Electrical Diagrams Help

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By — McGraw-Hill Professional
Updated on Sep 5, 2011

Electrical Diagrams

To understand how electric circuits work, you should be able to read electrical wiring diagrams, called schematic diagrams . These diagrams use schematic symbols . Here are the basic symbols. Think of them as something like an alphabet in a language such as Chinese or Japanese, where things are represented by little pictures. However, before you get intimidated by this comparison, rest assured that it will be easier for you to learn schematic symbology than it would be to learn Chinese (unless you already know Chinese!).

Basic Symbols

The simplest schematic symbol is the one representing a wire or electrical conductor: a straight solid line. Sometimes dashed lines are used to represent conductors, but usually, broken lines are drawn to partition diagrams into constituent circuits or to indicate that certain components interact with each other or operate in step with each other. Conductor lines are almost always drawn either horizontally across or vertically up and down the page so that the imaginary charge carriers are forced to march in formation like soldiers. This keeps the diagram neat and easy to read.

When two conductor lines cross, they are not connected at the crossing point unless a heavy black dot is placed where the two lines meet. The dot always should be clearly visible wherever conductors are to be connected, no matter how many of them meet at the junction.

A resistor is indicated by a zigzaggy line. A variable resistor, such as a rheostat or potentiometer, is indicated by a zigzaggy line with an arrow through it or by a zigzaggy line with an arrow pointing at it. These symbols are shown in Fig. 12-3.

Direct Current Electrical Diagrams Basic Symbols

Fig. 12-3 . (a) A fixed resistor. (b) A two-terminal variable resistor. (c) A three-terminal potentiometer.

An electrochemical cell is shown by two parallel lines, one longer than the other. The longer line represents the positive terminal. A battery , or combination of cells in series, is indicated by an alternating sequence of parallel lines, long-short-long-short. The symbols for a cell and a battery are shown in Fig. 12-4.

Direct Current Electrical Diagrams Some More Symbols

Fig. 12-4 . (a) An electrochemical cell, (b) A battery.

Some More Symbols

Meters are indicated as circles. Sometimes the circle has an arrow inside it, and the meter type, such as mA (milliammeter) or V (voltmeter), is written alongside the circle, as shown in Fig. 12-5a . Sometimes the meter type is indicated inside the circle, and there is no arrow (see Fig. 12-5b) . It doesn’t matter which way it’s done as long as you are consistent everywhere in a given diagram.

Direct Current Electrical Diagrams Some More Symbols

Fig. 12-5 . Meter symbols: (a) designator outside; (b) designator inside.

Some other common symbols include the lamp , the capacitor , the air-core coil , the iron-core coil , the chassis ground , the earth ground , the alternating-current (AC) source , the set of terminals , and the black box (which can stand for almost anything), a rectangle with the designator written inside. These are shown in Fig. 12-6.

Direct Current Voltage/Current/Resistance Circuits

Fig. 12-6 . More common schematic symbols: (a) incandescent lamp; (b) fixed-value capacitor; (c) air-core coil; (d) iron-core coil; (e) chassis ground; (f) earth ground; (g) ac source; (h) terminals; and (i) and black box.

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