A voltage regulator is designed to automatically maintain a
constant voltage level. A voltage regulator may be a simple
"feed-forward" design or may include negative feedback control loops.
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.
Measures of regulator quality
The output voltage can only be held roughly constant; the
regulation is specified by two measurements:
load regulation is
the change in output voltage for a given change in load current (for example:
"typically 15 mV, maximum 100 mV for load currents between 5 mA and 1.4 A,
at some specified temperature and input voltage").
line regulation or
input regulation is the degree to which output voltage changes with input
(supply) voltage changes - as a ratio of output to input change (for example
"typically 13 mV/V"), or the output voltage change over the entire
specified input voltage range (for example "plus or minus 2% for input
voltages between 90 V and 260 V, 50-60 Hz").
Other important parameters are:
Temperature
coefficient of the output voltage is the change with temperature (perhaps
averaged over a given temperature range).
Initial accuracy
of a voltage regulator (or simply "the voltage accuracy") reflects
the error in output voltage for a fixed regulator without taking into account
temperature or aging effects on output accuracy.
Dropout voltage is
the minimum difference between input voltage and output voltage for which the
regulator can still supply the specified current. A low drop-out (LDO)
regulator is designed to work well even with an input supply only a volt or so
above the output voltage. The input-output differential at which the voltage
regulator will no longer maintain regulation is the dropout voltage. Further
reduction in input voltage will result in reduced output voltage. This value is
dependent on load current and junction temperature.
Absolute maximum
ratings are defined for regulator components, specifying the continuous and
peak output currents that may be used (sometimes internally limited), the
maximum input voltage, maximum power dissipation at a given temperature, etc.
Output noise
(thermal white noise) and output dynamic impedance may be specified as graphs
versus frequency, while output ripple noise (mains "hum" or
switch-mode "hash" noise) may be given as peak-to-peak or RMS
voltages, or in terms of their spectra.
Quiescent current
in a regulator circuit is the current drawn internally, not available to the
load, normally measured as the input current while no load is connected (and
hence a source of inefficiency; some linear regulators are, surprisingly, more
efficient at very low current loads than switch-mode designs because of this).
Transient response
is the reaction of a regulator when a (sudden) change of the load current
(called the load transient) or input voltage (called the line transient)
occurs. Some regulators will tend to oscillate or have a slow response time
which in some cases might lead to undesired results. This value is different
from the regulation parameters, as that is the stable situation definition. The
transient response shows the behaviour of the regulator on a change. This data
is usually provided in the technical documentation of a regulator and is also
dependent on output capacitance.
Mirror-image
insertion protection means that a regulator is designed for use when a voltage,
usually not higher than the maximum input voltage of the regulator, is applied
to its output pin while its input terminal is at a low voltage, volt-free or
grounded. Some regulators can continuously withstand this situation; others
might only manage it for a limited time such as 60 seconds, as usually
specified in the datasheet. This situation can occur when a three terminal
regulator is incorrectly mounted for example on a PCB, with the output terminal
connected to the unregulated DC input and the input connected to the load.
Mirror-image insertion protection is also important when a regulator circuit is
used in battery charging circuits, when external power fails or is not turned
on and the output terminal remains at battery voltage.
Electronic voltage regulators
A simple voltage regulator can be made from a resistor in
series with a diode (or series of diodes). Due to the logarithmic shape of
diode V-I curves, the voltage across the diode changes only slightly due to
changes in current drawn or changes in the input. When precise voltage control
and efficiency are not important, this design may work fine.
Feedback voltage regulators operate by comparing the actual
output voltage to some fixed reference voltage. Any difference is amplified and
used to control the regulation element in such a way as to reduce the voltage
error. This forms a negative feedback control loop; increasing the open-loop
gain tends to increase regulation accuracy but reduce stability (stability is
avoidance of oscillation, or ringing, during step changes). There will also be
a trade-off between stability and the speed of the response to changes. If the
output voltage is too low (perhaps due to input voltage reducing or load
current increasing), the regulation element is commanded, up to a point, to
produce a higher output voltage–by dropping less of the input voltage (for
linear series regulators and buck switching regulators), or to draw input
current for longer periods (boost-type switching regulators); if the output
voltage is too high, the regulation element will normally be commanded to
produce a lower voltage. However, many regulators have over-current protection,
so that they will entirely stop sourcing current (or limit the current in some
way) if the output current is too high, and some regulators may also shut down
if the input voltage is outside a given range (see also: crowbar circuits).
Electromechanical regulators
Circuit design for a simple electromechanical voltage
regulator.
A voltage stabilizer using electromechanical relays for
switching.
Graph of voltage output on a time scale.
