Page 14 - Troubleshooting Analog Circuits

I. First Things First

The Philosophy of Troubleshooting

In this first chapter, I will make the point that a significant part of effective trouble-
shoDting lies in the way that you think about the problem. The next chapter will cover
the equipment you should buy and build to help you diagnose problems. Other chap-
ters will illuminate some of the more subtle and elusive characteristics of passive and
active components, and the PC boards and cables that interconnect them.

Troubleshooting Is More Efictive with the Right Philosophy
If you recall that the most boring class in school was a philosophy class, and you
think this book will be boring that way, well, WRONG. We are going to talk about
the real world and examples of mistakes, goofs, and how we can recover from these
mistakes. We are going to talk about all the nasty problems the world tries to inflict
on US. We are talking about Trouble with a capital T, and how to overcome it.
Here at National Semiconductor, we decided a couple of years ago that we ought
to write a book about switch-mode power supplies. Within the applications and de-
sign groups, nearly all of the engineers volunteered to write a chapter, and I volun-
teered to do a chapter on troubleshooting. At present, the status of that book is pretty
dubious. But, the “troubleshooting chapter” is going strong, and you readers are
among the first to benefit, because that one chapter has expanded to become this
entire book. Although I am probably not the world’s best analog-circuit trouble-
shooter, I am fairly good, and I just happened to be the guy who sat down and put all
these stories in writing.
Furthermore, the techniques you need to troubleshoot a switch-mode power supply
apply, in general, to a lot of other analog circuits and may even be useful for some
basic digital hardware. You don’t have to build switchers to find this book useful; if
you design or build any analog circuits, this book is for you.
Maybe there are some engineers who are knowledgeable about digital circuits,
computers, microprocessors, and software, who may someday write about the trou-
bleshooting of those types of circuits. That sure would suit me fine, because I am
certainly not going to talk about those circuits!! Everybody has to be ignorant about
something, and rhar is exactly what I am ignorant about.

If Only Everythihg Would Always Go Right.. .

Why are we interested in troubleshooting? Because even the best engineers take on
projects whose requirements are so difficult and challenging that the circuits don’t
work as expected-at least not the first time. I don’t have data on switching regula-
tors, but I read in an industry study that when disk drives are manufactured, the frac-
tion that fails to function when power is first applied typically ranges from 20 to
70%. Of course, this fraction may occasionally fall as low as 1 % and rise as high as
100%. But, on the average, production engineers and technicians must be prepared to

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