Page 39 - Troubleshooting Analog Circuits

3. Getting Down to the

Component Level

Resistors and Inductors

In earlier chapters, we’ve covered the philosophy of troubleshooting analog circuits,
and the tools and equipment you need to do so. But if you’re working on a circuit and
are not aware of what can cause component failure, finding the root of your problem
could be difficult. Hence, this chapter covers resistors, fuses, inductors, and trans-
formers; their possible modes of failure; and the unsuspected problems that may
occur if you use the wrong type of component. (Capacitors are waiting in the next
chapter. Kind of a shame to segregate them from the resistors . . . .)
Troubleshooting circuits often boils down to finding problems in passive compo-
nents. These problems can range from improper component selection in the design
phase to damaged components that hurt the circuit’s performance. Resistors, induc-
tors, and transformers can each be a source of trouble.
Resistors are certainly the most basic passive component, and, barring any extreme
or obscure situations, you won’t usually run into problems caused by the resistors
themselves. I don’t mean to say that you’ll never see any resistor problems, but most
of them will be due to the way you use and abuse and mis-specify resistors. In other
cases, some other part of the circuit may be causing damage to a resistor, and the
failure of the resistor is just a symptom of a larger problem.
You may eventually have to track down a wide variety of problems involving
resistors to achieve a working design. Some will seem obvious. For example, your
circuit needs a 10 kR resistor. The technician reaches into the drawer for one and
instead gets a 1 kR resistor, which then mistakenly gets inserted into your board.
This example illustrates the most common source of resistor trouble in our lab.
Consequently, I ask my technicians and assemblers to install resistors so that their
values are easy to read. And any time I find a 1 k resistor where a 10 k is supposed
to be, I check to see how many more 1 ks are in the 10 k drawer. Often there are
quite a few!
Sometimes a resistor gets mismarked; sometimes a resistor’s value shifts due to
aging, overheating, or temperature cycling. Recently, we found a batch of “1%”
metal-film resistors whose values had increased by 20 to 900% after just a few dozen
cycles of -55 to +125 “C. As it turned out, our QC department had okayed only cer-
tain resistors to be used in bum-in boards, and these particular resistors had not been
approved. The QC people, too, had spotted this failure mode.

Resistor Characteristics Can Vary Widely
You should be familiar with the different resistor types in order to select the most
appropriate type for your application; the most common types and some of their
characteristics are summarized in Table 3.1. A component type that’s good for one
application can be disastrous in another. For example, I often see an engineer specify

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