In electromechanical regulators, voltage regulation is
easily accomplished by coiling the sensing wire to make an electromagnet. The
magnetic field produced by the current attracts a moving ferrous core held back
under spring tension or gravitational pull. As voltage increases, so does the
current, strengthening the magnetic field produced by the coil and pulling the
core towards the field. The magnet is physically connected to a mechanical
power switch, which opens as the magnet moves into the field. As voltage decreases,
so does the current, releasing spring tension or the weight of the core and
causing it to retract. This closes the switch and allows the power to flow once
more.
If the mechanical regulator design is sensitive to small
voltage fluctuations, the motion of the solenoid core can be used to move a
selector switch across a range of resistances or transformer windings to
gradually step the output voltage up or down, or to rotate the position of a
moving-coil AC regulator.
Early automobile generators and alternators had a mechanical
voltage regulator using one, two, or three relays and various resistors to
stabilize the generator's output at slightly more than 6 or 12 V, independent
of the engine's rpm or the varying load on the vehicle's electrical system. Essentially,
the relay(s) employed pulse width modulation to regulate the output of the
generator, controlling the field current reaching the generator (or alternator)
and in this way controlling the output voltage produced.
The regulators used for DC generators (but not alternators)
also disconnect the generator when it was not producing electricity, thereby
preventing the battery from discharging back into the generator and attempting
to run it as a motor. The rectifier diodes in an alternator automatically
perform this function so that a specific relay is not required; this
appreciably simplified the regulator design.
More modern designs now use solid state technology
(transistors) to perform the same function that the relays perform in
electromechanical regulators.
Electromechanical regulators are used for mains voltage
stabilisation — see AC voltage stabilizers below.
Automatic Voltage Regulator
Voltage regulator for generators.
To control the output of generators (as seen in ships and
power stations, or on oil rigs, greenhouses and emergency power systems)
automatic voltage regulators are used. This is an active system. While the
basic principle is the same, the system itself is more complex. An automatic
voltage regulator (or AVR for short) consist of several components such as
diodes, capacitors, resistors and potentiometers or even microcontrollers, all
placed on a circuit board. This is then mounted near the generator and
connected with several wires to measure and adjust the generator.
How an AVR works: In the first place the AVR monitors the
output voltage and controls the input voltage for the exciter of the generator.
By increasing or decreasing the generator control voltage, the output voltage
of the generator increases or decreases accordingly. The AVR calculates how
much voltage has to be sent to the exciter numerous times a second, therefore
stabilizing the output voltage to a predetermined setpoint. When two or more
generators are powering the same system (parallel operation) the AVR receives
information from more generators to match all output.
Coil-rotation AC voltage regulator
Basic design principle and circuit diagram for the
rotating-coil AC voltage regulator.
This is an older type of regulator used in the 1920s that
uses the principle of a fixed-position field coil and a second field coil that
can be rotated on an axis in parallel with the fixed coil, similar to a
variocoupler.
When the movable coil is positioned perpendicular to the
fixed coil, the magnetic forces acting on the movable coil balance each other
out and voltage output is unchanged. Rotating the coil in one direction or the
other away from the center position will increase or decrease voltage in the
secondary movable coil.
This type of regulator can be automated via a servo control
mechanism to advance the movable coil position in order to provide voltage
increase or decrease. A braking mechanism or high ratio gearing is used to hold
the rotating coil in place against the powerful magnetic forces acting on the
moving coil.
AC voltage stabilizers
Magnetic mains regulator
Electromechanical
Electromechanical regulators called voltage stabilizers or
tap-changers, have also been used to regulate the voltage on AC power
distribution lines. These regulators operate by using a servomechanism to
select the appropriate tap on an auto transformer with multiple taps, or by
moving the wiper on a continuously variable auto transfomer. If the output
voltage is not in the acceptable range, the servomechanism switches the tap,
changing the turns ratio of the transformer, to move the secondary voltage into
the acceptable region. The controls provide a dead band wherein the controller
will not act, preventing the controller from constantly adjusting the voltage
("hunting") as it varies by an acceptably small amount.
Constant-voltage transformer
The ferroresonant transformer, ferroresonant regulator or
constant-voltage transformer is a type of saturating transformer used as a
voltage regulator. These transformers use a tank circuit composed of a
high-voltage resonant winding and a capacitor to produce a nearly constant
average output voltage with a varying input current or varying load. The
circuit has a primary on one side of a magnet shunt and the tuned circuit coil
and secondary on the other side. The regulation is due to magnetic saturation
in the section around the secondary.
The ferroresonant approach is attractive due to its lack of
active components, relying on the square loop saturation characteristics of the
tank circuit to absorb variations in average input voltage. Saturating
transformers provide a simple rugged method to stabilize an AC power supply.
