Do You Know Enough About Capacitors?
January 1960 Electronics World

January 1960 Electronics World Table of Contents  Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

Today's electronics and RF magazines tend to cater to engineers and managers, as opposed to technicians and hobbyists. That's not to say that techs do not benefit from the material presented, but that information is typically concerned with new product and system design with little attention paid to troubleshooting and maintenance. The predecessors to modern magazines much more often included articles on the latter. Publications like Popular Electronics, being intended for hobbyists, featured useful quizzes, "how to" articles, and troubleshooting tips along with product reports and an occasional design methodology piece. Electronics World, the predecessor to Popular Electronics, was more of an equal split between professional and hobby themes. This particular article tests the reader's knowledge of capacitors by proposing circuit failure examples which are due to a malfunctioning capacitor and challenges him/her to determine the likely cause based on observed symptoms.

Do You Know Enough About Capacitors?

By Sol Heller

In these "simple" troubleshooting and replacement problems, would you proceed correctly every time?

Do you feel there is nothing to replacing a defective capacitor except substituting a similar unit?

Or do you have the capacity to recognize that there is more to a capacitor than capacitance? Here is a quiz that may turn up some surprises for you. On the other hand, getting all the answers right should give you a charge.

Question: A service technician hesitates to use a 0.01-μf paper capacitor as a replacement because it has been lying around (unused) on his shelf for ten years. Is his hesitation justified?

Answer: With the exception of electrolytic units, capacitors deteriorate only during the time voltage is applied to them. The technician should show no hesitation about using the capacitor. Besides, if he does so little business that he still stocks a component he bought ten years ago, how can he afford to do anything else?

Question: Under what conditions is it preferable to connect an audio power output tube plate capacitor between plate and screen, rather than plate and ground, or plate and cathode?

Answer: When a capacitor of the proper working voltage is not available, or when concern is felt, for some reason, regarding the possibility of a breakdown in the replacement capacitor, the plate-to-screen connection may be used. The d.c. voltage across the capacitor is much smaller in the latter case than it would be if the negative end of the capacitor were connected to ground, or to the cathode of the audio power output tube.

Question: Can a conventional 10-μf electrolytic capacitor be used to replace L; in the C₂₈₁ in the circuit shown in Fig. 1?

Answer: No. When the receiver is fully warmed up, the negative side of the capacitor is at "B+" potential, while the positive side is at the higher boosted "B+" voltage. A conventional capacitor could be used if this condition prevailed at all times, since the negative side of the capacitor would always be at a lower potential than the positive side. The condition does not prevail at all times, however. During the time the receiver is warming up, the boost voltage is lower than "B+." The negative side of C₂₈₁ is consequently connected to a higher positive potential than the positive side. The capacitor operates at this reversed polarity for about 10 to 15 seconds. A conventional electrolytic capacitor would bite the dust prematurely if exposed to such topsy-turvy conditions. A special semi-polarized unit is used by some set makers and available from them.

Question: C₁ in the vertical oscillator circuit shown in Fig. 2 is a ceramic disc coupling capacitor. What type of capacitor can be substituted for it when vertical drift is a problem, and tests have indicated that the vertical oscillator tube is not responsible?

Answer: Substitution of a silver mica capacitor would be a logical procedure. The mica type has a very high electrical stability. A 1000-μμf. unit, for instance, will exhibit a capacitance change of less than .1% over a frequency range extending from low frequencies to 2 mc. The effect of a 1°C change in temperature is a change in capacitance of only 60 parts in a million. The tolerance of the replacement capacitor is not important - the vertical hold control setting will produce the correct frequency of operation even if the capacitor is 10 or 20% off its nominal capacitance. The important thing is that the capacitance, whatever it is, doesn't change with temperature.

Question: What harm, if any, is there in using a somewhat larger value of capacitance than called for in replacing the coupling capacitor between horizontal oscillator and horizontal output tube (C₁, Fig. 3).

Answer: A larger value of coupling capacitance will increase the amount of horizontal sweep signal applied to the horizontal output tube. The horizontal drive control may have to be reset to some point close to one end of its range, leaving an insufficient margin for readjustments necessitated by aging or replacement of tubes or components in this circuit.

Question: Why is it undesirable to mount an electrolytic capacitor in an area where considerable heat is likely to develop during receiver operation?

Answer: Heat is bad because it promotes drying out of the electrolyte. This increases the series resistance offered by the capacitor; and the power factor (ratio of resistance to impedance in the capacitor) goes up in consequence. A higher power factor is undesirable, since larger I²R losses occur across the increased resistance of the capacitor, and produce still further heating of the unit. Heat also tends to increase the leakage current of the capacitor (see Fig.  4), which promotes greater I²R losses in the capacitor, and additional heating of the unit.

