3.0 METHODOLOGY
3.1
List of equipment
The completion
of this project requires important equipment in order to ensure this project
successful. The equipment/components that were used during the construction of
this project are:
— Step Down Transformer (230 – 12v AC)
— Voltage regulator
— Menthol & LED
— Comparator
— 555 Timer
— Capacitor
— Rectifier
— Resistor
— LM 358
— IN4007
— Switch
— Diode
— Relay
3.1.1 TRANSFORMER
Transformers convert AC
electricity from one voltage to another with a little loss of power. Step-up
transformers increase voltage while step-down transformers reduce voltage. Most
power supplies use a step-down transformer to reduce the dangerously high
voltage to a safer low voltage. At input, it only has 2 port which is port 1 is
for live wire and port 2 for natural wire. The output is connected to the 12v
port and 0v port. Figure 1 shows the 230/12V Transformer.
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 that the
output power is (almost) equal to the power in [4].
The ratio of the number of turns on each coil, called the turn’s ratio will determine 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. A
small number of turns on its secondary (output) coil will give a low output
voltage. Equation below is the equation for ideal transformer.[4]
TURNS RATIO
= (Vp /
Vs) = ( Np
/ Ns ) -------------(1)
Where,
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.
If
the secondary coil is attached to the load that allows current to flow,
electrical power is transmitted from the primary circuit to the secondary circuit.
Ideally, the transformer is perfectly efficient and all of the incoming energy
is transformed from the primary circuit to the magnetic field
and into the secondary circuit [4].
3.1.2 COMPARATOR
Potential dividers are
connected to the inverting and non-inverting inputs of the op-amp to give some
voltage at these terminals. Supply voltage is given to +Vss and –Vss and it is
connected to the ground. The output of this comparator will be logic high (i.e.,
supply voltage) if the non-inverting terminal input is greater than the
inverting terminal input of the comparator. If the inverting terminal input is greater than
the non-inverting terminal input then the output of the comparator will be
logic low. In this project, an op-amp is a comparator. Figure 2 shows op-amp as
a comparator.
3.1.3
555 TIMER
The
555 Timer IC is an integrated
circuit
(chip) implementing a variety of timer
and multivibrator applications. The IC was
designed by Hans R. Camenzind in 1970 and brought to market in 1971 by Signetics (later acquired by Philips). The original name was
the SE555 (metal can)/NE555
(plastic DIP) and the part was
described as “The IC Time Machine”. It has been claimed that the 555 gets its
name from the three 5000 ohm
resistors used in typical early implementations but Hans Camenzind has stated
that the number was arbitrary. The part is still in wide use since it is ease
of use, low price and good stability. As of 2003[update], it is estimated that one
billion
units are manufactured every year.
Depending on the
manufacturer, the standard 555 package includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip
installed in an 8-pin mini dual-in-line package (DIP-8).
Variants available include the 556 (a 14-pin DIP combining two 555s on one
chip), and the 558 which is a 16-pin DIP combining four slightly modified 555s
with DIS & THR connected internally and TR falling edge sensitive instead
of sensitive level. Figure 3 shows 555 timer ic.
Ultra-low power versions of
the 555 are also available, such as the 7555 and TLC555. The 7555 is designed
to cause less supply flitching than the classic 555 and the manufacturer claims
that it usually does not require a “control” capacitor and in many cases does
not require a power supply bypass capacitor.
3.1.3.1 The 555 operating modes:
The 555
timer have three operating modes which are Monostable mode, Astable mode and
Bistable mode. The description of each mode is as below:
- Monostable mode: in this mode,
the 555 functions as a “one-shot”. Applications include timers, missing
pulse detection, bounce free switches, touch switches, frequency divider,
capacitance measurement, pulse-width modulation (PWM) etc.
- Astable – free running mode. The 555 can
operate as an oscillator. It usage include LED
and lamp flashers, pulse generation, logic clocks, tone generation,
security alarms, pulse position modulation,
etc.
