ENGR_FE_6_BasicCircu..

surprisesameSemiconductor

Nov 1, 2013 (3 years and 5 months ago)

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Basic Circuit
Components


Created by Jesse
Kuzy

for


Resistors


A resistor is a circuit component which has electrical
resistance; it slows the movement of electrons through it.


Resistors dissipate electrical energy, converting it to heat.

Resistors in Circuits


Resistors lower voltage across an active circuit; the voltage on
the positive end will be higher than the voltage on the
negative end.


The voltage across is a resistor is proportional to the current
flowing through it.


The symbol for resistors is a zigzagging line. It resembles a
type of resistor called the wire
-
wrapped resistor, which is wire
wrapped around a ceramic core.

Capacitors


Capacitors are circuit components that store electrical charge.


Capacitors have two conductors separated by an insulator
called the dielectric.


When there is an electric potential across the capacitor (a
difference in the voltage), electrons cannot flow across the
gap; instead, one end becomes positively charged and the
other becomes negatively charged, and an electric field forms
between the conductors.



Capacitors in Circuits


When a circuit first comes on, the charge in the capacitor
begins to build. Electrons gathering on one end and vacating
the other create a temporary current as they move. As they
do so, the voltage across the capacitor increases and the
current decreases.


After the circuit has been on for a long time (steady
-
state),
there is a voltage across the capacitor and no current through
it. At steady
-
state conditions, a capacitor acts like a break in
the circuit.


The symbol for a capacitor is like two plates near one another;
this resembles the construction of basic capacitors.




[Picture of plate
capacitor]

Inductors


An inductor is a circuit element that develops a magnetic field
as current flows through it. This field resists and slows the
movement of electrons in the inductor.


Most inductors consist of coiled wire.


Inductors in Circuits


The amount that an inductor resists electrical current depends
on the voltage difference across it, and vice
-
versa.


When a circuit first comes on, the voltage across an inductor is
high and no current flows through it.


Over time, the voltage drops and the current through the
inductor increases as the magnetic field develops.


At steady
-
state conditions, there is no voltage across an
inductor and current flows through at a constant rate. An
inductor behaves like wire at steady
-
state.


The symbol for an inductor is like coiled wire.


Comparing Inductors and
Capacitors


The properties of inductors and capacitors are complements
in many ways. Consider:

Circuit has

just

come on

Circuit has

come on
recently

Steady
-
state

Capacitor

Capacitor

has no
voltage and current
flows freely

Voltage increases,
current decreases,
electric

field forms

Capacitor has a
voltage and no
current flows across

Inductor

Inductor has a
voltage and no
current flows across

Voltage decreases,
current increases,
magnetic field forms

Inductor has no
voltage and current
flows freely

Comparing Resistors to
Inductors and Capacitors


Inductors and capacitors act differently when a circuit is going
from off to on or from on to off than when the circuit has
been on for a long time. This is called transient behavior.
Generally speaking, their behavior is time
-
dependent.


By contrast, resistors act the same at steady
-
state as they do
in changing systems.


Inductors and capacitors behave differently in AC circuits than
in DC circuits. Their behaviors are much more complicated in
AC circuits, where the current and voltage are constantly
fluctuating. Naturally in DC circuits, where the current and
voltage stay the same, they are much less complicated.
Resistors, because they are unaffected by current and voltage
changes, behave the same way in both AC and DC circuits

Ideal vs Real Components


Resistance, capacitance, and inductance are properties that all
circuit elements have. Well
-
designed elements tend to focus
on just one of these. It is possible to have a component
designed to focus on more than one property.


When represented in circuit diagrams, elements only have the
property that they are designed for; resistors don’t have
capacitance, inductors don’t have resistance, and so on.


If an actual component does have two or more of these
properties to a significant degree, it is often represented in
diagrams by multiple elements which together account for all
of the component’s properties. This keeps circuit analysis
clean and simple.


For example, if an inductor has a non
-
negligible resistance, it
may be represented in a diagram as an inductor in series with
a resistor.

Semiconductors


Semiconductors are materials that fall between conductors
and insulators.


They may act as insulators in some conditions and as
conductors in others.


Semiconductors can be doped; this is when another substance
is added to the semiconductor to change its properties.


Donor
dopants

produce an excess of electrons in the
semiconductor. Semiconductors doped with donors are called
n
-
type.


Acceptor
dopants

produce an excess of positive “holes” where
there are no electrons. Semiconductors doped with acceptors
are called p
-
type.

Diodes


A diode is a circuit element which essentially is a resistor with
polarity; it has a different resistance in one direction than in the
other.


Most diodes have no resistance in one direction and very high
resistance in the other, so that they only allow current to flow in one
direction. These diodes are called rectifiers.


Recall that semiconductors may change from insulators to
conductors under certain conditions. For semiconductor diodes, the
diode behaves as an insulator until a certain voltage is achieved
across the diode. It then behaves as a conductor, allowing current to
pass. When this happens, the diode is forward
-
biased.


The symbol for a diode looks like an arrow that points in the
direction of current flow. The diode shown below would allow
current to flow from left to right.



Transistors


Transistors are circuit components made of semiconductors
that amplify and switch currents.


A good example of how transistors work is the Bipolar
Junction Transistor (BJT). In the NPN BJT, a layer of p
-
type
semiconductor separates two sections of n
-
type
semiconductor. When there is a voltage across the two n
-
type
layers, no current can pass through. When positive voltage is
applied to the p
-
type layer, however, the transistor becomes
conductive, and current can pass through.


In PNP transistors, two p
-
type semiconductors are separated
by n
-
type semiconductor material. When positive voltage is
applied to the n
-
type layer, it is closed; when negative voltage
is applied, it is open.


Parts of a Transistor


The terminal that
receives current is
called the collector.


The terminal that
releases current is
called the emitter.


The terminal that
controls whether the
transistor is on is
called the base.

Collector

Emitter

Base