BASIC ELECTRONICS - OD1633 - LESSON 1/TASK 2
The major difference between an open in a parallel circuit and an open in a
series circuit is that in the parallel circuit the open would not
necessarily disable the circuit. If the open condition occurs in a series
portion of the circuit, there will be no current because theme is no
complete path for current flow. If, on the other hand, the open occurs in a
parallel path, some current will flow in the circuit. The parallel branch
where the open occurs will be effectively disabled, total resistance of the
circuit will increase, and total current will decrease.
To clarify these points, figure 58 on the following page illustrates a
series-parallel circuit. First, the effect of an open in a series portion
of this circuit will be examined. View A shows the normal circuit RT = 40
Ohms and IT = 3 amps. In view B, an open is shown in the series portion of
the circuit. There is no complete path for current, and the resistance of
the circuit is considered to be infinite.
In view C, an open is shown in the parallel branch of R3. There is no path
for current through R3.
In the circuit, current flows through R1 and R2
only.
Since there is only one path for current flow, R1 and R2 are
effectively in series.
Under these conditions, RT = 120 Ohms and IT = 1 amp. As you can see, when
an open occurs in a parallel branch, total circuit resistance increases and
total circuit current decreases.
A short circuit in a parallel network has an effect similar to short in a
series circuit. In general, the short will cause an increase in current and
the possibility of component damage, regardless of the type of circuit
involved. To illustrate this point, figure 59 on page 96 shows a series-
parallel network in which shorts are developed.
In view A, the normal
circuit is shown (RT = 40 Ohms and IT = 3 amps).
In view B, R1 has shorted. R1 now has zero Ohms of resistance. The total
of the resistance in the circuit is now equal to the resistance of the
parallel network of R2 and R3, or 20 Ohms. Circuit current has increased to
6 amps. All of this current goes through the parallel network (R2, R3), and
this increase in current would most likely damage the components.
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