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ABSTRACT
A voltage stabilizer is a device which is used to sense inappropriate voltage levels and correct
them to produce a reasonably stable output where the load is connected. Here we will study
the design of a simple automatic AC voltage stabilizer which can be used for the above
purpose. In this circuit, active and passive devices, such as diodes, transformer, resistor,
capacitor, relay, IC etc. have been used. One step down transformer and one step up
transformer have also been used in this work. In case the line voltage crosses a predetermined
threshold, the comparator detects it and its output immediately goes high, switching ON the
transistor and the relay for the desired actions. The relay contacts are appropriately integrated
to the transformer taps for executing the above actions as per the commands given by the op
amp output. The designed circuit operates successfully and the results obtained are
satisfactory. When installed, the relay trips whenever the input voltage crosses 230 volts,
bringing the output to 218 volts and keeps it until the voltage reaches higher levels. When the
voltage drops back to 225, the relay gets de-energized pulling the voltage to 238 volts and
maintains the difference as the voltage further goes down. The above action keeps the output
to the appliance well between 200 to 250 volts with fluctuations ranging from 180 to 265 volts.
Introduction
A voltage stabilizer is a device which is used to sense inappropriate voltage levels and correct
them to produce a reasonably stable output at the output where the load is connected. Which is
fabricated using transistor and other discrete components. It can be used to protect loads such as
TV, Refrigerator and VCR from undesirable over and under line voltages, as well as surges
caused due to sudden failure/resumption of mains power supply. This circuit can be directly as
standalone circuit between the main supply and the load, or it may be inserted between an
existing automatic/manual stabilizer and the load. In case the mains voltage crosses a
predetermined threshold, the ICs non inverting detects it and its output immediately goes high,
switching ON the transistor and the relay for the desired actions.
The relay, which is a DPDT type of relay, has its contacts wired up to a transformer, which is an
ordinary transformer modified to perform the function of a stabilizer transformer.
So if the input AC voltage tends to increase a set threshold value, the transformer deducts some
voltage and tries to stop the voltage from reaching dangerous levels and vice versa during low voltage situations.
Operation:
220V ac input supply from supply line and flow the ac voltage of rectifier. Rectifier chance the
voltage ac to dc and capacitor chance the filtering dc. Operational amplifier or comparator
compare the ac supply voltage. Relay is an Electrical switching device. Its Normally-open (NO)
contacts connect the circuit when the relay is activated the circuit is disconnected when the relay
is inactive. Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the
AC Input
Power
Supply
Rectifier
& Filter Amplifier Relay Output
2
circuit is connected when the relay is inactive. Change-over (CO), or double-throw (DT),
contacts control two circuits: one normally-open contact and one normally-closed contact with a
common terminal. Output voltmeter show the stable voltage.
1.3 Overview
A voltage stabilizer is a device which is used to sense inappropriate voltage levels and correct
them to produce a reasonably stable output at the output where the load is connected.
The power line fluctuations and cut-offs cause damages to electrical appliances connected to the
line. It is more serious in the case of domestic appliances like Fridge and Television. If a fridge
is operated on low voltage, excessive current flows through the motor, which heats up, and get
damaged.
The high/low voltage protection circuit with time delay presented here is a low cost and reliable
circuit for protecting such equipments from damages. Whenever the power line is switched on it
gets connected to the appliance only after a delay of a fixed time. If the power down time (time
for which the voltage is beyond limits) is less than the delay time, the power resumes after the
delay: If it is equal or more, then the power resumes directly.
This circuit has been designed, built and evaluated by me to use as a protector for my home
refrigerator. This is designed around readily available semi-conductor devices such as standard
bipolar medium power NPN transistor (BC547), an 8-pin type 741 op-amp IC and DPDT relay.
Its salient feature is that no relay hunting is employed. This draw back is commonly found in the
proctors available in the market.
The complete circuit is consisting of various stages. They are: - Dual rail power supply,
Reference voltage source, Voltage comparators for hi/low, Time delay stage and Relay driver
stage. Lets now look at the step-by-step design details
Description of Equipments
2.1 Required Component:
Resistor R1 & R2 = 10KΩ
Resistor R3 = 470KΩ
Variable Resistor = 10KΩ
Capacitor C1 = 1000 µF/25 V
Diode D1 & D2 = 1N 4007
Zener Diode Z1 & Z2 = 4.7 V/ 400mW
Transformer TR1 = 0V - 12 V , 500mA
Transformer TR2 = 9V-0V-9V, 5A.
Op-Amp = LM 741
Transistor = BC 547
Relay = DPDT, 12V, 200Ω.
LED = Red (1)
Voltmeter = 1 Pcs.
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2.2 Transformer:
Basic principles a transformer is an electrical device that transfers energy between two or more
circuits through electromagnetic induction.
A varying current in the transformer's primary winding creates a varying magnetic flux in the
core and a varying magnetic field impinging on the secondary winding. This varying magnetic
field at the secondary induces a varying electromotive force (emf) or voltage in the secondary
winding. Making use of Faraday's Law in conjunction with high magnetic permeability core
properties, transformers can thus be designed to efficiently change AC voltages from one
voltage level to another within power networks. Transformers have become essential for the AC
transmission, distribution, and utilization of electrical energy
Center Taps Transformer:
In electronics, a center tap is connection made to a point halfway along a winding of a
Transformer or inducer, or along the element of a resistor or a potentiometer. Taps are
Sometimes used on inductors for the coupling of signals, and may not necessarily be at the halfway
point , but rather, closer to one end. A common application of this is in the Hartley
oscillator. Inductors with taps also permit the transformation of the amplitude of alternating
current (AC) voltages for the purpose of power conversion, in which case, they are referred to as
autotransformers, since there is only one winding. An example of an autotransformers is an
automobile ignition coil. Potentiometer tapping provides one or more connections along the
device’s element, along with the usual connections at each of the two ends of the element, and
the slider connection. Potentiometer taps allow for circuit functions that would otherwise not be
available with the usual construction of just the two end connections and one slider connection.