- Bistable
mode or Schmitt trigger.
The 555 can operate as a flip-flop.
if the DIS pin is not connected and no capacitor is used. It usage include
bouncefree latched switches, etc.
The pin diagram for 555timer is shown in
Figure 4 while the connection of the 555 timer pins is shown in the table 1 below:
Table
1 : Connection of 555 timer pin
|
Pin
|
Name
|
Purpose
|
|
1
|
GND
|
Ground,
low level (0 V)
|
|
2
|
TRIG
|
OUT
rises, and interval starts, when this input falls below 1/3 VCC.
|
|
3
|
OUT
|
This
output is driven to +VCC
or GND.
|
|
4
|
RESET
|
A
timing interval may be interrupted by driving this input to GND.
|
|
5
|
CTRL
|
“Control”
access to the internal voltage divider (by default, 2/3 VCC).
|
|
6
|
THR
|
The
interval ends when the voltage at THR is greater than at CTRL.
|
|
7
|
DIS
|
|
|
8
|
V+, VCC
|
Positive
supply voltage is usually between 3 and 15 V.
|
FIGURE 4 : 555 TIMER
PIN DIAGRAM
The
555 timer IC is a simple 8 pin dual in line package (DIP-8) IC. It can be used
as below:
- As a monostable
- As an astable
- As a source or sink
100Ma
- Use supply voltages of
5v to 15v disrupt the power supply – use a decoupling capacitor!
3.1.3.3 555 as a buffer
A buffer circuit allows an
input circuit to be connected to an output circuit. It is like an interface
between one circuit and another. The buffer circuit requires very little input
current but it should be able to supply adequate output current. The 555 can
supply in excess of 100MA of current. Hence, it can be used as a convenient buffer
for logic gates which cannot supply much current. The 555 can also ‘sink’ a
similar amount of current. The circuit
used as a buffer is shown below in Figure 5.
FIGURE 5 : 555 TIMER AS A BUFFER
The circuit acts like an
inverter or NOT gate. When the input is held low, the output is high and will
provide (source) current. When the input is held high, the output is low and
will sink current. For a buffer, higher power devices require larger currents.
The 555 buffer can be used to drive a relay or a transistor circuit. Figure 5
shows 555 timer as a buffer.
In this mode, the 555
functions as a one-shot pulse generator. Applications include timers, missing
pulse detection, bounce free switches, touch switches, frequency divider, capacitance
measurement, pulse-width modulation (PWM) and so on. The 555 can be used as a monostable using the circuit
shown. Figure 6 shown 555 timer as a monostable mode.
The
description of the circuit in monostable mode is described as below:
- The output is normally low but will go
high for a short length of time depending on the values of the other
components.
- R
and C determine the time period of the output pulse.
- The
input is normally high and goes low to trigger the output (falling edge
triggered).
- The
length of the input pulse must be less than the length of the output
pulse.
- The
47Uf capacitor ‘decouples’ the supply to avoid affecting other parts of
the circuit.
- It is standard to add a 10Nf capacitor from pin5 to
gnd.
The
minimum value of R should be about 1kohm to avoid too much current flowing into the
555. The maximum value of R should be about 1Mohm so that enough current can
flow into the input of the 555 and there is also current needed to allow the
electrolytic capacitors leakage current. The minimum value of capacitor is 100Pf
will avoid the timing equation being too far off. The maximum value of
capacitor should be about 1000µF as any bigger capacitors will discharge too
much current through the chip. These maximum and minimum values give a minimum
period of 0.1 µs and a maximum period of 1000s.
3.1.3.5 Using the 555 as an astable
The 555 can operate
as an oscillator. Astable
mode also call a free running mode. It usage are include LED and lamp
flashers, pulse generation, logic clocks, tone generation, security alarms,pulse position modulation and so on. The 555 can be used as a simple ADC, converting an analog value to a pulse
length. Selecting a thermistor as timing resistor allows the use of the 555 in a temperature
sensor. The period of the output pulse is determined by the temperature. The
use of a microprocessor based circuit can then convert the pulse period to
temperature, linearize it and even provide calibration means. The 555 can be used as an
astable using the circuit shown. Figure 7 shown 555 timer as an astable mode.
