[ The PC Guide | Introduction to
the PC | PC Fundamentals ]
Basic Electrical Components
There are several important basic electrical components that are commonly found in the
circuits of virtually all PC parts and peripherals. These devices are the fundamental
building blocks of electrical and electronic circuits, and can be found in great numbers
on motherboards, hard disk logic boards, video cards and just about everywhere else in the
PC, including places that might surprise you. They can be used and combined with each
other and dozens of other devices, in so many different ways that I could not even begin
to describe them all. Still, it is useful to know a bit about how they work, and this page
will at least provide you with a basis for recognizing some of what you see on those
boards, and perhaps understanding the fundamentals of circuit schematics. Bear in mind
when reading the descriptions below that it would really take several full pages to fully
describe the workings of most of these components! Fortunately, this level of detail isn't
really necessary to provide the background necessary when working with PCs.
For each component, I provide a sample photo, as well as an illustration of the
component's symbol in an electrical schematic (diagram showing how a circuit is designed).
There are many variants of each of the components shown below; so the diagrams should only
be considered examples.
- Battery: A direct current electricity source of a specific voltage,
used primarily in small circuits.
A battery (in this case, a button cell on a PC
Original photo © Kamco Services
Image used with permission.
- Resistor: As you could probably guess from the name, a resistor
increases the resistance of a circuit. The main purpose of this is to reduce the flow of
electricity in a circuit. Resistors come in all different shapes and sizes. They dissipate
heat as a result of their opposing electricity, and are therefore rated both in
terms of their resistance (how much they oppose the flow of electrons) and their power
capacity (how much power they can dissipate before becoming damaged.) Generally, bigger
resistors can handle more power. There are also variable resistors, which can have their
resistance adjusted by turning a knob or other device. These are sometimes called potentiometers.
Magnified surface-mount resistor from a motherboard.
These small resistors are now much more common on PC
electronics than the older, larger pin type.
Note the "R10" designation.
- Capacitor: A capacitor is a component made from two (or two sets of)
conductive plates with an insulator between them. The insulator prevents the plates from
touching. When a DC current is applied across a capacitor, positive charge builds on one
plate (or set of plates) and negative charge builds on the other. The charge will remain
until the capacitor is discharged. When an AC current is applied across the capacitor, it
will charge one set of plates positive and the other negative during the part of the cycle
when the voltage is positive; when the voltage goes negative in the second half of the
cycle, the capacitor will release what it previously charged, and then charge the opposite
way. This then repeats for each cycle. Since it has the opposite charge stored in it each
time the voltage changes, it tends to oppose the change in voltage. As you can
tell then, if you apply a mixed DC and AC signal across a capacitor, the capacitor will
tend to block the DC and let the AC flow through. The strength of a capacitor is called capacitance
and is measured in farads (F). (In practical terms, usually microfarads and the
like, since one farad would be a very large capacitor!) They are used in all sorts of
electronic circuits, especially combined with resistors and inductors, and are commonly
found in PCs.
Three capacitors on a motherboard.
The two large capacitors in the background are 1500 microfarads
and 2200 microfarads respectively, as you can clearly see from
their labeling. The small silver-colored capacitor in the foreground is
a 22 microfarad electrolytic capactor. Electrolytics are commonly used in
computers because they pack a relatively high capacitance into a small
package. The plus sign indicates the polarity of the capacitor, which also has its
leads marked with "+" and "-". If you look closely you can see
the "+" marking
on the motherboard, just to the left of the capacitor. Note that very small
capacitors are also found in surface-mount packages just like the resistor above.
