Make Electronics

Switching Basics and More 63

Experiment 8: A Relay Oscillator

FundAmentAls

Capacitor basics (continued)

Ceramic capacitors have no polarity, meaning that you can apply negative volt-
age to either side of them. Electrolytics do have polarity, and won’t work unless
you connect them the right way around.

The schematic symbol for a capacitor has two significant variants: with two
straight lines (symbolizing the plates inside a capacitor), or with one straight
line and one curved line, as shown in Figure 2-69. When you see a curved line,
that side of the capacitor should be more negative than the other. The schemat-
ic symbol may also include a + sign. Unfortunately, some people don’t bother
to draw a curved plate on a polarized capacitor, yet others draw a curved plate
even on a nonpolarized capacitor.

Figure 2-69. The generic schematic for a capacitor is on the left. The version on the
right indicates a polarized capacitor which requires its left plate to be “more posi-
tive” than its right plate. The plus sign is often omitted.

Figure 2-70. A tantalum capacitor was plugged into this breadboard, accidentally
connected the wrong way around to a power source capable of delivering a lot of
current. After a minute or so of this abuse, the capacitor rebelled by popping open
and scattering small flaming pieces, which burned their way into the plastic of the
breadboard. Lesson learned: observe polarity!

Capacitor Polarity

You must connect an electrolytic

capacitor so that its longer wire is

more positive than its shorter wire.

The shell of the capacitor is usually

marked with a negative sign near the

shorter wire.

Some capacitors may behave badly

if you don’t observe their polarity.

One time I connected a tantalum

capacitor to a circuit, using a power

supply able to deliver a lot of current,

and was staring at the circuit and

wondering why it wasn’t working

when the capacitor burst open and

scattered little flaming fragments of

itself in a 3-inch radius. I had forgot-

ten that tantalum capacitors can be

fussy about positive and negative

connections. Figure 2-70 shows the

aftermath.