The description of circuit that act in
astable mode is describe below:
- The output will oscillate between high and low
continuously – the circuit is not stable in any state
- Ra, Rb and C determine the
time period of the output
- The reset, pin 4, must be held high for the circuit
to oscillate. If pin 4 is held low then the output remains low. Pin 4 can
be used to turn the astable ‘on’ and ‘off’ in effect
- The 47uf capacitor ‘decouples’ the supply to avoid
affecting other parts of the circuit
- It is standard to add a 10Nf capacitor from pin5 to
gnd.
As with the monostable, the
minimum value of Ra should be about 1k ohm to avoid too much current flowing into
the 555.The maximum value of Ra or Rb should be about 1M ohm so that enough
current can flow into the input of the 555 and there is also current to allow
for the electrolytic capacitors leakage current.
The minimum value of
capacitor is 100Pf in order to avoid the timing equation being too far off. The
maximum value of capacitor should be about 1000µF as any bigger capacitors will
discharge too much current through the chip. These maximum and minimum values
give a minimum frequency of 0.001 Hz and a maximum frequency of 4.8 MHz.
Considering the oscillations in more
detail:
- The output is controlled by the charging and
discharging of the capacitor.
- The capacitor charges through Ra and Rb.
- But discharges through the discharge pin (pin 7) and
thus only through Rb.
- The time that the capacitor takes to charge or
discharge is given as T = 0.7 R C.
- Thus the charge time is 0.7 (Ra + Rb) C.
- The discharge time is 0.7 Rb C.
- Giving a total time of (0.7 (Ra + Rb) C) + (0.7 Rb
C) = 0.7 (Ra + 2Rb) C.
- The time the output is high is thus always longer
than the time the output is low.
- The 555 astable cannot produce a square wave!
It is not necessary to know
how the 555 works. In fact a systems approach to electronics would never
consider how any such sub-block works. However, a knowledge of how the 555
functions is useful. A much simplified block diagram of the 555 timer. Figure 8
shown the operation of 555 timer.
The
description of 555 timer is describe as follows:
- The resistors
are arranged across the power supply to form a potential divider. The
voltages at the junctions of the potential divider are 2/3 Vcc and 1/3
Vcc. They are connected to the inputs to a pair of comparators.
- One comparator, switching at 2/3 Vcc is
controlled via the threshold
input.
- The voltage at which the threshold
comparator switches can be changed from 2/3 Vcc by applying a voltage to
the control pin.
This pin is usually decoupled to ground via a 10Nf capacitor and, in this
case, the comparator switches at 2/3 Vcc as expected.
- One comparator, switching at 1/3 Vcc is
controlled via the trigger
input.
- The outputs from the two comparators
control a set-reset flip flop (bistable).
- The reset pin of the 555 (not of the
bistable) is usually held high. Taking this pin momentarily low apply a
voltage to the reset pin of the flip flop and the output falls to zero.
- The output of the flip flop is connected
to the output pin via a power amplifier circuit which includes short
circuit protection etc.
- The output goes high when the trigger input is
less than 1/3 Vcc.
- The output then remains high until the threshold input
rises above 2/3 Vcc.
- When the output is low, the discharge pin is
connected to ground via a transistor. The capacitor can be organized to
discharge through this pin but the value of the capacitor should be less
than 1000µF to avoid damaging the transistor.
3.1.4
RELAY
A relay is an electrically operated switch. Many relays use an
electromagnet to operate a switching mechanism mechanically, but other
operating principles are also used. Relays are used where it is necessary to
control a circuit by a low-power signal (with complete electrical isolation
between control and controlled circuits), or where several circuits must be
controlled by one signal. Figure 9 shows the relay.
