Page 45 - Troubleshooting Analog Circuits

32 3. Getting Down to the Component Level

quarter of the element is required to dissipate this same amount of power, the pot will
fail quickly. For example, many years ago, the only ohmmeters available might put as
much as 50 mA into a 1 R resistor. When a 50 kR, 10-turn precision potentiometer
(think of an item costing $20) was tested at incoming inspection using such an ohm-
meter, the test technician would turn the pot down to the end where the 50 mA was
sufficient to bum out the delicate wirewound element. Then he would write in his
report that the potentiometer had failed. What a dumb way to do incoming inspection!
Some trimming potentiometers are not rated to carry any significant DC current
through the wiper. This DC current-even a microampere-could cause electromi-
gration, leading to an open circuit or noisy, unreliable wiper action. Other trim-pots
are alleged to be more reliable if a small amount of current-at least a microampere
DC-is drawn through the wiper, to prevent “dry failures.” Carbon pots are not likely
to be degraded by either of these failure modes. If you have any questions about the
suitability of your favorite trimming potentiometers for rheostat service, you or your
components engineer should ask the pot’s manufacturer.
How do you spot resistor problems? The most obvious way is to follow your nose.
When a resistor is dying it usually gets quite hot, and sometimes the strong smell of
phenolic leads right to the abused component. Just be careful not to bum your fin-
gers. You may also encounter situations in which a resistor hasn’t truly failed but
doesn’t seem to be doing its job, either. Something seems to be wrong with the cir-
cuit, and a resistor of the wrong value is the easiest explanation. So, you measure the
resistor in question, and 90% of the time the resistor is just fine-usually the trouble
is elsewhere. A resistor doesn’t usually fail all by itself. Its failure is often a symptom
that a transistor or circuit has failed; if you just replace the resistor, the new one will
also bum out or exhibit the same strange characteristics.
Around our lab, if anybody smells an “overheated resistor,” he makes sure that we
understand what it is. Usually when I holler, “Who has a resistor overheated??,” an
engineer or technician will sheepishly say, “I just cooked my circuit . . . .” But some-
times it is a failure in a piece of un-attended equipment, and the sooner we can turn
off its power or fetch a fire extinguisher, the better.
How do you check for resistor errors? If you’re desperate, you can disconnect one
end of the resistor and actually measure its value. It’s often easier to just measure the
I X R drops in the network and deduce which resistor, if any, seems to be of the
wrong value. If one resistor is suspected of being temperature-sensitive, you can heat
it with a soldering iron or cool it with freeze mist as you monitor its effect. In some
solid-state circuits, the signals are currents, so it’s not easy to probe the circuit with a
voltmeter. In this case, you may have to make implicit measurements to decide if a
resistor is the problem. Also, remember that a sneak path of current can often cause
the same effect as a bad resistor.
When you are trying to make precision measurements of resistors, you should be
aware that even the best ohmmeters-ven the ones with 4-wire connections and lots
of digits on the DVM-do not have as good accuracy or resolution as you can get by
forcing a current through a stable reference resistor RmF and then through the Rx
and comparing the voltages. This is especially true for low resistor values. See Figure
3.4. You also have complete control over the amount of current flowing through
the Rx.

Watch Out for Damaged Components
Damaged resistors can also be the source of trouble. A resistor that’s cracked can be
noisy or intermittent. When resistors are overheated with excess power, such as 2 or
3 W in a 1/4-W resistor, they tend to fail “open”-they may crack apart, but they