Why good resistors go bad!

[Kevin J Ward]

While reading the discussions on various reflectors about out-of-spec
carbon-composition resistors in boat anchors, I was reminded that
resistors have voltage ratings, as well as power ratings.  I went looking
for information on this and found the following discussion in "The Art of
Electronics", Second Edition,  by Paul Horowitz and Winfield Hill.  The
copyright on this particular edition is 1990.  I present only the most
pertinent portion of the discussion here.  Keep in mind we are talking
about ¼ watt resistors.

<snip>
"Resistors are so easy to use that they're often taken for granted.
They're not perfect, though, and it is worthwhile to look at some of
their defects.  The popular 5% composition type, in particular, although
fine for nearly all noncritical circuit applications, is not stable
enough for precision applications.  You should know about its limitations
so that you won't be surprised someday.  Its principal defects are
variations in resistance with temperature, voltage, time, and humidity.
Other defects may relate to inductance (which may be serious at high
frequencies), the development of thermal hot spots in power applications,
or electrical noise generation in low-noise amplifiers.  The following
specifications are worst-case values; typically you'll do better, but
don't count on it!

"SPECIFICATIONS FOR ALLEN-BRADLEY AB SERIES TYPE CB

"Standard tolerance is ±5% under normal conditions.  Maximum power for
70°C ambient temperature is 0.25 watt, which will raise the internal
temperature to 150°C.  The maximum applied voltage specification is
(0.25R)½ or 250 volts, whichever is less.  They mean it!  (See Fig.
6.53.)  A single 5 second overvoltage to 400 volts can cause a permanent
change in resistance by 2%."

           (The following chart is paraphrased.)

Soldering (350°C at 1/8 inch)                                   ±2
permanent change
Load cycling (500 ON/OFF cycles in 1000 hrs.)  +4%-6% permanent change
Vibration (20g) and shock (100g)                             ±2 permanent
change


"For applications that require any real accuracy or stability a 1%
metal-film resistor should be used.  They can be expected to have
stability of better than 0.1% under normal conditions and better than 1%
under worst-case treatment.  Precision wire-wound resistors are available
for the most demanding applications.  For power dissipation above about
0.1 watt, a resistor of higher power rating should be used.   <snip>
Allowable resistor power dissipation depends on air flow, thermal
conduction via the resistor leads, and circuit density; thus, a
resistor's power rating should be considered a rough guideline.  Note
also that resistor power ratings refer to average power dissipation and
may be substantially exceeded for short periods of time (a few seconds or
more, depending on the resistor's "thermal mass")."

           (And concerning the above mentioned figure:)

<snip>   "In high voltage circuits like this, it's easy to overlook the
fact that you may need to use 1 watt (or larger) resistors, rather than
the standard ¼ watt type.  A more subtle trap awaits the unwary, namely
the maximum voltage rating of 250 volts for standard ¼ watt composition
("carbon") resistors, regardless of power dissipation.  Carbon resistors
run at higher voltages show astounding voltage coefficients, not to
mention permanent changes of resistance.  For example, in an actual
measurement (Fig. 6.53) a 1000:1 divider (10Meg, 10k) produced a division
ratio of 775:1 (29% error!) when driven with 1kV; note that the power was
well within ratings.  This non-ohmic effect is particularly important in
the output-voltage-sensing divider of high-voltage supplies and
amplifiers  -  beware!  Companies like Victoreen make resistors in many
styles designed for high-voltage applications like this."

Take that formula for maximum voltage and plug in a few resistor values.
You might be surprised to see that the maximum voltage for a 1000 ohm ¼
watt resistor is 15.8 volts!  Even at that voltage the resistor will be
dissipating a quarter of a watt, which is only recommended for short
periods.

Now you know why all those old radios have so many resistors that need to
be replaced.

Kevin  N2IE

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