A
relay is an electrically operated
switch. Current flowing through the coil of the relay creates a magnetic
field which attracts a lever and changes the switch contacts. The coil current
can be on or off so the relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram. Figure 10 shows
relay coil and switch contacts.
Relays
allow one circuit to switch a second circuit which can be completely separate
from the first. For example a low voltage battery circuit can use a relay to
switch a 230V AC mains circuit. There is no electrical connection inside the
relay between the two circuits which are the link is between magnetic and
mechanical.
The
coil of a relay passes a relatively large current, typically 30MA for a 12V
relay. But, it can be as much as 100MA for relays designed to operate from
lower voltages. Most ICs (chips) cannot provide this current and a transistor
is usually used to amplify the small IC current to the larger value required
for the relay coil. The maximum output current for the popular 555 timer IC is
200MA so these devices can supply relay coils directly without amplification.
Relays
are usually Single Pole Double Throw (SPDT) or Double Pole Double Throw (DPDT)
but they can have many more sets of switch contacts such as relays with 4 sets
of changeover contacts that are already readily available.
Most
relays are designed for Printed Circuit Board (PCB) mounting but it can also
been solder the wires directly to the pins in order to avoid the plastic relay
from melting.
The
relay coil usually will be obvious and it may be connected either way round.
Relay coils produce brief high voltage ‘spikes’ when they are switched off and
this can destroy transistors and ICs in the circuit. Protection diode needs to
be connected across the relay coil to prevent any damage.
Figure
10 shows a relay with its coil and switch contacts from schematic diagram. The
lever on the left being attracted by magnetism when the coil is switched on.
This lever moves the switch contacts.
3.1.4.1 Applications of relays
Relays have various application in today’s
application. Generally it is used for:
·
Control a high-voltage circuit with a
low-voltage signal, as in some types of modems or audio amplifiers.
·
Control a high-current circuit with a
low-current signal, as in the starter solenoid of an automobile.
·
Detect and isolate faults on transmission and
distribution lines by opening and closing circuit breakers.
·
Time delay functions. Relays can be modified
to delay opening or delay closing for a set of contacts. A very short (a
fraction of a second) delay would use a copper disk between the armature and
moving blade assembly. Current flowing in the disk maintains magnetic field for
a short time, lengthening release time. For a slightly longer (up to a minute)
delay, a dashpot is used. A dashpot is a piston filled with fluid that is
allowed to escape slowly. The time period can be varied by increasing or
decreasing the flow rate. For longer time periods, a mechanical clockwork timer
is installed.
3.1.5 VOLTAGE REGULATOR
The LM78XX/LM78XXA series is a three-terminal positive
regulators that are available in the TO-220/D-PAK package and with several
fixed output voltages, making them useful in a wide range of applications. Each
type employs internal current limiting, thermal shutdown and safe operating
area protection, making it essentially indestructible. If adequate heat sinking
is provided, they can deliver over 1A output current. Although designed was primarily
as fixed voltage regulators, these devices can be used with external components
to obtain adjustable voltages and currents. The features of voltage regulator
are output current up to 1A, output voltages of 5V, 6V, 8V, 9V, 10V, 12V, 15V,
18V and 24V instead of thermal overload protection, short circuit protection, and
output transistor safe operating area protection. Figure 11 shows the circuit
of voltage regulator.
3.1.6
LM 358
The LM358 series consists of two independent,
high gain and internally frequency compensated operational amplifiers which
were designed specifically to operate from a single power supply over a wide
range of voltages. Operation from split power supplies is also possible and the
low power supply current is independent
of the magnitude of the power supply voltage.
Application
areas include transducer amplifiers, dc gain blocks and all the conventional
op-amp circuits which now can be more easily implemented in single power supply
systems. As an example, the LM358 series can be directly operated on the
standard of +5V power supply voltage which is usually used in digital systems
and will easily provide the required interface electronics without requiring
the additional ±15V power supplies. The unique characteristics of LM 358 are:
·
In the linear mode the input common-mode
voltage range includes ground and the output voltage can also swing to ground,
even though operated from only a single power supply voltage.