Older designs of ferroresonant transformers had an output
with high harmonic content, leading to a distorted output waveform. Modern devices
are used to construct a perfect sine wave. The ferroresonant action is a flux
limiter rather than a voltage regulator, but with a fixed supply frequency it
can maintain an almost constant average output voltage even as the input
voltage varies widely.
The ferroresonant transformers, which are also known as
Constant Voltage Transformers (CVTs) or ferros, are also good surge
suppressors, as they provide high isolation and inherent short-circuit
protection.
A ferroresonant transformer can operate with an input
voltage range ±40% or more of the nominal voltage.
Output power factor remains in the range of 0.96 or higher
from half to full load.
Because it regenerates an output voltage waveform, output
distortion, which is typically less than 4%, is independent of any input
voltage distortion, including notching.
Efficiency at full load is typically in the range of 89% to
93%. However, at low loads, efficiency can drop below 60%. The current-limiting
capability also becomes a handicap when a CVT is used in an application with
moderate to high inrush current like motors, transformers or magnets. In this
case, the CVT has to be sized to accommodate the peak current, thus forcing it
to run at low loads and poor efficiency.
Minimum maintenance is required, as transformers and
capacitors can be very reliable. Some units have included redundant capacitors
to allow several capacitors to fail between inspections without any noticeable
effect on the device's performance.
Output voltage varies about 1.2% for every 1% change in
supply frequency. For example, a 2 Hz change in generator frequency, which is
very large, results in an output voltage change of only 4%, which has little
effect for most loads.
It accepts 100% single-phase switch-mode power supply
loading without any requirement for derating, including all neutral components.
Input current distortion remains less than 8% THD even when
supplying nonlinear loads with more than 100% current THD.
Drawbacks of CVTs are their larger size, audible humming
sound, and the high heat generation caused by saturation.
DC voltage stabilizers
Many simple DC power supplies regulate the voltage using a
shunt regulator such as a Zener diode, avalanche breakdown diode, or voltage
regulator tube. Each of these devices begins conducting at a specified voltage
and will conduct as much current as required to hold its terminal voltage to
that specified voltage by diverting excess current from a non-ideal power
source to ground, often through a relatively low-value resistor to dissipate
the excess energy. The power supply is designed to only supply a maximum amount
of current that is within the safe operating capability of the shunt regulating
device.
If the stabilizer must provide more power, the shunt
regulator output is only used to provide the standard voltage reference for the
electronic device, known as the voltage stabilizer. The voltage stabilizer is
the electronic device, able to deliver much larger currents on demand.
Active regulators
Active regulators employ at least one active (amplifying)
component such as a transistor or operational amplifier. Shunt regulators are
often (but not always) passive and simple, but always inefficient because they
(essentially) dump the excess current not needed by the load. When more power
must be supplied, more sophisticated circuits are used. In general, these
active regulators can be divided into several classes:
Linear series
regulators
Switching
regulators
SCR regulators
Linear regulators
Linear regulators are based on devices that operate in their
linear region (in contrast, a switching regulator is based on a device forced
to act as an on/off switch). In the past, one or more vacuum tubes were
commonly used as the variable resistance. Modern designs use one or more transistors
instead, perhaps within an Integrated Circuit. Linear designs have the
advantage of very "clean" output with little noise introduced into
their DC output, but are most often much less efficient and unable to step-up
or invert the input voltage like switched supplies. All linear regulators
require a higher input than the output. If the input voltage approaches the
desired output voltage, the regulator will "drop out". The input to
output voltage differential at which this occurs is known as the regulator's
drop-out voltage.
Entire linear regulators are available as integrated
circuits. These chips come in either fixed or adjustable voltage types.
Switching regulators
Switching regulators rapidly switch a series device on and
off. The duty cycle of the switch sets how much charge is transferred to the
load. This is controlled by a similar feedback mechanism as in a linear
regulator. Because the series element is either fully conducting, or switched
off, it dissipates almost no power; this is what gives the switching design its
efficiency. Switching regulators are also able to generate output voltages
which are higher than the input, or of opposite polarity — something not
possible with a linear design.
Like linear regulators, nearly complete switching regulators
are also available as integrated circuits. Unlike linear regulators, these
usually require one external component: an inductor that acts as the energy
storage element. (Large-valued inductors tend to be physically large relative
to almost all other kinds of componentry, so they are rarely fabricated within
integrated circuits and IC regulators — with some exceptions.
Comparing linear vs. switching regulators
The two types of regulators have their different advantages:
Linear regulators
are best when low output noise (and low RFI radiated noise) is required
Linear regulators
are best when a fast response to input and output disturbances is required.
At low levels of
power, linear regulators are cheaper and occupy less printed circuit board
space.
