AC/DC POWER SUPPLY

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AC/DC POWER SUPPLY

• An alternating current (AC) is an electrical
  current whose magnitude and direction vary
  cyclically, as opposed to direct current (DC) or
  continuous current , whose direction remains
  constant. The usual waveform of an AC power
  circuit is a sine wave, as this results in the
  most efficient transmission of energy. However in
  certain applications different waveforms are
  used, such as triangular or square waves.

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AC/DC What's the Difference?

• In 1887 direct current (DC) was king. At that
  time there were 121 Edison power stations
  scattered across the United States delivering DC
  electricity to its customers.
• But DC had a great limitation -- namely, that
  power plants could only send DC electricity about
  a mile before the electricity began to lose
  power. So when George Westinghouse introduced his
  system based on high-voltage alternating current
  (AC), which could carry electricity hundreds of
  miles with little loss of power, people naturally
  took notice. A "battle of the currents" ensued.
  In the end, Westinghouse's AC prevailed.
• The American Experience Edison's Miracle of
  Light AC - DC What's the Difference?

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The amplitude of AC
An AC voltage v can be described mathematically
as a function of time by the following equation

Vpeak is the peak voltage (unit volt), ? is the
angular frequency (unit radians per second) The
angular frequency is related to the physical
frequency, f, which represents the number of
oscillations per second (unit hertz), by the
equation ?
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How to compare AC and DC

• AC voltage is usually expressed as a root mean
  square (RMS) value, written Vrms. For a
  sinusoidal voltage

Vrms is useful in calculating the power consumed
by a load. If a DC voltage of VDC delivers a
certain power P into a given load, then an AC
voltage of Vpeak will deliver the same average
power P into the same load if Vrms VDC. Because
of this fact, RMS is the normal means of
measuring AC voltage. 110VAC is actually the RMS
value which is used from the energy companies in
America. But the voltage peak value is 155.55V.
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AC/DC POWER SUPPLY
Block-Diagramm
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AC/DC POWER SUPPLY
Circuit diagram
Note This configuration applies for 230V
(Europe).
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Transformer

• Function A transformer is a device used to
  increase (step up) or decrease (step down) the AC
  voltage in a circuit.
• Properties Laminated in order to reduce power
  losses through Eddy Currents.
• Operation The way transformers operate is based
  on the principle that an alternating current in
  the primary coil will induce an alternating
  electro-magnetic-field (EMF) on the secondary
  coil due to their mutual inductance.

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What is happening inside a transformer?
1. Iron Core molecules before energizing.
2. Energizing the coil and thus magnetic flux
generation.
3. Iron Core molecules line up after energizing.
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Rectifier
Diode
Ideal Diode
Real Diode
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Half Wave Rectifier
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Full Wave Rectifier
Link to PDF with the bridge rectifier animation

• One disadvantage of the full-wave rectifier is
  that there is a voltage loss of 1.4V across the
  diodes.

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You could either build a full wave rectifier
bridge out of 4 diodes (i.e. 1N4001 capable of
50V and 1A) or an already made rectifier as the
one shown below
While the full-wave rectifier is an improvement
on the half-wave rectifier, its output still
isn't suitable as a power supply for most
circuits since the output voltage still varies
between 0V and Vs-1.4V.  So, if you input 12V AC,
you will get 10.6V DC output. This is why the
'smoothing' block, described in the next chapter
is required.
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Smoothing
Vr

• Where I Amps, C F, f Hz and Vr V

• Note The ripple voltage shouldn't be higher than
  10 of Vs.

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Ex 1 What should be the Capacitance to
appropriately smooth an 24Vpp AC signal, 60Hz and
1A?
Ex 2 How big would the Ripple voltage be if the
current gets reduced to 500mA?

• Compromise
• Availability of components
• Price of components
• Ripple voltage

Q3 What factors affect the amount of ripple?
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Regulation

• Reason While there are many circuits that will
  tolerate a smoothed power supply, some must have
  a completely regular supply with no ripple
  voltage.

If the ripple voltage is too large and the input
voltage to the regulator falls below the
regulated voltage of the regulator, then of
course the regulator will not be able to produce
the correct regulated voltage.  As a rule of
thumb the input voltage to a regulator should
usually be at least 2V above the regulated
voltage.
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By using the brochures of the components all the
requirements can be found.