Page 40 - Troubleshooting Analog Circuits

Resistor Characteristics Can Vary Widely 27

a carbon-composition resistor in a case where stability and low TC (Temperature
Coefficient) are required. Sometimes it was just a bad choice, and a conversion to a
stable metal-film resistor (such as an RN55D or RN60C) with a TC of 50 or 100
ppm/”C max considerably improves accuracy and stability, In other cases, the engi-
neer says. “No, I tried a metal-film resistor there, but, when I put in the carbon re-
sistor, the overall TC was improved.” In this case, the engineer was relying on the
carbon-composition resistor to have a consistent TC which must compensate for
some other TC problem. I have found that you can’t rely on consistent TC with the
carbon-composition type. and I do not recommend them in applications where preci-
sion and stability are required-even if you do see some TC improvement in your
circuit.
However. carbon-composition resistors do have their place. I was recently
reviewing a military specification that spelled out the necessary equipment for the
ESD (electrostatic discharge) testing of circuits. An accurate 1500 R resistor was
required for use as the series resistor during discharge of the high-voltage capacitor.
In this case, you would assume that a metal-film resistor would be suitable; however.
a metal-film resistor is made by cutting a spiral into the film on the resistor’s ceramic
core (Figure 3. la). Under severe overvoltage conditions, the spiral gaps can break
down and cause the resistor to pass a lot more current than Ohm’s Law predicts-the
resistor will start to destroy itself. Therefore, the spec should have called for the use
of a carbon-composition resistor. whose resistive element is a large chunk of resistive
material (Figure 3. lb). This resistor can handle large overloads for a short time
without any such flash-over. Even when you are applying a 200% to 400% overload
for just a short time, the nonuniform heating of the spiraled section of a metal-film
resistor can cause the resistor to become unreliable. You can also get around this
problem by using a series connection of metal-film resistors. If you put fifteen 100 0.
1/4-W metal-film resistors in series, each individual resistor would not see
overvoltage or excessive power.
Carbon-film resistors are now quite inexpensive and have become the most
common type of resistor around most labs. Their main drawback is that they are very
similar in appearance to metal-film resistors and have some similar characteristics:
Carbon-film resistors have 1 % tolerances, are normally manufactured with spiral
cuts, and have the same kind of voltage-overload limitations as metal-film types. But.
carbon-film resistors have much higher TCs-500 to 800 ppm/”C. It’s easy to erro-
neously insert a drifty carbon-film resistor for the intended metal-film type. Don’t
confuse the two.
Precision-film resistors, on the other hand, are available with greatly improved
accuracy and TC. Compared to ordinary RN55D and RN55C resistors with TCs of

Table 3. I. Typical Resistor Characteristics
TC
Resistor Type Range* (0) (fPPMPC) Parasitic Effects cost
Composition 1-22M High Low Low
Metal Film IO-IM Low Medium Medium
Carbon Film 10-IOM Medium Medium Medium
Wirewound, Precision I-1M Low High High
Wirewound. Power 0.01-100k Medium High Medium
Thin-Film 25-100k Low Low Medium
Thick-Film ICLIM Low Low Medium
Diffused 2&50k High High Low
*Ran_ee may vary by manufacturer.