2. Reason For Output Voltage Variation
Variation in Input Voltage
Increasing Load
Increase in Temperature
Fault
Circuit Break
3. Require Of Regulated Voltage circuit
The purpose of a voltage regulator is to keep the voltage in a circuit relatively close
to a desired value. Voltage regulators are one of the most common electronic
components, since a power supply frequently produces raw current that would
otherwise damage one of the components in the circuit. Voltage regulators have a
variety of specific functions, depending on their particular application.
A voltage regulator is a system designed to automatically maintain a constant
voltage level. A voltage regulator may use a simple feed-forward design or may
include negative feedback. 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.
5. Types Of Voltage Regulator
Zener Diode Voltage Regulator
Series Voltage Regulator
Shunt Voltage Regulator
6. Zener Diode as a Voltage regulator
At the time of initiation, choose the zener diode
based on the voltage required. Construction of a
voltage regulator is easy with the help of an ideal
Zener diode, merely connecting the diode
between the unregulated source of voltage and
ground.
The source resistance Rs is connected in series
with zener diode to limit the flow of current
through the diode with voltage source connected
across the combination. The cathode terminal of
zener diode is connected to the positive terminal
of the voltage source so that the zener diode is
biased in reverse condition and will be operating
in breakdown region.
8. Series Voltage regulator
The series voltage regulator or series pass voltage
regulator uses a variable element placed in series
with the load. By changing the resistance of the
series element, the voltage dropped across it can be
varied to ensure that the voltage across the load
remains constant.
The advantage of the series voltage regulator is that
the amount of current drawn is effectively that used
by the load, although some will be consumed by any
circuitry associated with the regulator. Unlike the
shunt regulator, the series regulator does not draw
the full current even when the load does not require
any current. As a result the series regulator is
considerably more efficient.
9. Series Voltage Regulator
Such a circuit is also named an emitter
follower voltage regulator. It is called so
because the transistor used is connected
in an emitter follower configuration. The
circuit consists of an N-P-N transistor and
a zener diode. As shown in the figure
below, the collector and emitter terminals
of the transistor are in series with the
load. Thus this regulator has the name
series in it. The transistor used is a series
pass transistor.
10. Continue..
When the input supply voltage Vin increases the output voltage Vload also
increases. This increase in Vload will cause a reduced voltage of the
transistor base emitter voltage Vbe as the zener voltage Vzener is
constant. This reduction in Vbe causes a decrease in the level of
conduction which will further increase the collector-emitter resistance of
the transistor and thus causing an increase in the transistor collector-
emitter voltage and all of this causes the output voltage Vout to reduce.
Thus, the output voltage remains constant. The operation is similar when
the input supply voltage decreases.
The next condition would be the effect of the output load change in
regard to the output voltage. Let us consider a case where the current is
increased by the decrease in load resistance Rload. This causes a decrease
in the value of output voltage and thus causes the transistor base emitter
voltage to increase. This causes the collector emitter resistance value to
decrease due to an increase in the conduction level of the transistor. This
causes the input current to increase slightly and thus compensates for the
decrease in the load resistance Rload.
11. Series Regulator Advantages & Dis-
Advantages
The series resistor causes a huge amount of power loss.
1. The supply current flow will be more through the transistor
than it is to be through the load.
2. The circuit may have problems regarding over voltage
mishaps.
13. Continue…
If the input voltage increases, the $V_{BE}$ and $V_O$ also gets increased. But this
happens initially. Actually when $V_{in}$ increases, the current $I_{in}$ also
increases. This current when flows through RS, causes a voltage drop $V_S$ across
the series resistor, which also gets increased with $V_{in}$. But this makes $V_o$ to
decrease. Now this decrease in $V_o$ compensates the initial increase maintaining
it to be constant. Hence $V_o$ is maintained constant. If the output voltage
decreases instead, the reverse happens.
If the load resistance decreases, there should be decrease in the output voltage
$V_o$. The current through the load increases. This makes the base current and
collector current of the transistor to decrease. The voltage across the series resistor
becomes low, as the current flows heavily. The input current will be constant.
The output voltage appears will be the difference between the applied voltage
$V_i$ and the series voltage drop $V_s$. Hence the output voltage will be
increased to compensate the initial decrease and hence maintained constant. The
reverse happens if the load resistance increases.
14. Advantages & Disadvantgaes
Advantages
The transistor shunt regulator has a good efficiency for small load currents.
It has ease to reject short circuit.
The voltage regulation is good.
It has constant DC output voltage.
Disadvtages
It has poor efficiency for large load currents.
It has high output impedance.
The output DC voltage is not absolutely constant because both VBB and
VZ voltages decrease with increase in room temperature.
The DC output voltage cannot be changed easily as no such means is provided.
It can be used for large currents loads.
15. Regulated Power IC
78xx (sometimes L78xx, LM78xx, MC78xx...) is a family of
self-contained fixed linear voltage regulator integrated
circuits. The 78xx family is commonly used in electronic
circuits requiring a regulated power supply due to their
ease-of-use and low cost.
There are common configurations for 78xx ICs, including
7805 (5 V), 7806 (6 V), 7808 (8 V), 7809 (9 V), 7810 (10 V),
7812 (12 V), 7815 (15 V), 7818 (18 V), and 7824 (24 V)
versions. The 7805 is the most common, as its regulated 5-
volt supply provides a convenient power source for
most TTL components.
Less common are lower-power versions such as the
LM78Mxx series (500 mA) and LM78Lxx series (100 mA)
from National Semiconductor. Some devices provide slightly
different voltages than usual, such as the LM78L62
(6.2 volts) and LM78L82 (8.2 volts) as well as the
STMicroelectronics L78L33ACZ (3.3 volts).
17. Continue…
LM340 Series Voltage Regulator
The voltage regulator using LM340 IC is the mostly used voltage regulator IC. A build
in reference voltage is shown in the block diagram of LM340 IC below.
18. Vref drives from the non-inverting input of the operational amplifier. There are
various stages of voltage gain of the op-amp used here. This high gain helps the
op-amp to build an error voltage between inverting and non-inverting terminals to
almost zero. Thus, the inverting input terminal value will be similar as the non-
inverting terminal, Vref. Thus, the current flowing through the potential divider can
be written as
I = Vref/R2
The resistor R2, as shown in the diagram, is not an exterior component connected
to the IC, but an internal resistor, which is built inside on the IC by manufacturer.
Due to the above conditions, the same current flows through the R1. Thus the
output voltage can be written as
Vout = Vref/R2 (R1 + R2)
19. LM317 Voltage Regulator
It is a type of positive-linear-voltage regulators used for voltage
regulation, which is invented by Robert C. Dobkin and Robert J. Widlar while
they worked at National Semiconductor in 1970. It is a three-terminal-adjustable-
voltage regulator and is easy to use because to set the output voltage it requires
only two external resistors in LM317 voltage regulator circuit. It is majorly used for
local and on card regulation. If we connect a fixed resistor between the output
and adjustment of LM317 regulator, then the LM317 circuit can be used as a
precise current regulator.
20. Features of LM317 Voltage Regulator
It is capable of providing excess current of 1.5A, hence it is conceptually
considered as operational amplifier with an output voltage ranging from 1.2V to
37V.
The LM317 voltage regulator circuit internally consists of thermal overload
protection and short circuit current limiting constant with temperature.
It is available in two packages as 3-Lead Transistor Package and surface mount
D2PAK-3.
Stocking of many fixed voltages can be eliminated.
