Tab Electronics Guide to Understanding Electricity ... - Sciences Club

68 Chapter Two
electronic circuit that you may construct in the future, so that you
will be capable of selecting an appropriate power supply to operate it.
Exercise 7 Figure 2-22c illustrates a series-parallel circuit. To visualize
this fact, use your pencil point to trace out the following two current
paths. Current flows from negative to positive, so start at the negative end
of the battery (the bottom end illustrated with a short horizontal line)
and follow the current path to the first connection point (labeled “node
1”), continue up through D2, then D1, through the next connection point
(labeled “node 2”), through R1, and finally ending at the positive side of
the battery. Tracing out the second current path, begin again at the negative
side of the battery and follow the current flow to node 1, but this
time follow the other current path through D3, then through R2,
through node 2, through R1, and finally back to the positive side of the
battery. As can be seen, there are two current “loops” in this circuit. One
consists of D2, D1, and R1. The other consists of D3, R2, and R1. Notice
that the total current flow from the battery branches at node 1, with
some of it flowing through the D2-D1 leg, while the rest of it flows
through the D3-R2 leg. However, at node 2, the two current flows combine
again, and the total current flows through R1. Since the total circuit
current flows through R1, R1 must be in series with the remainder of the
circuit. However, because the current flow branches, causing only a portion
of the total circuit current to flow through the other circuit components,
the remainder of the circuit must be in some type of parallel
configuration. Therefore, this circuit is referred to as a series-parallel circuit.
As stated previously, a certain portion of the current branches at node
1 and flows through the D2-D1 leg. Whatever value of current that happens
to flow through D2 must also flow through D1, since there are no
connections between them wherein the current could branch. Likewise,
whatever value of current that happens to flow through D3 must also
flow through R2. Therefore, D2 and D1 are in series with each other, and
D3 and R2 are also in series with each other. In addition, the series leg of
D2 and D1 is in parallel with the series leg of D3 and R2.
Refer to Fig. 2-22d and compare it to the circuit of Fig. 2-22c. Note
that they are exactly the same from the electrical perspective. The only difference
is in the way R2 is illustrated. Both methods of drawing the
circuit are correct, but the Fig. 2-22d illustration makes it easier to visualize
that the D3-R2 series leg is connected directly across from, or parallel
with, the D2-D1 series leg.
What would be the desired resistance value of R2 in Fig. 2-22c Going
back to the general rules that govern a parallel circuit, you already know