机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 22.pptVIP

Lesson 22 Basic circuit measurements Text Voltage, current, and resistance measurements are commonly required in electronics work. Special types of instruments are used to measure these basic electrical quantities. The instrument used to measure voltage is a voltmeter, the instrument used to measure current is an ammeter ,and the instrument used to measure resistance is an ohmmeter①. Commonly, all three instruments are combined into a single instrument known as a multimeter, or VOM (volt-ohm-milliammeter),in which you can choose what specific quantity to measure by selecting the switch setting②. Typical multimeters are shown in Figure 22.1. Part (a) shows an analog meter, that is, with a needle pointer, and Part (b) shows a digital multimeter (DMM), which provides a digital readout of the measured quantity. METER SYMBOLS Throughout this book, certain symbols will be used to represent the different meters, as shown in Figure 22.2. You may see any of three types of symbols for voltmeters, ammeters, and ohmmeters, depending on which symbol most effectively conveys the information required③. Generally, the pictorial analog symbol is used when relative measurements or changes in quantities ate to be depicted by the position or movement of the pointer. The pictorial digital symbol is used when fixed values are to be indicated in a circuit. The general schematic symbol is used to indicate placement of meters in a circuit when no values or value changes need to be shown. HOW TO MEASURE CURRENT WITH AN AMMETER Figure 22.3 illustrates how to measure current with an ammeter. Part (a) shows the simple circuit in which the current through the resistor is to be measured④. Connect the ammeter in the current path by first opening the circuit, as shown in Part (b). Then insert the meter as shown in Part (c). As you will learn later, such a connection is a series connection. The polarity of the meter must be such that the current flows in at the positive terminal and out at the nega