切换至交流电压档（ A C V \mathrm{ACV} ACV），交流信号无固定极性，表笔交换不改变读数。

测量 220 V 220\ \mathrm{V} 220 V、 380 V 380\ \mathrm{V} 380 V 市电，选用不低于 750 V 750\ \mathrm{V} 750 V 的交流量程。

带电测量执行单手操作，另一只手脱离导电体，禁止双手同时握持两支表笔探针，防止人体形成导电回路。

5. 电阻测量

电阻测量的前置条件：被测回路完全断电，大容量电容充分放电。带电测量会引入外部电压，造成读数失真或烧毁仪表内部分压元件。

表笔配置：红表笔接入 V Ω \mathrm{V\Omega} VΩ 孔，黑表笔接入 C O M \mathrm{COM} COM 孔。

手动量程仪表按预估阻值选择倍率，每更换一次量程，都需要短接两支表笔完成欧姆调零；无法调零代表内部电池电量不足。

电路板上在路测量时，优先断开被测元件其中一端引脚，规避周边并联元件的分流效应；测量时手指不要同时触碰元件两端金属引脚，防止人体电阻并联引入偏差。

6. 电流测量

表笔配置：黑表笔固定接入 C O M \mathrm{COM} COM 孔；预估电流小于 200 m A 200\ \mathrm{mA} 200 mA，红表笔接入 m A \mathrm{mA} mA 孔；预估电流达到安培级，红表笔接入 10 A 10\mathrm{A} 10A 孔。

档位切换至对应直流或交流电流档，仪表串联接入被测回路。

电流测量完成后，优先将表笔从电流插孔取出，恢复至 V Ω \mathrm{V\Omega} VΩ 插孔配置，避免后续误操作。

7. 仪表收纳与长期保养

单次测量结束，档位旋钮旋至交流 750 V 750\ \mathrm{V} 750 V 最高量程，不要长期停留在电阻档或电流档。

闲置时长超过 30 30 30 天，取出内部 9 V 9\ \mathrm{V} 9 V 叠层电池，避免电解液渗漏腐蚀电路板。

潮湿环境（机房、发射塔区域）使用完毕，擦干表笔金属探针与仪表外壳，降低潮气侵入带来的精度漂移。

8. 共性故障与风险汇总

表笔插孔与测量类型不匹配：电流插孔测电压，属于高风险操作。
量程低于被测信号幅值，超量程过载。
带电切换档位，内部触点打火。
电阻档带电测量，外部电压灌入仪表。
在路测量不隔离并联元件，读数偏离真实值。
10 A 10\mathrm{A} 10A 大电流档位长时间通电，分流器过热。
长期存放不取出电池，漏液腐蚀。
弱电测量触碰探针金属部分，静电损伤芯片。

How To Use a Multimeter

如何使用万用表

2023-07-12 | By Maker.io Staff

This article, however, will explain the basics of operating a standard digital multimeter and how to measure common physical properties most of us will find helpful when diagnosing circuits in our projects.

本文将讲解标准数字万用表的基础操作方法，以及几种常用物理量的测量方式，这些测量手段在排查项目电路故障时十分实用。

The Parts of a Multimeter

万用表的组成部分

Before measuring, you should familiarize yourself with your newly acquired equipment. While most devices will slightly differ from other multimeters available for purchase, they typically all share some standard features, as illustrated by the following image:

开始测量前，你需要熟悉新到手的设备。市面上绝大多数万用表外观略有差异，但一般都具备下图所示的基础组件：

This figure explains the main parts of a common handheld multimeter.

本图标注了常见手持万用表的主要部件。

The main display, which might be either a more straightforward LCD or a more complex full-color readout, shows the current measurement results. The hold button locks the currently displayed value for later reference. Some meters may also include memory and recall buttons to store a few readout values. The function selection dial allows you to select which physical property to measure. Finally, the probes plug into the ports, depending on the property to measure. However, there's typically a common ground port for all functions. Note that some meters may include different probes, such as alligator clips, or they may not have any probes.

