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.
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?
The amplitude of AC
V
Veff 
2
An AC voltage v can be described mathematically as a function of time by the
following equation:
Vpeak is the peak voltage (unit: volt),
ω 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 ω =
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.
AC/DC POWER SUPPLY
Block-Diagramm
AC/DC POWER SUPPLY
Circuit diagram :
Note: This configuration applies for 230V (Europe).
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 electromagnetic-field (EMF) on the secondary coil due to their mutual inductance.
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.
Rectifier:
Diode
Ideal Diode
Real Diode
Half Wave Rectifier:
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.
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.
Smoothing:
I
Vr 
2C. f
 Where: I [Amps], C [F], f [Hz]
and Vr [V]
 Note: The ripple voltage shouldn't
be higher than 10% of Vs.
Vr
Ex 1: What should be the Capacitance to appropriately smooth an 24Vpp AC
signal, 60Hz and 1A?
I
1A
C

 6950F
2  Vr  f 2  0,112V 60Hz
Ex 2: How big would the Ripple voltage be if the current gets reduced to
500mA?
I
0,5 A
Vr 

 0.6V
3
2  C  f 2  6,95e  60Hz
Compromise:
• Availability of components
• Price of components
• Ripple voltage
Q3: What factors affect the amount of ripple?
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.
By using the brochures of the components all the requirements can be found.