A capacitor (originally known as a condenser) is a passive two-terminal electrical component
used to store energy electro statically in an electric field. The forms of practical capacitors vary
widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e.
insulator). The conductors can be thin films, foils or sintered beads of metal or conductive
electrolyte, etc. The "non-conducting" dielectric acts to increase the capacitor's charge capacity.
A dielectric can be glass, ceramic, plastic film, air, vacuum, paper, mica, oxide layer etc.
Capacitors are widely used as parts of electrical circuits in many common electrical devices.
Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a capacitor stores energy
in the form of an electrostatic field between its plates.
A capacitor consists of two conductors separated by a non-conductive region. The nonconductive
region is called the dielectric. In simpler terms, the dielectric is just an electrical
insulator. Examples of dielectric media are glass, air, paper, vacuum, and even a semiconductor
depletion region chemically identical to the conductors. A capacitor is assumed to be selfcontained
and isolated, with no net electric charge and no influence from any external electric
field. The conductors thus hold equal and opposite charges on their facing surfaces, and the
dielectric develops an electric field. In SI units, a capacitance of one farad means that one
coulomb of charge on each conductor causes a voltage of one volt across the device.
An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of
charge ±Q on each conductor to the voltage V between them:
Because the conductors (or plates) are close together, the opposite charges on the conductors
attract one another due to their electric fields, allowing the capacitor to store more charge for a
given voltage than if the conductors were separated, giving the capacitor a large capacitance.
Sometimes charge build-up affects the capacitor mechanically, causing its capacitance to vary.
In this case, capacitance is defined in terms of incremental changes:
Resistor:
A resistor is a passive two-terminal electrical component that implements electrical resistance
as a circuit element. Resistors act to reduce current flow, and, at the same time, act to lower
voltage levels within circuits. In electronic circuits resistors are used to limit current flow, to
adjust signal levels, bias active elements, terminate transmission lines among other uses. Highpower
resistors that can dissipate many watts of electrical power as heat may be used as part of
motor controls, in power distribution systems, or as test loads for generators. Resistors may have
fixed resistances that only change a little with temperature, time or operating voltage. Variable
resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or
as sensing devices for heat, light, humidity, force, or chemical activity. Resistors are common
elements of electrical networks and electronic circuits and are ubiquitous in electronic
equipment. Practical resistors as discrete components can be composed of various compounds
and forms. Resistors are also implemented within integrated circuits. The electrical function of a
resistor is specified by its resistance: common commercial resistors are manufactured over a
range of more than nine orders of magnitude. The nominal value of the resistance will fall within
a manufacturing tolerance.
Potentiometer applications:
Potentiometers are rarely used to directly control significant amounts of power (more than a watt
or so). Instead they are used to adjust the level of analog signals (for example volume controls
on audio equipment), and as control inputs for electronic circuits. For example, a light dimmer
uses a potentiometer to control the switching of a TRIAC and so indirectly to control the
brightness of lamps. Preset potentiometers are widely used throughout electronics wherever
adjustments must be made during manufacturing or servicing.
User-actuated potentiometers are widely used as user controls, and may control a very wide
variety of equipment functions. The widespread use of potentiometers in consumer electronics
declined in the 1990s, with rotary encoders, up/down push-buttons, and other digital controls
now more common. However they remain in many applications, such as volume controls and as
position sensors. Low-power potentiometers, both linear and rotary, are used to control audio
equipment, changing loudness, frequency attenuation and other characteristics of audio signals.
The 'log pot' is used as the volume control in audio power amplifiers, where it is also called an
"audio taper pot", because the amplitude response of the human ear is approximately
logarithmic. It ensures that on a volume control marked 0 to 10, for example, a setting of 5
sounds subjectively half as loud as a setting of 10. There is also an anti-log pot or reverse audio
taper which is simply the reverse of a logarithmic potentiometer. It is almost always used in a
ganged configuration with a logarithmic potentiometer, for instance, in an audio balance control.
Television:
Potentiometers were formerly used to control picture brightness, contrast, and color response. A
potentiometer was often used to adjust "vertical hold", which affected the synchronization
between the receiver's internal sweep circuit (sometimes a multivibrator) and the received
picture signal, along with other things such as audio-video carrier offset, tuning frequency (for
push-button sets) and so on.
Motion control:
Potentiometers can be used as position feedback devices in order to create "closed loop" control,
such as in a servomechanism. This method of motion control used in the DC Motor is the
simplest method of measuring the angle or speed.
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Transducers:
Potentiometers are also very widely used as a part of displacement transducers because of the
simplicity of construction and because they can give a large output signal.
Computation:
In analog computers, high precision potentiometers are used to scale intermediate results by
desired constant factors, or to set initial conditions for a calculation. A motor-driven
potentiometer may be used as a function generator, using a non-linear resistance card to supply
approximations to trigonometric functions. For example, the shaft rotation might represent an
angle, and the voltage division ratio can be made proportional to the cosine of the angle.
2.6 Diode:
The most common function of a diode is to allow an electric current to pass in one direction
(called the diode's forward direction), while blocking current in the opposite direction (the
reverse direction).
Thus, the diode can be viewed as an electronic version of a check valve. This unidirectional
behavior is called rectification, and is used to convert alternating current to direct current,
including extraction of modulation from radio signals in radio receivers—these diodes are forms
of rectifiers.