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Electrical Power Supplies
Unit 5
Background
• You already learned in Unit 2 that conductors are materials that allow
electricity to flow through them. It is important to remember that
conductors are not perfect. All materials block, or resist, the flow of current
to some degree.
• Resistance is the measure used to tell how much a material blocks the flow
of current. Resistance is measured in units called ohms.
• Electrons flow only when there is a difference in potential (voltage) across
the ends of a conductor.
• Voltage drop is the difference between the two ends of a conductor through
which electrons are flowing. Voltage drop is due to the resistance of a circuit.
• In a circuit, the flow of electrons will continue until both
ends of the conductor reach a common potential. To
maintain a flow of electrons in a conductor, some
arrangement must be provided to maintain a difference in
potential while electrons flow from one end to the other.
• A sustained electric current requires some sort of “electrical
pump” to maintain this difference in potential. A device that
provides this potential difference is known as a voltage
source.
Direct Current (DC)
• Electrical current can flow in either of two directions through a conductor.
If the current flow, either steadily or in pulses, in only one direction, it is
called direct current (DC).
• There a number of ways to produce direct current. One way is to use
chemical generators. For example, if you dissolve table salt in water, the
salt will break down into positive sodium ions and negative chloride ions. If
two dissimilar metal plates (such as zinc and copper) are immersed in the
salt solution, the positive ions will migrate toward one plate which is
positive (copper) and the negative ions will migrate toward the other plate
which is negative (zinc). If the two plates are connected by a conductor, a
current will flow through the solution (as ions) and through the conductor
(as electrons). The solution is called the electrolyte. This kind of chemical
generator is called a wet cell. Cells in which the electrolyte is absorbed by
paper or formed into a paste are called dry cells. You can connect two or
more cells together to form a battery.
A battery produces direct current in a
circuit. Electrons always move from the
repelling negative terminal toward the
attracting positive terminal. However,
the conventional method for current
flow is from positive terminal to
negative terminal, opposite to the
electron flow. The MB100 Kit contains
four 1.5 volt (AA) batteries. These
batteries are composed of an electrolyte
paste and a carbon rod in a zinc
container.
Batteries will produce an electric current
only as long as chemical reactions within
the battery take place. When the
chemical reactants are used up, the
battery will no longer produce a current
and we commonly say the battery is
“dead”.
Alternating Current (AC)
• Alternating current (AC) acts as it name indicates – it
alternates in direction or polarity. Electrons in the
circuit are moved first in one direction and then in the
opposite direction, alternating back and forth within
relatively fixed positions. This movement of electrons
is accomplished by alternating the direction of voltage
at the power source. Alternating current is usually
produced by rotating a coil in a magnetic field.
Alternating current is better
suited than direct current for
transmission through power
lines because AC can be
transmitted long distances at
high voltages and low
currents, lowering heat loss in
the wires. Nearly all
commercial AC circuits in
North America have voltages
and currents that alternate
back and forth at a frequency
of 60 cycles per second.
Generators
• Electric generators are devices which convert mechanical energy into
electrical energy. In 1831, Michael Faraday made one of the first DC
generators. He took a copper disk, mounted it on a shaft, then
rotated it between the poles of a horseshow magnet. He found that
he had a direct, continuous voltage between a contact on the shaft
and one on the outer edge of the disk. Faraday’s generator, however,
had a low output voltage and was very inefficient.
• Thomas Edison’s first direct current generator was displayed at the
Paris World Exhibition in 1881. A year later Edison set up a
commercial electric generating station in New Your City to provide
power for the first electric street lights.
Two Main Types of Generators
• DC Generator
• DC generator rotates the
conductor in a stationary
magnetic field.
• AC Generator
• AC generator rotates a magnetic
field that is cut by the stationary
conductors.
The current induced in the conductors of all generators is alternating
current. However, the current taken from the generator may be AC or
DC , depending on how the generator is constructed.
