Page 56 - Troubleshooting Analog Circuits

Extended Foil Offers Extensive Advantages 43

set, the "clearing" of the shorts would make the signals very noisy. Likewise. when
used as audio coupling capacitors, "dry" tantalum capacitors would sometimes make
a lot of noise as they "cleared" their leaky spots. These parts have therefore become
unpopular for audio coupling. Similarly. you might use an electrolytic capacitor with
a small reverse voltage-perhaps 0.5 V-with no harm or problems. BUT a friend
told me of the time he was using an electrolytic Capacitor as an audio-coupling capac-
itor with 2 V of reverse bias. Because of the reverse bias. it was producing all sort.~ of
low-frequency noise and jitter. So, excess noise is often a clue that something is
going wrong-perhaps it is trying to tell you about a misapplication. or a part in-
stalled backwards.

Extended Foil Offers Extensive Advantages

Another aspect of the film capacitor is whether or not it uses "extended-foil" con-
struction. The leads of many inexpensive wound-foil capacitors are merely connected
to the tip ends of the long strip of metal foil. However, in an extended-foil capacitor.
the foils extend out on each side to form a direct low-resistance, low-inductance path
to the leads.
This construction is well suited for capacitors that must provide low ESR (equiva-
lent series resistance) in applications such as high-frequency filters. Then if you
substituted a capacitor without extended foil, the filter's performance would be dras-
tically degraded.
So there are several methods of construction and several dielectrics that are impor-
tant considerations for most capacitor applications. If an aggressive purchasing agent
wants to do some substituting to improve cost or availability. the components engi-
neer or design engineer may have to do a lot of work to make sure that the substitu-
tion won't cause problems. If a substitution is made, the replacement part is a good
place to start looking for trouble. A capacitor with higher-than-planned-for ESR can
cause a feedback loop to oscillate-for example, when a capacitor without cxtended-
foil construction is substituted for one with such construction. Substitution of capaci-
tors with higher ESR than the designer intended can also cause filters to fail to prop-

BODY OF
END CAP CAPACITOR END CAP
/
BUVY ut

FILM
LEAD
Figure 4. I (a) When the tabs connect to one end of a long foil, some elements of the capacitor will be
IO or 20 feet away from the leads. The series Rs and Ls are poor. This construction was
adequate for low-fidelity audio circuits, but is uncommon these days. (b) When the exposed
edges of the extended foils are crimped together, no element of the capacitor is more than an
inch or two from the leads and connections. Most film capacitors are made with extended
foils these days.