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INTRODUCTION
The project is based on the working of temperature controller using LM35 and fan as
cooler. The circuit automatically senses the temperature and works normally within a particular
temperature range. Above that range the sensor produces a signal and automatically turns on the
cooling fan to control the testing temperature. 0-100°C electronic temperature controlled relay is
a circuit using which the temperature can be controlled with the help of a LM35 temperature
sensor.
The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is
linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus has an advantage
over linear temperature sensors calibrated in˚ Kelvin, as the user is not required to subtract a
large constant voltage from its output to obtain convenient Centigrade scaling. The LM35 does
not require any external calibration.
Using a precision reference (TL431) a temperature is set and an accurate comparator in order to
construct a simple thermal controlled switch. The temperature that is set is compared with the
output of the LM35 which decides whether to energize or de-energize the relay.
An LED and an alarm are used to indicate when the device crosses the set temperature.
The circuit is very useful in practical areas like heater, iron box etc.
Block Diagram
BLOCK DIAGRAM EXPLANATION
The functional block diagram is shown. It comprises of a LM35, comparator, relay and a reference voltage to set the reference temperature.
LM35: Temperature sensor which is calibrated in the Celsius (Centigrade) scale with a linear degree to volt conversion.
Comparator: An accurate comparator (A1 of LM358) in order to construct a complete thermal-controlled switch
Reference voltage: The preset (VR1) & resistor (R3) from a variable voltage divider which sets a reference voltage (V ref) form 0V ~ 1.62V. The op-amp (A2) buffers the reference voltage so as to avoid loading the divider network (VR1 & R3).
Relay: The circuit switches a miniature relay ON or OFF according to the temperature detected by the single chip temperature sensor LM35DZ. When the LM35DZ detects a temperature higher than the preset level (set by VR1), the relay is actuated. When the temperature falls below the preset temperature, relay is de-energized.
REFERENCE
VOLTAGE
LM 35 COMPARATOR
RELAY
List of Components
Fig 1: Circuit diagram of temperature controller using LM35dz
The fig1 shows the circuit diagram of temperature controller using LM35dz and following are
the list of components of temperature controller:
IC1: LM35DZ Precision Celsius (Centigrade) Temperature sensor
IC2: TL431 +2.5V precision voltage reference
IC3: LM358 Dual single supply Op-amp.
LED1: 3mm or 5mm LED
Q1: General purpose PNP transistor ( A1015,...) with E-C-B pin-out
D1, D2: 1N4148 silicon diodes (or 1SS133)
D3, D4: 1N400x (x=2,,,,.7) rectifier diodes
ZD1: Zener diode, 13V, 400mW
Preset (trim pot) : 2.2K (Temperature set point)
(# 222 or 2k2)
Resistor: ( 1/4W or 1/6W)
R1 -- 10K
R2 -- 4.7M
R3 -- 1.2K
R4 -- 1K
R5 -- 1K
R6 -- 33Ω
Capacitors:
C1 -- 0.1 µF ceramic or Mylar cap
(# 104 or 100n)
C2 -- 470 µF or 680 µF electrolytic cap.
(16V min)
Miscellaneous items:
8-pin socket -- x 1 pcs
Miniature relay -- DC12V SPDT, Coil=400 Ω or higher
Temperature sensor (LM35)
The LM35 series are precision integrated-circuit temperature sensors, whose output voltage is
linearly proportional to the Celsius (Centigrade) temperature. The LM35 thus has an advantage
over linear temperature sensors calibrated in ° Kelvin, as the user is not required to subtract a
large constant voltage from its output to obtain convenient Centigrade scaling. The LM35 does
not require any external calibration or trimming to provide typical accuracies of ±/4°C at room
temperature and ± /4°C over a full -55 to +150°C temperature range. Low cost is assured by
trimming and calibration at the wafer level. The LM35’s low output impedance, linear output,
and precise inherent calibration make interfacing to readout or control circuitry especially easy.
It can be used with single power supplies, or with plus and minus supplies. As it draws only 60
µA from its supply, it has very low self-heating, less than 0.1°C in still air. The LM35 is rated to
operate over a -55° to +150°C temperature range, while the LM35C is rated for a -40° to +110°C
range (-10° with improved accuracy). The LM35 series is available packaged in hermetic TO-46
transistor packages, while the LM35C, LM35CA, and LM35D are also available in the plastic
TO-92 transistor package. The LM35D is also available in an 8-lead surface mount small outline
package and a plastic TO-220 package.