Switching
regulators are best when power efficiency is critical (such as in portable
computers), except linear regulators are more efficient in a small number of
cases (such as a 5V microprocessor often in "sleep" mode fed from a 6V
battery, if the complexity of the switching circuit and the junction
capacitance charging current means a high quiescent current in the switching
regulator).
Switching
regulators are required when the only power supply is a DC voltage, and a
higher output voltage is required.
At high levels of
power (above a few watts), switching regulators are cheaper (for example, the
cost of removing heat generated is less).
SCR regulators
Regulators powered from AC power circuits can use silicon
controlled rectifiers (SCRs) as the series device. Whenever the output voltage
is below the desired value, the SCR is triggered, allowing electricity to flow
into the load until the AC mains voltage passes through zero (ending the half
cycle). SCR regulators have the advantages of being both very efficient and
very simple, but because they can not terminate an on-going half cycle of
conduction, they are not capable of very accurate voltage regulation in
response to rapidly changing loads. An alternative is the SCR shunt regulator
which uses the regulator output as a trigger, both series and shunt designs are
noisy, but powerful, as the device has a low on resistance.
Combination (hybrid) regulators
Many power supplies use more than one regulating method in
series. For example, the output from a switching regulator can be further
regulated by a linear regulator. The switching regulator accepts a wide range
of input voltages and efficiently generates a (somewhat noisy) voltage slightly
above the ultimately desired output. That is followed by a linear regulator
that generates exactly the desired voltage and eliminates nearly all the noise
generated by the switching regulator. Other designs may use an SCR regulator as
the "pre-regulator", followed by another type of regulator. An
efficient way of creating a variable-voltage, accurate output power supply is
to combine a multi-tapped transformer with an adjustable linear post-regulator.
Example linear regulators
Transistor regulator
In the simplest case a common collector transistor (emitter
follower) is used with the base of the regulating transistor connected directly
to the voltage reference:
Voltage stabiliser transistor, IEC symbols.svg
A simple transistor regulator will provide a relatively
constant output voltage, Uout, for changes in the voltage of the power source,
Uin, and for changes in load, RL, provided that Uin exceeds Uout by a
sufficient margin, and that the power handling capacity of the transistor is
not exceeded.
The output voltage of the stabilizer is equal to the zener
diode voltage less the base–emitter voltage of the transistor, UZ − UBE, where
UBE is usually about 0.7 V for a silicon transistor, depending on the load
current. If the output voltage drops for any external reason, such as an
increase in the current drawn by the load (causing a decrease in the
Collector-Emitter junction voltage to observe KVL), the transistor's
base–emitter voltage (UBE) increases, turning the transistor on further and
delivering more current to increase the load voltage again.
Rv provides a bias current for both the zener diode and the
transistor. The current in the diode is minimum when the load current is
maximum. The circuit designer must choose a minimum voltage that can be
tolerated across Rv, bearing in mind that the higher this voltage requirement
is, the higher the required input voltage, Uin, and hence the lower the
efficiency of the regulator. On the other hand, lower values of Rv lead to
higher power dissipation in the diode and to inferior regulator characteristics.[3]
where VR min is the minimum voltage to be maintained across
Rv
ID min is the minimum current to be maintained through the
zener diode
IL max is the maximum design load current
hFE is the forward current gain of the transistor,
ICollector / IBase[3]
Regulator with an operational amplifier
The stability of the output voltage can be significantly
increased by using an operational amplifier:
Voltage stabiliser OA, IEC symbols.svg
In this case, the operational amplifier drives the
transistor with more current if the voltage at its inverting input drops below
the output of the voltage reference at the non-inverting input. Using the
voltage divider (R1, R2 and R3) allows choice of the arbitrary output voltage
between Uz and Uin.
Commercial Voltage Regulators
The Voltage regulators or stabilizers are used and
commercially sold in third world countries like India, Bangladesh,
Pakistan,Afghanistan etc. due to high voltage fluctuations. Automatic Voltage
Regulators are used on generator sets on ships, emergency power supplies,
oilrigs etc to stabilize fluctuations in power need. For example, when a large
machine is turned on, the demand for power is suddenly a lot higher. The
voltage regulator senses this (decrease in voltage) and starts to send more
power to the exiter[4] Voltage regulators normally commercially produced
normally operate on a range of voltage i.e. 150V-240V, 90V-280V etc. Servo
Stabilizers are also manufactured and used widely in-spite of the fact that
they are obsolete and use back dated technology.
Voltage regulators are used in devices like air
conditioners, refrigerators, televisions etc. in order to protect them from
input fluctuating voltage. The major problem faced is the use of relays in
voltage regulators. Relays create sparks which result in faults in the product.
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