DC GENERATOR
• DC generators have a commutator mounted on one end of an
armature shaft. The commutator changes (or rectifies) the alternating
current in the conductors into direct current. Brushes mounted in
holders ride on the rotating commutator bars and carry the direct
current from the commutator to the external circuit.
The DC generator in an
automobile consists of an
armature mounted on a
shaft. The armature is
composed of wires coiled
around slots in a soft iron
core and is rotated
between magnetic field
poles and connected to a
commutator. It delivers
direct current to external
terminals.
AC Generator/DC
Generator
An AC generator is the
same as a DC generator
except instead of a
commutator it uses
continuous rings that are
mounted side by side and
insulated from the shaft.
One end of the coil is
attached to either of the
two rings. The voltage is
picked up between them.
DIODES
• A diode is an electrical device
allowing current to move through
it in one direction with far greater
ease than in the other. The most
common kind of diode in modern
circuit design is the semiconductor
diode.
• Diode behavior is analogous to the
behavior of a hydraulic device
called a check valve. A check valve
allows fluid flow through it in only
one direction as in Figure
Diode Summary
• A diode is an electrical
component acting as a one-way
valve for current.
• In the Figure here 
(b) Is the schematic
representation of a diode and (c)
is the real component
appearance.
Rectifiers
• A rectifier is a circuit that converts alternating current (AC) to direct current
(DC) This means that the component permits passage of only the positive
or negative portion of an alternating current. All rectifiers perform the
same basic function of rectifying an electric current, that is changing an
alternating current containing both positive and negative components into
a direct current that has only a positive or negative component.
• This conversion is critical for all sorts of household electronics. AC signals
come out of your house’s wall outlets, but DC is what powers most
computers and other microelectronics.
• Current in AC circuits literally alternates – quickly switches between
running in the positive and negative directions – but current in a DC signal
only runs in one direction. So to convert from AC to DC you just need to
make sure current can’t run in the negative direction. Sounds like a job for
DIODES!
Half-Wave Rectifer
• A half-wave rectifier can
be made out of just a
single diode. If an AC
signal, like a sine wave
for example, is sent
through a diode any
negative component to
the signal is clipped out.
Input (red/left) and output (blue/right) voltage waveforms,
after passing through the half-wave rectifier circuit (middle).
Full Wave Rectifier
• A full-wave bridge rectifier uses
four diodes to convert those
negative humps in the AC signal
into positive humps.
• These circuits are a critical
component in AC-to-DC power
supplies, which turn the wall
outlet’s 120/240VAC signal into
3.3V, 5V, 12V, etc. DC signals. If you
tore apart a wall adapter, you’d
most likely see a handful of diodes
in there, rectifying it up.
The bridge rectifier circuit (middle), and the output wave form it
creates (blue/right).
Can you spot the four diodes making a bridge rectifier in this?
Impedance
• Impedance is the apparent resistance to current flow
in AC circuits that corresponds to true (ohmic)
resistance in DC circuits. Impedance (Z) is measured in
ohms and is related to the voltage (V) and the current
(I) by the equation.
• Impedance = Voltage / Current
or
Z=V/I
Many electronic components, such as
loudspeakers, radio transmitters, and generators
are rated according to their impedances. This
enables users to match impedances on
components easily. For example, in a home
television installation the best reception is
obtained when the antenna and the cable are
selected to match the impedance of the receiver.
Voltage Regulator
A voltage regulator is a device used to control the voltage in a DC
circuit. The regulator establishes the maximum voltage level for the
circuit. However, the voltage in the circuit can never exceed the
voltage of the power supply (unless some sort of amplifier is used). For
example, a 5 VDC voltage regulator connected to a 12 VDC power
supply will limit the voltage in the circuit to 5 VDC. Motor vehicles use a
voltage regulator to control the charging rate of the battery by the
vehicles' generator. Without the use of a voltage regulator, the battery
could be damaged by overcharging.