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 R2 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 R2 (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 R2 is no matter to us in this example; we only
care about its effect upon the circuit:
With R2 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 R1
and R3 (which haven't changed resistances)
increase as well, so that the two resistors are dropping the
whole 9 volts. R2, 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 R2,
even with the increased total current, is zero volts.
On the other hand, if R2 were to
fail "open" -- resistance increasing to nearly infinite
levels -- it would also create wide-reaching effects in the
rest of the circuit:
With R2 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 R1 or R3.
R2, 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 R2 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 R2
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 (IR1 and IR3)
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 R2 would
result in almost all the voltage being dropped across the
internal resistance of the battery, with almost no voltage
left over for resistors R1, R2, and R3:
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 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.
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REVIEW:
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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!
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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).
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