Page 102 - Teach Yourself Electricity and Electronics

5

CHAPTER

Direct-current circuit

analysis

IN THIS CHAPTER, YOU’LL LEARN MORE ABOUT DC CIRCUITS AND HOW THEY
behave. These principles apply to almost all circuits in utility ac applications, too.
Sometimes it’s necessary to analyze networks that don’t have obvious practical use.
But even a passive network of resistors can serve to set up the conditions for operation
of a complex electrical device such as a radio amplifier or a digital calculator, by pro-
viding specific voltages or currents.

Current through series resistances

Have you ever used those tiny holiday lights that come in strings? If one bulb burns out,
the whole set of bulbs goes dark; then you have to find out which bulb is bad, and re-
place it to get the lights working again. Each bulb works with something like 10 V; there
are about a dozen bulbs in the string. You plug in the whole bunch and the 120-V utility
mains drive just the right amount of current through each bulb.
In a series circuit, such as a string of light bulbs (Fig. 5-1), the current at any given
point is the same as the current at any other point. The ammeter, A, is shown in the line
between two of the bulbs. If it were moved anywhere else along the current path, it would
indicate the same current. This is true in any series dc circuit, no matter what the com-
ponents actually are and regardless of whether or not they all have the same resistance.
If the bulbs in Fig. 5-1 were of different resistances, some of them would consume
more power than others. In case one of the bulbs in Fig. 5-1 burns out, and its socket is
then shorted out instead of filled with a replacement bulb, the current through the whole
chain will increase, because the overall resistance of the string would go down. This
would force each of the remaining bulbs to carry too much current. Another bulb would
probably burn out before long. If it, too, were replaced with a short circuit, the current

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