Power Supply Section: Description

POWER SUPPLY SECTION
Description:-
The Power Supply is a Primary requirement for any system to start. The
required DC power supply for the base unit as well as for the recharging unit is
derived from the mains line. For this purpose center tapped secondary of 12V-0-
12V transformer is used. From this transformer we getting 5V power supply. The
+5V output is a regulated output and it is designed using 7805 positive voltage
regulator. This is a 3 Pin voltage regulator, can deliver current up to 800
milliamps. Rectification is a process of rendering an alternating current or voltage
into a unidirectional one. The component used for rectification is called
‘Rectifier’. A rectifier permits current to flow only during positive half cycles of
the applied AC voltage. Thus, pulsating DC is obtained .To obtain smooth DC
power additional filter circuits required.
There are many types of power supply. Most are designed to convert high voltage
AC mains electricity to a suitable low voltage supply for electronic circuits and
other devices. A power supply can by broken down into a series of blocks, each of
which performs a particular function.
For example a 5V regulated supply:

Each of the blocks is described in more detail below:
 Transformer - steps down high voltage AC mains to low voltage AC.

 Rectifier - converts AC to DC, but the DC output is varying.
 Smoothing - smoothes the DC from varying greatly to a small ripple.
 Regulator - eliminates ripple by setting DC output to a fixed voltage.
TRANSFORMER:-
Transformers convert AC electricity from one voltage to another with little loss
of power. Transformers work only with AC and this is one of the reasons why
mains electricity is AC.
Circuit symbol of transformer:-
Step-up transformers increase voltage, step-down transformers reduce voltage.

Most power supplies use a step-down transformer to reduce the dangerously high
mains voltage (230V in UK) to a safer low voltage.
The input coil is called the primary and the output coil is called the secondary.
There is no electrical connection between the two coils, instead they are linked by
an alternating magnetic field created in the soft-iron core of the transformer. The
two lines in the middle of the circuit symbol represent the core.
Transformers waste very little power so the power out is (almost) equal to the
power in. Note that as voltage is stepped down current is stepped up.
The ratio of the number of turns on each coil, called the turns ratio, determines
the ratio of the voltages. A step-down transformer has a large number of turns on
its primary (input) coil which is connected to the high voltage mains supply, and a
small number of turns on its secondary (output) coil to give a low output voltage.
power out = power in
turns ratio Vp Np and
=
=
Vs Ns Vs × Is = Vp × Ip
Vp = primary (input) voltage Vs = secondary (output) voltage
Np = number of turns on primary coil Ns = number of turns on secondary coil
Ip = primary (input) current Is = secondary (output) current
RECTIFIER:-
There are several ways of connecting diodes to make a rectifier to convert AC
to DC. The bridge rectifier is the most important and it produces full-
wave varying DC. A full-wave rectifier can also be made from just two diodes if a
centre-tap transformer is used, but this method is rarely used now that diodes are
cheaper. A single diode can be used as a rectifier but it only uses the positive (+)
parts of the AC wave to produce half-wave varying DC.
Bridge rectifier:-
A bridge rectifier can be made using four individual diodes, but it is also available
in special packages containing the four diodes required. It is called a full-wave
rectifier because it uses all the AC wave (both positive and negative sections).
1.4V is used up in the bridge rectifier because each diode uses 0.7V when
conducting and there are always two diodes conducting, as shown in the diagram
below. Bridge rectifiers are rated by the maximum current they can pass and the
maximum reverse voltage they can withstand (this must be at least three times the
supply RMS voltage so the rectifier can withstand the peak voltages).
Bridge rectifier Output: full-wave varying DC

Alternate pairs of diodes conduct, (using all the AC wave)
changing over
the connections so the alternating
directions of
AC are converted to the one direction of
DC.
Single diode rectifier:-

A single diode can be used as a rectifier but this produces half-wave varying DC
which has gaps when the AC is negative. It is hard to smooth this sufficiently well
to supply electronic circuits unless they require a very small current so the
smoothing capacitor does not significantly discharge during the gaps.
Output: half-wave varying DC

Single diode rectifier
(using only half the AC wave)
SMOOTHING:-
Smoothing is performed by a large value electrolytic capacitor connected across
the DC supply to act as a reservoir, supplying current to the output when the
varying DC voltage from the rectifier is falling. The diagram shows the
unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The
capacitor charges quickly near the peak of the varying DC, and then discharges as
it supplies current to the output.

Note that smoothing significantly increases the average DC voltage to almost the
peak value (1.4 ×RMS value). For example 6V RMS AC is rectified to full wave
DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this
increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.
Smoothing is not perfect due to the capacitor voltage falling a little as it

discharges, giving a small ripple voltage. For many circuits a ripple which is 10%
of the supply voltage is satisfactory and the equation below gives the required
value for the smoothing capacitor. A larger capacitor will give less ripple. The
capacitor value must be doubled when smoothing half-wave DC.
Smoothing capacitor for 10% ripple, 5 × Io

C= Vs × f
C = smoothing capacitance in farads (F)

Io = output current from the supply in amps (A)
Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC
f = frequency of the AC supply in hertz (Hz), 50Hz in the UK
VOLTAGE REGULATION:-
Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or
variable E voltages. They are also rated by the maximum current they can pass.
Negative voltage regulators are available, mainly for use in dual supplies. Most
regulators include some automatic protection from excessive current ('overload
protection') and overheating ('thermal protection').
Many of the fixed voltage regulator ICs have 3 leads and look like power
transistors, such as the 7805 +5V 1A regulator shown on the above. If adequate
heat sinking is provided then it can deliver up to maximum 1A current. For an
output voltage of 5v-18v the maximum input voltage is 35v and for an output
voltage of 24V the maximum input voltage is 40V.For 7805 IC, for an input of 10v
the minimum output voltage is 4.8V and maximum output voltage is 5.2V. The
typical dropout voltage is 2V.
TOTAL CIRCUIT DIAGRAM

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POWER SUPPLY SECTION

For example a 5V regulated supply:

 Transformer - steps down high voltage AC mains to low voltage AC.

Step-up transformers increase voltage, step-down transformers reduce voltage.

Bridge rectifier Output: full-wave varying DC

Single diode rectifier:-

Output: half-wave varying DC

Smoothing is not perfect due to the capacitor voltage falling a little as it

Smoothing capacitor for 10% ripple, 5 × Io

C = smoothing capacitance in farads (F)

TOTAL CIRCUIT DIAGRAM

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