DC POWER SUPPLY
A power supply is a device that supplies electric power to an electrical load. The term is most
commonly applied to electric power converters that convert one form of electrical energy to another,
though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to
electrical energy. A regulated power supply is one that controls the output voltage or current to a
specific value; the controlled value is held nearly constant despite variations in either load current or the
voltage supplied by the power supply's energy source.
A compound DC motor connects the armature and fields windings in a shunt and a series combination to
give it characteristics of both a shunt and a series DC motor. This motor is used when both a high
starting torque and good speed regulation is needed. The motor can be connected in two arrangements:
cumulatively or differentially. An AC powered unregulated power supply usually uses a transformer to
convert the voltage from the wall outlet (mains) to a different, nowadays usually lower, voltage. If it is
used to produce DC, a rectifier is used to convert alternating voltage to a pulsating direct voltage,
followed by a filter, comprising one or more capacitors, resistors, and sometimes inductors, to filter out
(smooth) most of the pulsation. A small remaining unwanted alternating voltage component at mains or
twice mains power frequency (depending upon whether half- or full-wave rectification is used)—
ripple—is unavoidably superimposed on the direct output voltage.
BLOCK DIAGRAM OF POWER SUPPLY
TRANSFORMER
A transformer is a static electrical device that transfers energy by inductive coupling between
its winding circuits. A varying current in the primary winding creates a varying magnetic flux in
the transformer's core and thus a varying magnetic flux through the secondary winding. This
varying magnetic flux induces a varying electromotive force (EMF) or voltage in the secondary
winding.
Transformers range in size from thumbnail-sized used in microphones to units weighing
hundreds of tons interconnecting the power grid. A wide range of transformer designs are used in
electronic and electric power applications. Transformers are essential for the transmission,
distribution, and utilization of electrical energy.
CENTER TAP TRANSFORMER
In electronics, a center tap is a contact made to a point halfway along a winding of a transformer or
inductor, or along the element of a resistor or a potentiometer. Taps are sometimes used on inductors
for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one
end. A common application of this is in the Hartley oscillator. Inductors with taps also permit the
transformation of the amplitude of alternating current (AC) voltages for the purpose of power
conversion, in which case, they are referred to as autotransformers, since there is only one winding. An
example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides one or
more connections along the device's element, along with the usual connections at each of the two ends
of the element, and the slider connection. Potentiometer taps allow for circuit functions that would
otherwise not be available with the usual construction of just the two end connections and one slider
connection.
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In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that
allows both half-cycles of the AC waveform to contribute to the direct current, making it
smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a
diode bridge, but has poorer utilization of the transformer windings. Center-tapped two-diode
rectifiers were a common feature of power supplies in vacuum tube equipment. Modern
semiconductor diodes are low-cost and compact so usually a four-diode bridge is used (up to a
few hundred watts total output) which produces the same quality of DC as the center-tapped
configuration with a more compact and cheaper power transformer. Center-tapped
configurations may still be used in high-current applications, such as large automotive battery
chargers, where the extra transformer cost is offset by less costly rectifiers. Center-tapped
transformers are also used for dual-voltage power supplies. When a center-tapped transformer
is combined with a bridge (four diode) rectifier, it is possible to produce a positive and a
negative voltage with respect to a ground at the tap. Dual voltage supplies are important for all
sorts of electronics equipment.
A full-wave rectifier using two diodes and a center tap transformer.
RECTIFIER
A rectifier is an electrical device that converts alternating current (AC), which periodically
reverses direction, to direct current (DC), which flows in only one direction. The process is
known as rectification. Physically, rectifiers take a number of forms, including vacuum tube
diodes, mercury-arc valves, copper and selenium oxide rectifiers, solid-state diodes, siliconcontrolled rectifiers and other silicon-based semiconductor switches. Historically, even
synchronous electromechanical switches and motors have been used. Early radio receivers,
called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead
sulfide) to serve as a point-contact rectifier or "crystal detector".
Rectifiers have many uses, but are often found serving as components of DC power supplies and
high-voltage direct current power transmission systems. Rectification may serve in roles other
than to generate direct current for use as a source of power. As noted, detectors of radio signals
serve as rectifiers. In gas heating systems flame rectification is used to detect presence of flame.
The simple process of rectification produces a type of DC characterized by pulsating voltages
and currents (although still unidirectional). Depending upon the type of end-use, this type of DC
current may then be further modified into the type of relatively constant voltage DC
characteristically produced by such sources as batteries and solar cells.