BACKGROUNG OF LM35 SENSOR
Most commonly-used electrical temperature sensors are difficult to apply. For example,
thermocouples have low output levels and require cold junction compensation. Thermistors are
nonlinear. In addition, the outputs of these sensors are not linearly proportional to any
temperature scale. Early monolithic sensors, such as the LM3911, LM134 and LM135, overcame
many of these difficulties, but their outputs are related to the Kelvin temperature scale rather than
the more popular Celsius and Fahrenheit scales. Fortunately, in 1983 two I.C.’s, the LM34
Precision Fahrenheit Temperature Sensor and the LM35 Precision Celsius Temperature Sensor,
were introduced. This application note will discuss the LM34, but with the proper scaling factors
can easily be adapted to the LM35. The LM35/LM34 has an output of 10 mV/°F with a typical
nonlinearity of only ±0.35°F over a −50 to +300°F temperature range, and is accurate to within
±0.4°F typically at room temperature (77°F). The LM34’s low output impedance and linear
output characteristic make interfacing with readout or control circuitry easy. An inherent strength
of the LM34 sensor over other currently available temperature sensors is that it is not as
susceptible to large errors in its output from low level leakage currents. For instance, many
monolithic temperature sensors have an output of only 1 μA/°K. This leads to a 1°K error for
only 1 μ-Ampere of leakage current. On the other hand, the LM34 sensor may be operated as a
current mode device providing 20 μA/°F of output current. The same 1 μA of leakage current
will cause an error in the LM34’s output of only 0.05°F (or 0.03°K after scaling).
Low cost and high accuracy are maintained by performing trimming and calibration
procedures at the wafer level. The device may be operated with either single or dual supplies.
With less than 70 μA of current drain, the LM34 sensor has very little self-heating (less than
0.2°F in still air), and comes in a TO-46 metal can package, a SO-8 small outline package and a
TO-92 plastic package.
The LM35/LM34 is a versatile device which may be used for a wide variety of
applications, including oven controllers and remote temperature sensing. The device is easy to
use (there are only three terminals) and will be within 0.02°F of a surface to which it is either
glued or cemented. The TO-46 package allows the user to solder the sensor to a metal surface,
but in doing so, the GND pin will be at the same potential as that metal. For applications where a
steady reading is desired despite small changes in temperature, the user can solder the TO-46
package to a thermal mass. Conversely, the thermal time constant may be decreased to speed up
response time by soldering the sensor to a small heat fin.
Features:
Calibrated directly in ° Celsius (Centigrade)
Linear + 10.0 mV/°C scale factor
0.5°C accuracy guarantee able (at +25°C)
Rated for full -55° to +150°C range
Suitable for remote applications
Low cost due to wafer-level trimming
Operates from 4 to 30 volts
Less than 60 µA current drain
Low self-heating, 0.08°C in still air
Nonlinearity only ±/4°C typical
Dual single supply OP-AMP (LM358)
The LM358 series consists of two independent, high gain, 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 drain is independent of the magnitude of the power supply voltage.
The LM358 and LM2904 are available in a chip sized package (8-Bump micro SMD) using
National’s micro SMD package technology.
Fig2: Pin diagram of LM358 OP-AMP
LM358 Features:
Available in 8-Bump micro SMD chip sized package
Internally frequency compensated for unity gain
Large dc voltage gain: 100 dB
Wide bandwidth (unity gain): 1 MHz (temperature compensated)
Wide power supply range:
Single supply: 3V to 32V
or dual supplies: ±1.5V to ±16V
Very low supply current drain (500 μA)—essentially independent of supply
voltage
Low input offset voltage: 2 mV
Input common-mode voltage range includes ground
Differential input voltage range equal to the power supply voltage
Large output voltage swing
The LM358 (dual), is voltage feedback amplifiers that are internally frequency compensated to
provide unity gain stability. At unity gain (G=1), the amplifier offer 550 kHz of bandwidth.
They consume only 0.5mA of supply current over the entire power supply operating range. The
LM358 is specifically designed to operate from single or dual supply voltages.
The LM358 offer a common mode voltage range that includes ground and a wide output voltage
swing. The combination of low-power, high supply voltage range, and low supply current make
these amplifiers well suited for many general purpose applications and as alternatives to several
industry standard amplifiers on the market today.
