Page 85 - Teach Yourself Electricity and Electronics

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CHAPTER

Basic dc circuits

YOU’VE ALREADY SEEN SOME SIMPLE ELECTRICAL CIRCUIT DIAGRAMS. SOME OF
these are the same kinds of diagrams, using the same symbols, that professional tech-
nicians and engineers use. In this chapter, you’ll get more acquainted with this type of
diagram. You’ll also learn more about how current, voltage, resistance, and power are
related in direct-current (dc) and low-frequency alternating-current (ac) circuits.

Schematic symbols

In this course, the plan is to familiarize you with schematic symbols mainly by getting
you to read and use them “in action,” rather than by dryly drilling you with them. But
it’s a good idea now to check Appendix B and look over the various symbols. Some of
the more common ones are mentioned here.
The simplest schematic symbol is the one representing a wire or electrical conduc-
tor: a straight, solid line. Sometimes dotted lines are used to represent conductors, but
usually, dotted lines are drawn to partition diagrams into constituent circuits, or to in-
dicate 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 for-
mation like soldiers. This keeps the diagram neat and easy to read.
When two conductor lines cross, they aren’t connected at the crossing point unless
a heavy, black dot is placed where the two lines meet. The dot should always 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 zig-zaggy line. A variable resistor, or potentiometer, is in-
dicated by a zig-zaggy line with an arrow through it, or by a zig-zaggy line with an arrow
pointing at it. These symbols are shown in Fig. 4-1.
A cell is shown by two parallel lines, one longer than the other. The longer line rep-
resents the plus terminal. A battery, or combination of cells in series, is indicated by
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