Page 54 - Troubleshooting Analog Circuits

Nonpolar Capacitors Can Be a Bear 41

The last problem with old vacuum-tube equipment is that the heat tends to dry up
the capacitors’ electrolyte, thus causing their capacitance to decrease. This decrease
is evidenced by excessive ripple, or “hum,” on various signals and, of course, on the
power-supply output of unregulated supplies. Although I have presented these mal-
adies as problems afflicting old equipment, you should consider them even in new
designs.
In modem power-supply designs, it is critical that you choose a filter capacitor
whose effective series impedance is low at all rated temperatures and frequencies.
Otherwise the rms filter current multiplied by the resistive component of the series
impedance can cause excessive self-heating. And if the heat can’t flow out of the
capacitor, the temperature will rise and cause early failure. Excessive heating is one
of the most common causes of poor reliability in electrolytic capacitors.
For instance, at 120 Hz, which is the frequency of the ripple current flowing in the
filter capacitors that follow a full-wave rectifier operated from a 60-Hz AC source,
some manufacturers rate their capacitors at 2 A rms for each loo0 kF. Because the
rms current in the capacitor is nearly 2 A rms when the DC output is 1 A, this rating
is consistent with the rule of thumb for an ordinary full-wave bridge rectifier: Provide
at least loo0 pF of filter capacitance for each 1 A of DC output. At 20 or 40 kHz,
which is the ripple-current frequency in many switch-mode power-supply filters, the
capacitor will have a higher series resistance. Thus, that loo0 pF capacitor won’t be
suitable for handling even 1 A rms. If you insist on using a 120-Hz-rated capacitor as
a filter in a switch-mode supply, you will probably have to contact the capacitor
vendor for data or advice on de-rating.
Of course, if you install an electrolytic capacitor with reversed polarity and apply
working voltage, the reliability will be poor and the failure mode will probably be
dramatic. So, please be careful working with big power supplies and big filter capaci-
tors that store large amounts of energy. Wear protective goggles or glasses with
safety lenses for protection because a capacitor in a high-energy supply might decide
to blow up while you are peering at it. In fact, a friend of mine pointed out that a 6-V
electrolytic capacitor of even a few microfarads can blow out as explosively as a
shotgun blast if you apply 6 V DC of the wrong polarity or 6 V AC to it. So, again, be
very careful with your polar electrolytic capacitors.

Nonpolar Capacitors Can Be a Bear

You can buy nonpolar electrolytic capacitors made of either aluminum or tantalum.
They are bigger and more expensive than ordinary, polar capacitors, so they are fairly
uncommon. But, have you seen the little 3-leaded electrolytic types recently brought
to market? The lead in the center is the positive terminal and the other two leads are
negative. This configuration not only gives you lower inductance but also allows you
to insert the device into a board two ways-and both are correct-neither way is
wrong!
Tantalum capacitors have many characteristics similar to those of aluminum elec-
trolytic capacitors; and, for the extra price you pay, you can get less leakage and
somewhat lower series resistance. Designers often try out a timing circuit using a
tantalum capacitor and a high-value resistor. But when they try to buy a tantalum
capacitor with leakage guaranteed low enough to make the circuit work every time,
they get quite angry when nobody is interested in selling such a device. Of course. if
you were a manufacturer of tantalum capacitors and someone asked you to measure
the leakage, you would refuse the business, too, because testing is so difficult. Even