·
The unity gain cross frequency is temperature
compensated.
·
The input bias current is also temperature
compensated.
3.2 Block Diagram
FIGURE 12 : Block Diagram
3.2.1 Block Diagram Description
This system is built using three single phase transformers
which are wired in star input and star output, and 3 transformers are connected
in delta connections, having input 220 volt and output at 12 volt. This concept
followed the low voltage testing of fault conditions as it is not advisable to
create on mains line. The 555 timers are used for handling short duration and
long duration fault conditions. A set of switches are used to create the LL, LG
and 3L fault in low voltage side, for activating the tripping mechanism. Short
duration fault returns the supply to the load immediately and is called as
temporary trip while long duration shall result in permanent trip. Figure 13 shows
the block diagram of Auto ELCB.
3.3
Schematic Diagram
FIGURE 13 : Schematic Diagram
The
project uses six numbers of step-down transformers for handling the entire
circuit under low voltage conditions of 12v to test the three phase fault
analysis. The primaries of three transformers are connected to a three phase supply
in star configuration, while the secondary of the same is connected in star
configuration. The other set of three transformers with its primary connected
in star to three phase have their secondary’s connected in delta configuration.
The outputs of all the six transformers are rectified and filtered individually
and are given to six relay coils. Six
push buttons, one each connected across the relay coil is meant to create a
fault condition either at star i.e. Line to Line Fault or three Line Fault. The
Normally Closed (NC) contacts of all the relays are made parallel while all the
common points are grounded. The parallel connected point of NC are given to
pin2 through a resistor R5 to a 555 timer i.e. wired in monostable mode. Figure
13 shows a schematic diagram of Auto ELCB.
The
output of the same timer is connected to the reset pin 4 of another 555 timer
wired in astable mode. LED’S are connected at their output to indicate their
status. The output of the U3 555 timer from pin 3 is given to an Op-amp LM358
through wire 11 and d12 to the non-inverting input pin 3, while the inverting
input is kept at a fixed voltage by a potential divider RV2. The voltage at pin
2 coming from the potential divider is held so that it is higher than the pin 3.
The Op-amp is used as a comparator so that pin 1 develops zero logic so that it
fails to operate the relay through the driver transistor Q1.
All the six relay coils will obtain DC
voltage while the board is powered from a three phase supply and their common
point disconnects from the NC and moves on to the Normally Open (NO) points by
providing logic high at pin 2 of 555 timer U1 i.e. that is kept on monostable
mode. During the push button across the relay is pressed, it disconnects the
relay. During the process, in common contacts moves to the NC position to
provide a logic low at trigger pin of 555 timer in order to develop an output
that brings the U3 555 timer where it is used in astable mode for its reset pin
to high. The astable operation takes place at its output which is also
indicated by flashing D11 LED.
If the fault is off temporary in
nature i.e. if the push button pressed is released immediately the U1
monostable disables the output U3 which will goes to zero in the event of any
push button kept pressed for a longer duration the monostable output. This
could provides a longer duration active situation for U3. The astable timer of the
output of which the charges capacitor C13 through R11 will cause the output of
the comparator goes high and drives the relay to switch off three phase load.
The output of Op-amp remains high indefinitely through a
positive feedback that was provided for its pin 1 to pin 3 through a forward
biased diode and a resistor in series. This will cause the relay to be permanently
switched on in order to disconnect the load connected as the NC contacts
permanently off. In order to maintain the flow of DC supply, the star connected
secondary set DC’S are connecting in parallel through D8, D9 & D10 in order
for the supply to be uninterrupted to the circuit voltage of 12v DC and 5v DC that
were derived out of voltage regulator IC 7805.
3.4
PCB Layout
FIGURE 14 : PCB Layout
PCB layout were
designed and obtained from the simultion of the Auto-ELCB circuit and later it
was printed on the PCB board to construct the hardware. Figure 14 shows PCB
layout of Auto ELCB.