主显示屏可以是简易液晶显示屏，也可以是功能更丰富的彩屏，用于实时显示测量结果。hold 保持键可以锁定当前读数，方便后续查看。部分万用表配备存储与读取按键，可保存多组测量数值。功能选择旋钮用来切换待测量的物理量。表笔根据测量项目接入对应的插孔；万用表一般设有一个公共接地插孔，供多种测量模式共用。注意：部分万用表会搭配鳄鱼夹等替代表笔，也有部分型号不附带表笔。

Using a Multimeter to Measure Voltage

使用万用表测量电压

Start by plugging the black probe into the common port and the red probe into the voltage port on the meter to measure the voltage in a circuit or across a component:

测量电路或元器件两端电压时，先将黑色表笔插入公共插孔，红色表笔插入电压专用插孔：

Plug the probes into the multimeter's voltage measurement port, as indicated by the markings on the device.

按照机身标识，将表笔接入万用表的电压测量插孔。

Note that switching the red and black probes will not cause any harm. This will, however, negate the result shown on the screen, as the meter always measures the voltage relative to the common probe. Either way, use the function selection dial next to switch the meter into voltage mode. Make sure to choose DC or AC mode, accordingly:

红黑表笔互换不会损坏设备，但屏幕读数会变为负值，因为万用表以公共表笔作为电压参考基准。接好表笔后，转动功能旋钮切换至电压档位，根据被测对象选择直流 DC 或交流 AC 模式：

Note the markings next to the numbers to switch between AC and DC measuring. 留意数字旁的标识，以此切换交流、直流测量模式。

When measuring voltage, always measure the voltage in parallel. Essentially, in order to gauge the voltage across a component, you place one of the probes on each of the component's legs. Similarly, when determining the potential difference between two traces in a circuit, each lead needs to be connected to one of the traces. The circuit does not need to be opened in order to determine the voltage:

电压测量必须采用并联方式。简单来说，测量元器件两端电压时，两支表笔分别接在元器件的两个引脚；测量电路中两条走线之间的电势差时，两支表笔分别接触两条走线。测量电压不需要断开电路：

This diagram illustrates how to measure the voltage between the two resistors, R1, and R2. (Use the tabs at the bottom to select the various circuit diagrams)

本示意图演示如何测量电阻 R1 与 R2 之间的电压。（点击底部标签切换不同电路图）

Finally, note that when measuring any physical size with a multimeter, the value on the dial indicates the maximum value. Selecting 200 V, for example, allows measuring voltages of up to 200 V. It's advisable not to massively overshoot the actual voltage in the circuit, however, as larger limits tend to deliver less accurate results. When the real value is unknown, start with the largest value on the dial and work your way down to determine the true value.

另外需要注意：万用表旋钮上标注的数值代表该档位的最大量程。例如选择 200 V 档位，最多可测量 200 V 的电压。尽量不要选用远高于实际电压的量程，大量程档位的测量精度更低。如果不清楚被测电压大小，先选用最大量程，再逐步下调档位，直至读出准确数值。

Measuring the Resistance

测量电阻

Measuring the resistance of a component is very similar to determining the voltage differential between two points. Start by connecting the probes as shown above, and then select the ohmmeter setting with an appropriate range on the multimeter:

元器件电阻的测量方式和两点间电压测量较为接近。先按上文方式接好表笔，再在万用表上选择合适量程的电阻档：

Make sure to switch the meter into resistance mode and choose an appropriate upper limit.

确认万用表切换至电阻模式，并选择合适的量程上限。

Then, measure the resistance across a component in parallel:

之后以并联方式测量元器件两端电阻：

This diagram illustrates how to measure the total resistance of the circuit. (Use the tabs at the bottom to select the various circuit diagrams)

本示意图演示如何测量电路总电阻。（点击底部标签切换不同电路图）

Finding Open and Short Circuits Using a Multimeter

使用万用表查找断路与短路故障

Most multimeters can also help you find short and open circuits in your projects. A short circuit occurs when there's an electrical connection that shouldn't be present, while an open circuit describes the state where there's a missing or bad connection between two components that should be connected. In either case, a multimeter can determine these problems by measuring resistance. Very low resistance indicates a short circuit, while very high resistance points toward an open circuit. Start by switching the meter into continuity mode, which is often denoted by a symbol similar to the one shown in the following image:

多数万用表可以用来排查电路里的短路与断路。短路指电路出现本不该存在的电气连通；断路指原本应当连通的两点之间出现断开、接触不良。两种故障都可以通过电阻测量判断：电阻数值极低代表短路，电阻数值极高代表断路。先将万用表切换至通断档，该档位标识一般和下图类似：

Look for a marking similar to the one outlined in this picture to switch the meter into continuity mode.

