■ CAPACITOR FUNDAMENTALS
Is a FARAD?
One farad (1 F) is a large value as capacitors go, but
what does that mean? Well, if a 1 F capacitor is
charged up to 12 V, it contains enough energy to accelerate a 165-gram Frisbee™ to 67 miles per hour! That’s a
pretty stiff toss! Similarly, to get that 1 F of capacitance,
two plates separated by air and only 1/10 mm apart would
have to be 11,300,000 square meters in area! That’s 4. 36
RP except in very low-power and high-impedance circuits.
FIGURE 3. Roll capacitors are made from strips of metal and
dielectric materials rolled into a cylinder.
be ignored, but as the frequency increases, so does its
reactance. In fact, at a sufficiently high frequency, LS and
C form a series-resonant circuit. This is the self-resonant
frequency, fO of the capacitor. Above fO, the capacitor acts
more like a small inductor! RS (also called ESR for
Equivalent Series Resistance) acts just like a separate
resistor, dissipating a little energy as heat when current
flows. RS can be as high as several tens of ohms, but is
generally only important when the capacitor current is
high. RP provides a leakage path for current across the
dielectric and is typically in the megohms. You can ignore
There are two common ways of efficiently making capacitors: the Roll and the Stack. Figure 3 shows a typical roll
capacitor — two strips of very thin aluminum foil separated by
a dielectric. After leads are attached to the foil strips, the
sandwich is rolled up and either placed in a metal can or coated with plastic. Radial leads both stick out of one end. Axial
leads stick out from both ends along the roll’s axis. Because
of the long rolled strips, the roll capacitor’s LS is high.
The materials that make up a capacitor expand
and contract with temperature, meaning that the
capacitance changes with temperature, as well. In many
applications, such as IC bypassing or DC blocking, a
change of a few percent is of no concern. However, in a
timing circuit or an oscillator, it’s a big deal!
A capacitor’s change in value with temperature is
called its temperature coefficient or tempco and is denoted by an industry standard code. (The nominal value is
specified at 25°C.) The three most common tempcos are:
Z5U = +22%/-56% change over - 10°C to + 85°C
X7R = ±15% change over - 55°C to +125°C
NP0 = 0± 30 ppm/°C over - 55°C to + 125°C
As you can see, the Z5U rating allows quite a bit
of change, but these are inexpensive capacitors used
for garden variety applications or where temperature
won’t be changing much. The NP0 (that’s a zero, not
a capital O) capacitors are only used where holding
a steady value is important. You can find a nice display
of temperature coefficients at www.niccomp.com/
Circle #96 on the Reader Service Card.