Full-wave rectification
A full-wave rectifier converts the whole of the input waveform to one of constant polarity
(positive or negative) at its output. Full-wave rectification converts both polarities of the input
waveform to pulsating DC (direct current), and yields a higher average output voltage. Two
diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC
source (including a transformer without center tap), are needed.[3] Single semiconductor diodes,
double diodes with common cathode or common anode, and four-diode bridges, are
manufactured as single components.
Twice as many turns are required on the transformer For single-phase AC, if the transformer is
center-tapped, then two diodes back-to-back (cathode-to-cathode or anode-to-anode, depending
upon output polarity required) can form a secondary to obtain the same output voltage than for a
bridge rectifier, but the power rating is unchanged.
Full-wave rectifier using a center tap transformer and 2 diodes.
CAPACITOR
Filter capacitors are capacitors used for filtering of undesirable frequencies. They are common
in electrical and electronic equipment, and cover a number of applications, such as:
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•
•
•
•
Glitch removal on Direct current (DC) power rails
Radio frequency interference (RFI) removal for signal or power lines entering or leaving
equipment
Capacitors used after a voltage regulator to further smooth dc power supplies
Capacitors used in audio, intermediate frequency (IF) or radio frequency (RF) frequency
filters (e.g. low pass, high pass, notch, etc.)
Arc suppression, such as across the contact breaker or 'points' in a spark-ignition engine
Filter capacitors are not the same as reservoir capacitors, the tasks the two perform are different,
albeit related
The simple capacitor filter is the most basic type of power supply filter. The application of the simple
capacitor filter is very limited. It is sometimes used on extremely high-voltage, low-current power
supplies for cathode-ray and similar electron tubes, which require very little load current from the
supply. The capacitor filter is also used where the power-supply ripple frequency is not critical; this
frequency can be relatively high. The capacitor (C1) shown in figure 4-15 is a simple filter connected
across the output of the rectifier in parallel with the load.
Figure 4-15. - Full-wave rectifier with a capacitor filter.
When this filter is used, the RC charge time of the filter capacitor (C1) must be short and the RC
discharge time must be long to eliminate ripple action. In other words, the capacitor must charge up
fast, preferably with no discharge at all. Better filtering also results when the input frequency is high;
therefore, the full-wave rectifier output is easier to filter than that of the half-wave rectifier because of
its higher frequency.
For you to have a better understanding of the effect that filtering has on E avg , a comparison of a rectifier
circuit with a filter and one without a filter is illustrated in views A and B of figure 4-16. The output
waveforms in figure 4-16 represent the unfiltered and filtered outputs of the half-wave rectifier circuit.
Current pulses flow through the load resistance (R L ) each time a diode conducts. The dashed line
indicates the average value of output voltage. For the half-wave rectifier, E avg is less than half (or
approximately 0.318) of the peak output voltage. This value is still much less than that of the applied
voltage. With no capacitor connected across the output of the rectifier circuit, the waveform in view A
has a large pulsating component (ripple) compared with the average or dc component. When a
capacitor is connected across the output (view B), the average value of output voltage (E avg ) is increased
due to the filtering action of capacitor C1.
Operation of the simple capacitor filter using a full-wave rectifier is basically the same as that discussed
for the half-wave rectifier. Referring to figure 4-18, you should notice that because one of the diodes is
always conducting on. either alternation, the filter capacitor charges and discharges during each half
cycle. (Note that each diode conducts only for that portion of time when the peak secondary voltage is
greater than the charge across the capacitor.)
Figure 4-18. - Full-wave rectifier (with capacitor filter).
Another thing to keep in mind is that the ripple component (E r ) of the output voltage is an ac voltage
and the average output voltage (E avg ) is the dc component of the output. Since the filter capacitor offers
a relatively low impedance to ac, the majority of the ac component flows through the filter capacitor.
The ac component is therefore bypassed (shunted) around the load resistance, and the entire dc
component (or E avg ) flows through the load resistance. This statement can be clarified by using the
formula for X C in a half-wave and full-wave rectifier. First, you must establish some values for the circuit.
Remember, also, that the load resistance is an important consideration. If load resistance is made small,
the load current increases, and the average value of output voltage (E avg ) decreases. The RC discharge
time constant is a direct function of the value of the load resistance; therefore, the rate of capacitor
voltage discharge is a direct function of the current through the load. The greater the load current, the
more rapid the discharge of the capacitor, and the lower the average value of output voltage. For this
reason, the simple capacitive filter is seldom used with rectifier circuits that must supply a relatively
large load current. Using the simple capacitive filter in conjunction with a full-wave or bridge rectifier
provides improved filtering because the increased ripple frequency decreases the capacitive reactance
of the filter capacitor.