找到图中标注样式的标识，切换到通断测量模式。

Then, place the two probes on the two points of the circuit that should or shouldn't be connected, keeping in mind that the order of the probes doesn't matter. The meter will then make a beeping sound when the two points are electrically connected. Otherwise, it will remain silent.

将两支表笔接触电路中待检测的两个点位，表笔顺序不影响检测结果。如果两点之间电气连通，万用表会发出蜂鸣；不连通则无提示音。

Measuring the Current in an Electric Circuit

测量电路中的电流

Multimeters typically also measure the electric current that flows through a circuit. Until now, the meter was always used in parallel to the components of interest. Yet when determining the current, the meter must be placed in series with the point of interest in the circuit so that the current flows through the meter. To use a water pipe analogy: The water must flow through the measuring device to measure the water flow through a pipe.

万用表一般也可以测量电路电流。前面介绍的测量方式，万用表都是和被测元器件并联；而电流测量时，万用表必须串联在被测支路中，让电流流过万用表。用水管类比：想要测量管道流量，水流必须穿过测量装置。

Some meters may require plugging the red probe into a dedicated port for current measurements. However, the particular meter used throughout this article supports two modes --- a fused one for up to 200 mA and an unfused mode for measuring currents of up to 10 A.

部分万用表需要把红色表笔换到电流专用插孔。本文示例万用表包含两种电流档位：带保险丝档位，最大量程 200 mA；无保险丝档位，最大量程 10 A。

Make sure to plug the probes into the correct port when measuring current.

测量电流时，务必将表笔接入正确插孔。

Note that measuring too high an amperage in the fused mode will blow the protective fuse in the multimeter. In that case, the fuse must be replaced before the meter can make any other fused amperage measurements.

注意：在带保险丝档位测量超过量程的电流，会烧断万用表内部保护保险丝。保险丝熔断后，必须更换新保险丝，才能继续使用该电流档位测量。

Once done selecting the mode, open the circuit that needs to be tested and place the multimeter's leads in series so that the probes close the circuit, as shown in the following diagram:

选好档位后，断开待测电路，把万用表表笔串联接入断点，让表笔接通电路，如下图所示：

Placing the multimeter as shown allows measuring the current flow through the entire circuit. (Use the tabs at the bottom to select the various circuit diagrams)

按图示方式接入万用表，可以测量整个回路的电流。（点击底部标签切换不同电路图）

Summary

小结

Multimeters are essential to every maker's toolbox, as they allow you to troubleshoot circuits in many different ways. This article discussed how to measure the voltage across components, the resistance of parts, and the current flow through a circuit.

万用表是创客工具箱里必备工具，它提供多种方式排查电路故障。本文介绍了元器件两端电压、元器件电阻、回路电流的测量方法。

To summarize, measure voltages, resistance, and continuity parallel without opening the circuit. In contrast, measuring the current must happen in series so that the meter closes the previously open circuit.

总结：电压、电阻、通断检测采用并联测量，无需断开电路；电流测量必须串联接入，万用表用来接通断开的回路。

When measuring currents, make sure not to overload the meter. Doing so will blow a fuse, which must then be replaced before performing any other current measurements.

测量电流时，严禁超量程使用。过载会熔断保险丝，更换保险丝后才能再次进行电流测量。

Reference

How To Use a Multimeter
https://www.digikey.cn/zh/maker/tutorials/2023/how-to-use-a-multimeter
Scheme It | Create or Find Schematics and Diagrams | DigiKey
https://www.digikey.at/en/schemeit/home