Excessive heating spells death to electrolytic capacitors. In some types, every 10°C increase in temperature causes a 50% decrease in life expectancy; others are less affected by temperature. In general, however, keeping cool is as important for capacitors as it is for people.

Question: A technician, in replacing a power supply electrolytic capacitor that has failed prematurely, decides to leave off the insulating cardboard that was present around the original unit. Does this procedure make sense?

Answer: It does. The cardboard insulation prevented the capacitor from dissipating heat adequately, and could be responsible for premature failure. But watch for exposed voltage.

Question: What kind of electrolytic capacitor will function more satisfactorily than other kinds under high-temperature conditions?

Answer: A capacitor that has a hermetically sealed metal can for its container. This construction minimizes loss of electrolyte, and therefore increases life expectancy. Any capacitor with a voltage rating 50 to 100 volts higher than the operating voltage can be used at temperatures up to 185°F. Temperatures in excess of 140°F may damage a conventional capacitor if its voltage rating is not much above the operating voltage.

Question: Are there any applications in which it would make a difference whether an electrolytic capacitor was fabricated with an etched-foil anode or a plain-foil anode?

Answer: Yes. Dry electrolytic capacitors have anodes made of plain, etched, sprayed, or fabricated foil. An etched-foil anode is one that has been made very rough by special processing. The resultant surface undulations give the anode (which forms one plate of the capacitor) a much larger effective area. The electrolyte (which forms the second plate of the capacitor) follows the undulations. The capacitor, in consequence, has a much larger capacitance than a plain-foil unit of the same size would have, permitting compactness.

However, an etched-foil capacitor will have a considerably higher total impedance than a plain-foil equivalent, making the former inferior for certain applications. A unit that does not have an etched foil would be preferable for bypassing and decoupling in audio-frequency and vertical sweep circuits. The catalogues of parts supply houses are often useful in determining whether a specific electrolytic capacitor does or does not use an etched-foil anode.

Question: What effects, if any, do low temperatures have on electrolytic capacitors?

Answer: For most conventional types, the capacitance decreases rapidly below temperatures of -5°C. We can conceive of a situation where a battery portable in which an electrolytic capacitor has been replaced is returned to the service technician for further work because it didn't operate satisfactorily outdoors in very cold weather. This temporary condition clears up when the receiver and its owner sensibly return to a comfortable position by the fireside, or when the weather gets a little warmer. If necessary, higher-cost tantalum capacitors may be used. Some of these are rated for operation at -55°C, or even lower.

Question: In the case of a ceramic feed-through type of capacitor (as shown in Fig. 5A), are there any visual indications that might point to the need of replacement?

Answer: This type should be replaced if the silver-coated surface is peeled, or if the ceramic is cracked, or if the center conductor of the unit is loose.

Question: Does it make a difference in which position an electrolytic capacitor is mounted?

Answer: It may, especially if the unit is a wet electrolytic. An incorrect mounting position may reduce the effective capacitance. The unit should be mounted in an upright position in such a way that the vent (for escaping gas) is unobstructed.

Question: True or False: When an electrolytic capacitor begins to cause circuit malfunction, it is always necessary to replace it.

Answer: False. Take the case where the only change causing the trouble is an increase in r.f. impedance of the unit, impairing its ability to bypass higher frequencies. Bridging it with a non-electrolytic type of smaller value but of the proper voltage rating (for example, a 0.1μf paper unit) will generally eliminate the trouble.

Question: True or False: If a replacement filter capacitor doesn't function properly as soon as it is put into a radio (i.e., hum is heard), it should be replaced immediately.

Answer: False. It should be given a chance to reform. When an electrolytic capacitor has been idle for several months, initial leakage current may be high enough to introduce hum because the component has deformed while not in use. If the receiver is left in operation for 20 minutes or so, the capacitor will generally be restored to normal functioning.

Question: True or False: When a capacitor that is mounted on a printed board is to be replaced, it may become necessary to consider more of its characteristics than is ordinarily the case.

Answer: True. The physical configuration may become important in addition to the electrical properties. Take the paper tubular capacitor with phenolic case shown in Fig. 5B. Parallel leads permit the unit to be plugged into a printed board, after which it is soldered in place. It would be quite difficult to fit a conventional capacitor of the same nominal value with axial leads into the proper space, in most cases.

Posted May 9, 2019