Experiment 8: A Relay Oscillator
62 Chapter 2
FundAmentAls
Capacitor basics
DC current does not flow through a capacitor, but voltage can accumulate very
quickly inside it, and remains after the power supply is disconnected. Figures
2-67 and 2-68 may help to give you an idea of what happens inside a capacitor
when it is fully charged.Figure 2-67. When DC voltage reaches a capacitor, no current flows, but the capaci-
tor charges itself like a little battery. The positive and negative charges are equal
and opposite.Figure 2-68. You can imagine positive “charge particles” accumulating on one side
of the capacitor and attracting negative “charge particles” to the opposite side.In most modern electrolytic capacitors, the plates have been reduced to two
strips of very thin, flexible, metallic film, often wrapped around each other,
separated by an equally thin insulator. Disc ceramic capacitors typically consist
of just a single disc of nonconductive material with metal painted on both sides
and leads soldered on.
The two most common varieties of capacitors are ceramic (capable of storing a
relatively small charge) and electrolytic (which can be much larger). Ceramics
are often disc-shaped and yellow in color; electrolytics are often shaped like
miniature tin cans and may be just about any color. Refer back to Figures 2-13
and 2-14 for some examples.Getting Zapped by Capacitors
If a large capacitor is charged with
a high voltage, it can retain that
voltage for a long time. Because the
circuits in this book use low voltages,
you don’t have to be concerned
about that danger here, but if you
are reckless enough to open an old
TV set and start digging around
inside (which I do not recommend),
you may have a nasty surprise. An
undischarged capacitor can kill you
as easily as if you stick your finger
into an electrical outlet. Never touch
a large capacitor unless you really
know what you’re doing.