Page 53 - Troubleshooting Analog Circuits

4. Getting Down to the

Component Level

Capacitor Problems

Earlier chapters have described a good analog troubleshooter’s mind-set, armamen-
tarium of test equipment, and requisite knowledge of resistors, inductors, and trans-
formers. Next, we reveal some of the secrets of an often-underestimated class of
components4apacitors. And much of what you need to know to troubleshoot ca-
pacitor-related problems is not in any book-it’s not even in data sheets.
Capacitors are rather remarkable. We consider capacitors, like resistors, to be
“passive.” But if you charge up a really good capacitor-such as a 47 IJ.F polypropy-
lene capacitor-to 10 V, and then take a 2-week vacation, when you come back the
voltage may not have decreased by as much as 20% or even 10%. The capacitor may
have stored and retained enough energy to run a nano-power circuit for hours or to
light an LED for a shorter interval. Calling components with such exceptional prop-
erties “passive” is more than a little unfair!
Ordinary, aluminum, electrolytic capacitors are most often used for power-supply
filtering and bypassing. In the old vacuum-tube days, electrolytic capacitors were
often used at levels of 150 V, 300 V, 500 V, or more. There are several basic prob-
lems with these old circuits. First, if the voltage across a capacitor is much higher
than 350V, the capacitor’s reliability is not nearly as good as that of units operated
below 350 V. Also, if a piece of old equipment has not been powered up for years, it
is advisable to apply the AC power gradually by cranking up the line voltage slowly
with a variable transformer so that the electrolytic film has a chance to “form” up. If
you hit it with full voltage instantly, an old capacitor may fail. Of course, if you are
hit by high voltage, you may fail, too.
At this point, I should remind you that when working on high-voltage circuits,
probe with one hand only and keep the other hand in your pocket. Avoid grounding
your body at any other place, and stand or sit on an insulating slab of dry material.
These precautions can prevent a shock from causing you serious harm. When I start
work on a high-voltage circuit, I solder a neon lamp in series with a 100 kR resistor
across the high-voltage power supply as a glowing reminder that this circuit is pow-
ered by a voltage much higher than 15 V. I mean, I stick my fingers into low-voltage
circuits all the time, but when I see the glow of a neon lamp, I stop FAST.
After you operate a high-voltage power supply at full voltage, if you turn off the
power and decide that for safety’s sake you should short out the filters with a few
hundred ohms, be careful. A few minutes later, the voltage on the capacitors may
come back up to 60 or 80 V and give you a shocking experience. The partial recovery
of voltage on a discharged capacitor is caused by “soakage,” or dielectric absorption,
which causes the dielectric of the capacitor to “remember” the voltage it was recently
charged up to. In high-voltage equipment, it is wise to install a 2 W resistor of a few
hundred kilohms across each large high-voltage filter capacitor, to bleed off the
charge continuously and decrease the chance of shocks (Ref. 1).

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