KATHMANDU UNIVERSITY
SCHOOL OF ENGINEERING
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
ASSIGNMENT-IV
On/Off Control Design Of Electric Iron
Submitted By:
Anil Poude(31049)
Submitted To:
Dr. Samundra Gurung, PhD
24 september 2022
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ABSTRACT
This report, Mini project report on control of elctric iron is a third year project report which
reflects our understanding about the design of simple ON/ OFF control using the concept that we
have built so far. It is a basic design with simple calcultaion and doesn’t include the most
efficient way of designing the ON/OFF control but the basic foundation above which other
concept are developed.
This project sole purpose is to reflect the understanding of how a simple switch control can be
made using relay, operational amplifier, comparator and basic components. Here the ON/OFF
switch control of electic iron is designed which turns on and off automatically depending upon
its temperature. So as temperature of electric iron become 150 c it autometically turns off.
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1. Block Diagram
Thermi
stor
Signal
condtitioni
ng
Circuit
comparator
Relay
divier
Heating
coil of
Iron
Refrance volage
Figure 1. Overall Block Diagram.
The overall block diagram represents how our ON/OFF control switch works surfacially. Each
block have their own design mechanism which together make the control of a switch possible. It
is a simple concept which is the foundation of many complex switch mechanism. This concept
has been implemented on the automated on and off electric iron. This gives us basic knowledge
of ON/OFF switch control but doesn’t actually represents the exact circuit used in the electric
iron that we see but simpler version of it.
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2. Sensor Block
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so
than in standard resistors. The word thermistor is a portmanteau of thermal and resistor.
Thermistors are divided based on their conduction model. Negative Temperature Coefficient
(NTC) thermistors have less resistance at higher temperatures, while Positive Temperature
Coefficient (PTC) thermistors have more resistance at higher temperatures. Hence, a PTC
thermistor's resistance is directly proportional to temperature.
The temperature sensors in the KTY81 series have a positive temperature coefficient of
resistance and are suitable for use in measurement and control systems.
i.e
Resistance α temperature
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Figure 2. Sensor Block.
So thermistor sensor has been used in switch control design for automated street light. It can
operate at 5V and maximum rating voltage is 100V. It is also known as temperature depending
resistor.
Figure 3: Thermistor
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3. . Signal Conditioning Block
The KTY series of silicon PTC thermistors from NXP Semiconductors are ideal for use as
temperature sensing devices in measurement and control systems. Whilst non-linear, they exhibit
highly repeatable, predictable and stable resistance versus temperature characteristics. The KTY
sensors have a wide operating range and long operating life and are available in various package
styles and initial tolerances.
Fail-safe positive temperature coefficient
Semi-logarithmic R/T characteristic
Close resistance tolerance (KTY81/110 = ±1%)
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First we have to determinr the range in which our sensor work. And our range of operation is
shown in above table. ie -55c to 150c. And our designed circuit will cutoff the supply to the
heating coil of electric iron when the temperature of iron become 150c.
So we can design our signal in such a way that whenever the sensor senses above 150c heating
of electric iron should be off. Now since the sensor gives value in Ohm so its corresponding
nature with resistance mush be known. This can be done by looking at temperature Vs
Resistance graph.
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Temperature
0
150
Resistace
0
2.1k
Volt
0
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Now using the above data we develop the equation that satisfies the characteristics of the sensor.
We use linear equation to do so as the curve between resistance and temparatuere is linear. And
we know the voltage and resistance has linear relation.
We know,
𝑖)𝑉𝑜𝑙𝑎𝑡𝑎𝑔𝑒 𝛼 𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑖𝑖)𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝛼 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
So, from above relation :
𝑖. 𝑒 𝑉𝑜𝑙𝑎𝑡𝑔𝑒 𝛼 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
4 Use of Comparators and Op amps
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For signal conditioning , refrance voltage generation and comparision of the sensor voltage and
refrance voltage I used two op amp LM741 and one comparator LM311. By the calculation the
output voltage of the singnal conditioning circuit is 2.22 volt at 150c so to compare this refrace
voltage, we have to generate the refrace voltage as 2.22v. so to do that we used LM 741 with R1
1k value and Rf= 0.45k value.
5. Comparator Block
In this block two signals are compared and a output is given after their comparison. If the signal
at +ve terminal is greater than –ve terminal then the output is high while if vice versa then the
signal is low. In this way the signal after conditioning are compared and if the signal coming
from sensor is higher than reference signal meaning its temperature is 150c. This high output of
comparator drives the relay switch circuit. Here we have used LM311 as the comparator.
4.1 Why LM311?
It is used due to following advantages:
• Maximum Current from Vcc +: 7.5mA.
•
Single supply for two op-amps enables reliable operation.
•
Hence capable of driving load up to 50V and 50 mA.
•
Can drive most of the TTL and MOS loads.
•
Output can be Isolated from System Ground.
Figure 3. Comparator Block.
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4. Relay Driver Block
Figure 4. Relay Block.
In this circuit there is a NPN transistor to which the processed signal is given. This signal is then
given to relay. The relay is an inductor switch which is connected to AC source to operate the
heating coil. To prevent the back flow of current in an inductor the diode is used which prevents
back flow of current. Finally this all block is connected to make the automated electric iron work
based on ON/OFF switch control.
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Register is used before the transistor so that enough current can flow through transistor to drive
it. To know the resistor value of R1.
We Know :
𝐼𝐵 = 1𝑚𝐴,
𝑉𝐵 = 𝑉𝐵𝐸 − 𝑉𝐸 = 0.7,
𝐼𝐵 =
12−𝑉𝐵
𝑅𝐵 =
𝑅
,
12−0.7
1
= 11.3 ≈ 12 𝐾𝑂ℎ𝑚,
Advantages of using Diode across relay:
•
It reduces the harmonics and it also reduces sparking and arching across the mechanical
switch so that it reduces the voltage spike seen in a inductive load.
•
Freewheeling diode is used to protect the circuit from unusual damage caused due to
abrupt reduction in the current flowing through the circuit.
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6. Overall Circuit Simulation
Figure 5. Proteus Simulation.
In this way all the blocks are connected with each other and overall circuit is tesed on proteus.
Here the signal from the sensor is high as the temperature is below 150c. So the signal is high at
the comparator output which is given to transistor and the relay switch is ON. This causes the
AC source connected to the heatig coil is turnes ON
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Figure 6. LM311 Datasheet
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Figure 7. LM741 Datasheet
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