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Chapter 5: SERIES AND PARALLEL CIRCUITS

# Component Failure Analysis

The job of a technician frequently entails "troubleshooting" (locating and correcting a problem) in malfunctioning circuits. Good troubleshooting is a demanding and rewarding effort, requiring a thorough understanding of the basic concepts, the ability to formulate hypotheses (proposed explanations of an effect), the ability to judge the value of different hypotheses based on their probability (how likely one particular cause may be over another), and a sense of creativity in applying a solution to rectify the problem. While it is possible to distill these skills into a scientific methodology, most practiced troubleshooters would agree that troubleshooting involves a touch of art, and that it can take years of experience to fully develop this art.

An essential skill to have is a ready and intuitive understanding of how component faults affect circuits in different configurations. We will explore some of the effects of component faults in both series and parallel circuits here, then to a greater degree at the end of the "Series-Parallel Combination Circuits" chapter.

Let's start with a simple series circuit:

With all components in this circuit functioning at their proper values, we can mathematically determine all currents and voltage drops:

Now let us suppose that R_{2} fails shorted. *Shorted*
means that the resistor now acts like a straight piece of wire, with
little or no resistance. The circuit will behave as though a "jumper"
wire were connected across R_{2} (in case you were wondering,
"jumper wire" is a common term for a temporary wire connection in a
circuit). What causes the shorted condition of R_{2} is no matter to us in this example; we only care about its effect upon the circuit:

With R_{2} shorted, either by a jumper wire or by an internal resistor failure, the total circuit resistance will *decrease*.
Since the voltage output by the battery is a constant (at least in our
ideal simulation here), a decrease in total circuit resistance means
that total circuit current *must increase*:

As the circuit current increases from 20 milliamps to 60 milliamps, the voltage drops across R_{1} and R_{3} (which haven't changed resistances) increase as well, so that the two resistors are dropping the whole 9 volts. R_{2},
being bypassed by the very low resistance of the jumper wire, is
effectively eliminated from the circuit, the resistance from one lead to
the other having been reduced to zero. Thus, the voltage drop across R_{2}, even with the increased total current, is zero volts.

On the other hand, if R_{2} were to fail "open" -- resistance
increasing to nearly infinite levels -- it would also create
wide-reaching effects in the rest of the circuit:

With R_{2} at infinite resistance and total resistance being the
sum of all individual resistances in a series circuit, the total
current decreases to zero. With zero circuit current, there is no
electron flow to produce voltage drops across R_{1} or R_{3}. R_{2}, on the other hand, will manifest the full supply voltage across its terminals.

We can apply the same before/after analysis technique to parallel circuits as well. First, we determine what a "healthy" parallel circuit should behave like.

Supposing that R_{2} opens in this parallel circuit, here's what the effects will be:

Notice that in this parallel circuit, an open branch only affects the
current through that branch and the circuit's total current. Total
voltage -- being shared equally across all components in a parallel
circuit, will be the same for all resistors. Due to the fact that the
voltage source's tendency is to hold voltage *constant*, its
voltage will not change, and being in parallel with all the resistors,
it will hold all the resistors' voltages the same as they were before: 9
volts. Being that voltage is the only common parameter in a parallel
circuit, and the other resistors haven't changed resistance value, their
respective branch currents remain unchanged.

This is what happens in a household lamp circuit: all lamps get their operating voltage from power wiring arranged in a parallel fashion. Turning one lamp on and off (one branch in that parallel circuit closing and opening) doesn't affect the operation of other lamps in the room, only the current in that one lamp (branch circuit) and the total current powering all the lamps in the room:

In an ideal case (with perfect voltage sources and zero-resistance connecting wire), shorted resistors in a simple parallel circuit will also have no effect on what's happening in other branches of the circuit. In real life, the effect is not quite the same, and we'll see why in the following example:

A shorted resistor (resistance of 0 Ω) would theoretically draw infinite
current from any finite source of voltage (I=E/0). In this case, the
zero resistance of R_{2} decreases the circuit total resistance
to zero Ω as well, increasing total current to a value of infinity. As
long as the voltage source holds steady at 9 volts, however, the other
branch currents (I_{R1} and I_{R3}) will remain unchanged.

The critical assumption in this "perfect" scheme, however, is that the
voltage supply will hold steady at its rated voltage while supplying an
infinite amount of current to a short-circuit load. This is simply not
realistic. Even if the short has a small amount of resistance (as
opposed to absolutely zero resistance), no *real* voltage source
could arbitrarily supply a huge overload current and maintain steady
voltage at the same time. This is primarily due to the internal
resistance intrinsic to all electrical power sources, stemming from the
inescapable physical properties of the materials they're constructed of:

These internal resistances, small as they may be, turn our simple
parallel circuit into a series-parallel combination circuit. Usually,
the internal resistances of voltage sources are low enough that they can
be safely ignored, but when high currents resulting from shorted
components are encountered, their effects become very noticeable. In
this case, a shorted R_{2} would result in almost all the
voltage being dropped across the internal resistance of the battery,
with almost no voltage left over for resistors R_{1}, R_{2}, and R_{3}:

Suffice it to say, intentional direct short-circuits across the terminals of any voltage source is a bad idea. Even if the resulting high current (heat, flashes, sparks) causes no harm to people nearby, the voltage source will likely sustain damage, unless it has been specifically designed to handle short-circuits, which most voltage sources are not.

Eventually in this book I will lead you through the analysis of circuits *without the use of any numbers*,
that is, analyzing the effects of component failure in a circuit
without knowing exactly how many volts the battery produces, how many
ohms of resistance is in each resistor, etc. This section serves as an
introductory step to that kind of analysis.

Whereas the normal application of Ohm's Law and the rules of series and
parallel circuits is performed with numerical quantities (*"quantitative"*), this new kind of analysis without precise numerical figures is something I like to call *qualitative* analysis. In other words, we will be analyzing the *qualities* of the effects in a circuit rather than the precise *quantities*. The result, for you, will be a much deeper intuitive understanding of electric circuit operation.

**REVIEW:**- To determine what would happen in a circuit if a component fails, re-draw that circuit with the equivalent resistance of the failed component in place and re-calculate all values.
- The ability to intuitively determine what will happen to a circuit with any given component fault is a
*crucial*skill for any electronics troubleshooter to develop. The best way to learn is to experiment with circuit calculations and real-life circuits, paying close attention to what changes with a fault, what remains the same, and*why*! - A
*shorted*component is one whose resistance has dramatically decreased. - An
*open*component is one whose resistance has dramatically increased. For the record, resistors tend to fail open more often than fail shorted, and they almost never fail unless physically or electrically overstressed (physically abused or overheated).