Handbook for Sound Engineers

Resistors, Capacitors, and Inductors 263

supercapacitors to replace the battery as we know it
today.
Presently supercapacitors need batteries to store the
energy and are basically used as a buffer between the
battery and the device. Supercapacitors can be charged
and discharged hundreds of thousands of times where a
battery cannot do that.

• Calculating Backup Time. To calculate the desired
  backup time the supercapacitor will provide if the
  power goes off, the starting and ending voltage on the
  capacitor, the current draw from the capacitor, and
  the capacitor size must be known.
  Assuming that the load draws a constant current
  while running from VBACKUP, then the worst-case
  backup time in hours would use the equation:

(10-24)

where,

C is the capacitor value in farads,

VBACKUPSTART is the initial voltage in volts. The

voltage applied to VCC, less the voltage drop from

the diodes, if any, used in the charging circuit,

VBACKUPMIN is the ending voltage in volts,

IBACKUPMAX is the maximum VBACKUP current in

amperes.

For example, to determine how long the backup

time will be under the following conditions:

• 0.2 F capacitor


• VBACKUPSTART is 3.3 V


• VBACKUPMIN is 1.3 V


• IBACKUPMAX is 1000 nA, then:

10.3 Inductors

Inductance is used for the storage of electrical energy in
a magnetic field, called magnetic energy. Magnetic
energy is stored as long as current keeps flowing
through the inductor. The current of a sine wave lags the
voltage by 90° in a perfect inductor. Figure 10-23 shows
the color code for small inductors.

10.3.1 Types of Inductors

Inductors are constructed in a variety of ways,

depending on their use.

10.3.1.1 Air Core Inductors

Air core inductors are either ceramic core or phenolic

core.

10.3.1.2 Axial Inductor

An axial inductor is constructed on a core with concen-

tric leads on opposite ends, Fig.10-24A. The core mate-

rial may be phenolic, ferrite, or powdered iron.

10.3.1.3 Bobbin Core Inductor

Bobbin core inductors have the shape of a bobbin and

may come with or without leads. They may be either

axial or radial, Fig. 10-24B.

10.3.1.4 Ceramic Core

Ceramic core inductors are often used in high

frequency applications where low inductance, low core

losses, and high Q values are required. Ceramic has no

magnetic properties so there is no increase in permea-

bility due to the core material.

Backup time

CV (^) BACKUPSTART – VBACKUPMIN
IBACKUPMAX

3600
=-------------------------------------------------------------------------------------
Backup time
0.2 3.3 – 1.3
10 6–

300
=---------------------------------
=111.1 h
Figure 10-23. Color code for small inductors (in “H).
Mil spec (if required—larger band)
1st digit
2nd digit
Multiplier
Tolerance (narrower)
Color 1st 2nd Multiplier Tolerance
Black 0 0 1
Brown 1 1 10
Red 2 2 100
Orange 3 3 1000
Yellow 4 4 10,000
Green 5 5
Blue 6 6
Violet 7 7
Gray 8 8
White 9 9
Gold – – ±5%
Silver – – – ±10%
No band – – – ±20%
Inductance
digits