Page 55 - Troubleshooting Analog Circuits

42 4. Getting Down to the Component Level: Capacitor Problems

though this leakage is usually quite low, nobody wants to have to measure it in pro-
duction, nor to guarantee it for the lifetime of the component.
Wound-film and stacked-film capacitors cover wide ranges, from small signal-
coupling capacitors to large high-power filters. The different dielectrics are their
most interesting ingredients. Often a designer installs a polyester capacitor (techni-
cally, polyethylene terephthalate, often called Mylar-a trademark of E. I. DuPont de
Nemours and Co.) and wonders why something in the circuit is drifting 2 or 3% as the
circuit warms up. What’s drifting is probably the polyester capacitor; its TC of 600 to
900 ppm/OC is 10 times as high as that of a metal-film resistor.
If you give up on polyester and go to polystyrene, polypropylene, or Teflon, (also
a trademark of DuPont) the TC gets better-about -120 ppm/“C. Polystyrene and
polypropylene have low leakage and good dielectric absorption-almost as good as
Teflon’s, which is the best (Ref. 1). But Teflon is quite expensive and rather larger in
package size than the other types. Be careful with polystyrene; its maximum temper-
ature is +85 “C, so you might damage it during ordinary wave-soldering unless you
take special precautions to keep the capacitors from over-heating. Polycarbonate,
polysulfone, and polyphenylene have good TCs of about +lo0 ppm/”C, and their
names have enough syllables that they sound as if they should be pretty good, but
actually they have inferior soakage. Glass and porcelain are dielectrics that sound
like they ought to have some really fancy characteristics, and excellent dielectric
absorption. But they don’t, not very good at all. Many years ago, wound-film capaci-
tors were made with oil-impregnated paper, but you won’t see them unless you are
working on ancient radios. They were pretty crummy, just adequate for audio coupling
on low-fidelity radios.

Foiled Again!
Now let’s discuss the difference between a polyester foil capacitor and a metallized
polyester capacitor. The foil capacitor is made of alternating layers of film and foil,
where both the delicate film and the metal foil are just a couple of tenths of a milli-
inch thick. This construction makes a good capacitor at a nominal price and in a
nominal size. The metallized-film capacitor is made with only a very thin film of
polyester-with the metal deposited on the polyester in a very thin layer. This con-
struction leads to an even smaller size for a given capacitance and voltage rating, but
the deposited metal is so thin that its current-carrying capacity is much less than that
of the metal in the foil capacitor. This offers advantages and disadvantages. If a pin-
hole short develops in this metallized-polyester capacitor’s plastic film, the metal
layer in the vicinity of the pinhole will briefly carry such a high current density that it
will vaporize like a fuse and “clear” the short.
For many years, metallized polyester capacitors were popular in vacuum-tube
television sets because they were small and cheap. These metallized capacitors would
recover from pinhole flaws not just once but several times. However, at low voltages,
the energy stored in the capacitors would often prove insufficient to clear a fault.
Thus, the capacitors’ reliability at low voltages was often markedly worse than it was
at their rated voltage. You could safely use a cheap, compact, metallized-polyester
capacitor in a 100-V TV circuit but not in a 2-V circuit. Fortunately, there are now
classes of metallized-polycarbonate, metallized-polyester, and metallized-polypropy-
lene capacitors that are reliable and highly suitable for use at both low and high volt-
ages. I was reading one of these data sheets the other day, and it said that at low volt-
ages, any pin-hole fault is cleared by means of oxidation of the ultra-thin metal film.
When the old metallized-polyester capacitors began to become unreliable in a TV