Topic 1 Linear DC Power Supply
Topic 1 Linear DC Power Supply
Topic 1 Linear DC Power Supply
TRANSFORMER
Circuit symbols
TRANSFORMER
Transformers convert AC electricity from one voltage to another
with little loss of power.
TRANSFORMER
• The input coil is called the primary and the output coil is called
the secondary.
• The two lines in the middle of the circuit symbol represent the
core.
DC POWER SUPPLY
TRANSFORMER
Ideal Power Equation
Vp ; Primary (input) Voltage
If this condition is met, the incoming electric power must equal the
outgoing power:
RECTIFIER
RECTIFIER
Types Of Rectifiers :
Half-wave rectifier
Full-wave rectifier
Bridge rectifier
DC POWER SUPPLY
HALF-WAVE RECTIFIER
An a.c voltage is applied to a single diode connected in
series with a load resistor, RL.
Schematic diagram :
DC POWER SUPPLY
CIRCUIT OPERATION :
The AC voltage polarity across the secondary winding changes after every half
cycle.
b) The input voltage during the positive half-cycles is directly applied to the
load, R.
c) The waveforms of the output voltage during t0 to t1 are of the same shape
as that of the input AC voltage.
c) For the negative half cycles t1 to t2, no power is delivered to the load, R.
INPUT AND OUTPUT VOLTAGE
@ VP(sec) – 0.7V
DC POWER SUPPLY
FULL-WAVE RECTIFIER
The full-wave rectifier circuit using two diodes and a centre-tapped
transformer, which usually taken as the ground or zero voltage
reference point.
The purpose of centre-tapped transformer is to get two a.c output with the
same value but in inverse phase.
INPUT & OUTPUT VOLTAGE
OPERATION :
a). During positive half cycle, D1 is forward biased & D2 is reverse biased.
b). During negative half cycle, D2 is forward biased & D1 is reverse biased.
a). During positive half-cycle, D1 & D2 are forward biased and conduct current.
D3 & D4 are reverse-biased.
b). During negative half-cycle, D3 & D4 are forward biased and conduct current.
D1 & D2 are reverse-biased.
• 1.4V is used up in the bridge rectifier because each diode uses
0.7V when conducting and there are always two diodes
conducting.
Vpsec – 0.7V
2
EXAMPLES
Example 1:
Vpsec
220Vpp G
Vpsec 2k
EXAMPLES
EXAMPLES
Example 3:
A full wave rectifier with a 120 Vrms sinusodial input had a load
resistor of 1KΩ. If silicone diodes are employed, determine:-
i. Average voltage available at the load.
ii. Average current
EXAMPLES
DC POWER SUPPLY
FILTER
DC POWER SUPPLY
FILTER
A pulsating DC wave
A ripple DC wave
Rectifier Filter
•The output of the rectifier is a pulsating DC wave.
•We need a constant DC output.
•Pulsating DC is not pure because it contains an AC component.
DC POWER SUPPLY
FILTER
• The AC component in DC power supply is called ripple. Ripple is usually to be
considered as unwanted effect.
• Ripple is the small unwanted residual periodic variation of the direct
current (DC) output of a power supply which has been derived from
an alternating current (AC) source.
• Filter is the circuit used to remove the ripple and produce a very smooth
waveform.
• To do this, we need to filter out the oscillations from the pulsating DC wave.
• This is obtained with a diode and capacitor combination.
The capacitor can more effectively reduce the ripple when the time between peaks
is shorter.
CAPACITOR FILTER
• The capacitor discharge through RL after peak of positive alteration when the diode
is reverse-biased.
• This discharging occurs during the portion of the input voltage indicated by solid
curve.
• The input begins to decrease below its peak to 0V, while the capacitor is begin to
discharge.
• The discharging time is longer than decreasing the voltage to 0V.
DC POWER SUPPLY
10 X
5
t
0 A 2 4 C 6 8 Z 10
Time Constant, RLC1
Ripple voltage of C1
Time Constant, RLC2
Rectifier C1 C2 RL Vk
• Large voltage drops in the series resistor R (poor voltage regulation). DC output
voltage across load, RL will also be diminished to a lower value.
• Power is wasted in R and is dissipated in the form of unwanted heat.
• The RC filter helps to reduce the ripple voltage, but it introduces output DC
voltage losses when the load current is higher. Thus R-C filter is suitable only for
light loads (small load current or large load resistance).
b). π FILTER(C-L-C FILTER)
L
Rectifier C1 C2 RL Vk
Rectifier C2 RL Vk
Type
Type Number Output
Output Voltage Number
78XX Voltage
79XX
7805 +5 V 7905 -5 V
7806 +6 V 7905.2 -5.2 V
7808 +8 V 7906 -6.0 V
7809 +9 V 7908 -8.0 V
7812 +12 V 7912 -12 V
7815 +15 V 7915 -15 V
7818 +18 V 7918 -18 V
7824 +24 V 7924 -24 V
DC POWER SUPPLY
VOLTAGE DIVIDER
VOLTAGE DIVIDER
SCHEMATIC CIRCUITS :