RESISTOR
A resistor is a passive two terminal electrical component that implements electrical resistor as a
circuit element.The current through a resistor is in direct proportion to the voltage across the
resistor's terminals. This relationship is represented by Ohm's law.
where I is the current through the conductor in units of amperes V is the potential difference
measured across the conductor in units of volts and R is the resistance of the conductor in units
of ohm .The ratio of the voltage applied across a resistor's terminals to the intensity of current in
the circuit is called its resistance, and this can be assumed to be a constant (independent of the
voltage) for ordinary resistors working within their ratings.
Resistors are common elements of electrical networks and electronic circuits and are ubiquitous
in electronic equipment. Practical resistors can be made of various compounds and films, as well
as resistance wire (wire made of a high-resistivity alloy, such as nickel-chrome). Resistors are
also implemented within integrated circuits particularly analog devices, and can also be
integrated into hybrid and printed circuits.
Variable resistor
adjust
the
resistance
between
two
A variable resistor is a
potentiometer with
only two connecting
wires instead of three.
However,
although
the actual component
is the same, it does a
very different job. The
pot allows us to
control the potential
passed through a
circuit. The variable
resistance lets us
points
in
a
circuit.
A variable resistance is useful when we don't know in advance what resistor value
will be required in a circuit. By using pots as an adjustable resistor we can set the
right value once the circuit is working. Controls like this are often called 'presets'
because they are set by the manufacturer before the circuit is sent to the customer.
A resistor constructed so that its resistance value may be changed without
interrupting the circuit to which it is connected. Also known as variable resistor.
VOLTAGE REGULATOR
A voltage regulator is designed to automatically maintain a constant voltage level. A voltage
regulator may be a simple "feed-forward" design or may include negative feedback control
loops. It may use an electromechanical mechanism, or electronic components. Depending on the
design, it may be used to regulate one or more AC or DC voltages.
Electronic voltage regulators are found in devices such as computer power supplies where they
stabilize the DC voltages used by the processor and other elements. In automobile alternators and
central power station generator plants, voltage regulators control the output of the plant. In an
electric power distribution system, voltage regulators may be installed at a substation or along
distribution lines so that all customers receive steady voltage independent of how much power is
drawn from the line.
VOLTAGE REGULATOR (7805)
PROJECT WORK
Objective: To make a 5V DC power supply and operate DC MOTOR FAN with this dc power
supply & varying the speed of fan using the variable resistor.
Requirements:
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Centre tap transformer(12-0-12)
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Two diodes (1N4007)
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Capacitor(1000µf,25V)
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Voltage regulator (7805)
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Two current limiting resistors
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Light emitting diodes
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DC motor fan
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variable resistor(0-100)
DC MOTOR
A DC motor is a mechanically commutated electric motor powered from direct current (DC). The stator
is stationary in space by definition and therefore its current. The current in the rotor is switched by the
commutator to also be stationary in space. This is how the relative angle between the stator and rotor
magnetic flux is maintained near 90 degrees, which generates the maximum torque.
DC motors have a rotating armature winding (winding in which a voltage is induced) but non-rotating
armature magnetic field and a static field winding (winding that produce the main magnetic flux) or
permanent magnet. Different connections of the field and armature winding provide different inherent
speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the
voltage applied to the armature or by changing the field current. The introduction of variable resistance
in the armature circuit or field circuit allowed speed control. Modern DC motors are often controlled by
power electronics systems called DC drives.
The introduction of DC motors to run machinery eliminated the need for local steam or internal
combustion engines, and line shaft drive systems. DC motors can operate directly from rechargeable
batteries, providing the motive power for the first electric vehicles. Today DC motors are still found in
applications as small as toys and disk drives, or in large sizes to operate steel rolling mills and paper
machines.
DC Motors Characteristics
• When the first start up, they draw a lot more current, up to 10x more.
• If you “stall” them (make it so they can’t turn), they also draw a lot of current
• They can operate in either direction, by switching voltage polarity
• Usually spin very fast: >1000 RPM
• To get slower spinning, need gearing.
How to Select a DC Motor
Selecting a DC motor for a particular application can be a rather involved process and should be
done in close consultation with MicroMo's application engineers. Hower, it is often useful to be
able to "ballpark" a motor selection on one's own. A few rules relating to the physics and the
practical application of motors should be kept in mind.
The major constraint on motor operation is thermal in nature. The heat a motor must dissipate
can always be calculated as follows:
Pdis = I2 x R
Heat dissipated = current through the motor squared, multiplied by the terminal resistance.
The current through a motor is solely determined by the torque the motor produces. Current and
torque are related by the torque constant of the motor.
I = Mo / kM
Current through motor = torque produced divided by the torque constant