- Inductor: An inductor is essentially a coil of wire. When current flows
through an inductor, a magnetic field is created, and the inductor will store this
magnetic energy until it is released. In some ways, an inductor is the opposite of a
capacitor. While a capacitor stores voltage as electrical energy, an inductor stores
current as magnetic energy. Thus, a capacitor opposes a change in the voltage of a
circuit, while an inductor opposes a change in its current. Therefore, capacitors block DC
current and let AC current pass, while inductors do the opposite. The strength of an
inductor is called--take a wild guess--its inductance, and is measured in henrys
(H). Inductors can have a core of air in the middle of their coils, or a ferrous (iron)
core. Being a magnetic material, the iron core increases the inductance value, which is
also affected by the material used in the wire, and the number of turns in the coil. Some
inductor cores are straight in shape, and others are closed circles called toroids.
The latter type of inductor is highly efficient because the closed shape is conducive to
creating a stronger magnetic field. Inductors are used in all sorts of electronic
circuits, particularly in combination with resistors and capacitors, and are commonly
found in PCs.
A toroidal core inductor from a PC motherboard.
The two bars in the symbol represent the iron core;
an air-core inductor would not have the bars.
Note that very small inductors are also found in
surface-mount packages just like the resistor above.
- Transformer: A transformer is an inductor, usually with an iron core,
that has two lengths of wire wrapped around it instead of one. The two coils of wire do
not electrically connect, and are normally attached to different circuits. One of the most
important components in the world of power, it is used to change one AC voltage into
another. As described above, when a coil has a current passed through it, a magnetic field
is set up proportional to the number of turns in the coil. This principle also works in
reverse: if you create a magnetic field in a coil, a current will be induced in it,
proportional to the number of turns of the coil. Thus, if you create a transformer with
say, 100 turns in the first or primary coil, and 50 turns in the second or secondary
coil, and you apply 240 VAC to the first coil, a current of 120 VAC will be induced in the
second coil (approximately; some energy is always lost during the transformation). A
transformer with more turns in its primary than its secondary coil will reduce voltage and
is called a step-down transformer. One with more turns in the secondary than the
primary is called a step-up transformer. Transformers are one of the main reasons
we use AC electricity in our homes and not DC: DC voltages cannot be changed using
transformers. They come in sizes ranging from small ones an inch across, to large ones
that weigh hundreds of pounds or more, depending on the voltage and current they must
A transformer from the interior of a PC power supply.
Note the large heat sink fins above and below it.
- Diode / LED: A diode is a device, typically made from semiconductor
material, that restricts the flow of current in a circuit to only one direction; it will
block the bulk of any current that tries to go "against the flow" in a wire.
Diodes have a multitude of uses. For example, they are often used in circuits that convert
alternating current to direct current, since they can block half the alternating current
from passing through. A variant of the common diode is the light-emitting diode
or LED; these are the most well-known and commonly-encountered kind of diode,
since they are used on everything from keyboards to hard disks to television remote
controls. An LED is a diode that is designed to emit light of a particular frequency when
current is applied to it. They are very useful as status indicators in computers and
battery-operated electronics; they can be left on for hours or days at a time because they
run on DC, require little power to
operate, generate very little heat and last for many years even if run continuously. They
are now even being made into low-powered, long-operating flashlights.
A diode (top) and a light-emitting diode (bottom). Note
symbol on the circuit board above the diode, and the "CR3"
designation. The LED shown is an older, large diode from a
system case. LEDs are now more often round and usually smaller.
- Fuse: A fuse is a device designed to protect other components from
accidental damage due to excessive current flowing through them. Each type of fuse is
designed for a specific amount of current. As long as the current in the circuit is kept
below this value, the fuse passes the current with little opposition. If the current rises
above the rating of the fuse--due to a malfunction of some sort or an accidental
short-circuit--the fuse will "blow" and disconnect the circuit. Fuses are the
"heroes" of the electronics world, literally burning up or melting from the high
current, causing a physical gap in the circuit and saving other devices from the high
current. They can then be replaced when the problem condition has been corrected.
All fuses are rated in amps for the amount of current they can tolerate before blowing;
they are also rated for the maximum voltage they can tolerate. Always replace a blown
fuse only with another of the same current and voltage rating.
A fuse, sitting in its fuse holder,
from the interior of a PC power supply.
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