Fig3:LM358 Pin Configuration
LM358 Pin Configuration:
Pin No. Pin Name Description
1 OUT1 Output, channel 1
2 -IN1 Negative input, channel 1
3 +IN1 Positive input, channel 1
4 –Vs Negative supply
5 +IN2 Positive input, channel 2
6 -IN2 Negative input, channel 2
7 OUT2 Output, channel 2
8 +Vs Positive supply
Precision Voltage Reference (TL431)
Three terminal shunt regulated family with an output voltage ranges between V ref and
36V, to be set by two external resistors.
• The TL431xDBZR types feature an enhanced stability area with a very low load capacity
requirement.
• The TL431xFDT types offer an enhanced stability area and a higher Electromagnetic
Interference (EMI) ruggedness, for example, for Switch Mode Power Supply (SMPS)
applications.
• The TL431xSDT types are designed for standard requirements and linear applications.
The TL431 is three-terminal adjustable Shunt Regulators with specified thermal stability
over applicable automotive, commercial, and military temperature ranges. The output
voltage can be set to any value between Vref (approximately 2.5 V) and 36 V, with two external
resistors. These devices have a typical output impedance 0.2. Active output circuitry provides a
very sharp turn-on characteristic, making these devices excellent replacements for Zener
Diodes in many applications, such as onboard regulation, adjustable power supplies, and
switching power supplies. The TL432 has exactly the same functionality and electrical
specifications as the TL431 but has different pinouts for the DBV, DBZ, and PK packages.
Both the TL431 and TL432 devices are offered in three grades, with initial tolerances (at 25C) of
0.5%, 1%, and 2%, for the B, A, and standard grade, respectively. In addition, low output drifts
vs temperature ensures good stability over the entire temperature range.
The TL43xxC devices are characterized for operation from 0C to 70C, the TL43xxI devices are
characterized for operation from 40C to 85C, and the TL43xxQ devices are characterized for
operation from 40C to 125C.
Fig 4: TL431
TL431 adjustable shunt references the characteristics of the chip and its application in a number
of functional circuits. Texas Instruments (TI) TL431 is the first production of a good thermal
stability of three-terminal adjustable shunt reference. Its output voltage with two resistors can be
arbitrarily set to from Vref (2.5V) to 36V range of any value. The device's typical dynamic
impedance of 0.2Ω, in many applications can use it instead of Zener diodes, for example, digital
voltage meter, amplifier circuit, voltage regulator power supply, switching power supply and so
on. The left is the symbol of the device.3 pins are: the cathode (Cathode), anode (anode) and the
reference terminal (REF).
Fig5: TL431 reference
Can be seen from the fig 4, VI is an internal 2.5V reference, connected to the inverting input of
op amp. Shows the characteristics of the op amp, and only when the REF terminal (inverting
input) voltage is very close to the VI (2.5V), the transistor will have a stable in the non-saturation
current through, and with small changes in voltage REF, through the transistor in Figure 1 from 1
to 100mA of current changes. Of course, the figure is by no means the actual internal structure of
the TL431; it cannot simply replace it with this combination.
Features and benefits:
Programmable output voltage up to 36 V
Three different reference voltage tolerances:
Standard grade: 2 %
A-Grade: 1 %
B-Grade: 0.5 %
Applications:
Typical temperature drift: 6 mV (in a range of 0 C up to 70 C)
Low output noise
Typical output impedance: 0.2ohm
Sink current capability: 1 mA to 100 mA
AEC-Q100 qualified (grade 1)
Adjustable precision shunt regulator:
Shunt regulator
Precision current limiter
Precision constant current sink
Isolated feedback loop for Switch Mode Power Supply (SMPS)
PNP Transistor (A1015)
The type of BJT is the PNP, consisting of a layer of N-doped semiconductor between two layers
of P-doped material. A small current leaving the base is amplified in the collector output. That is,
a PNP transistor is "on" when its base is pulled low relative to the emitter.
The arrows in the NPN and PNP transistor symbols are on the emitter legs and point in the
direction of the conventional current flow when the device is in forward active mode.
A device for remembering the PNP transistor symbol is pointing in (proudly), based on the
arrows in the symbol and the letters in the name. That is, the PNP transistor is the BJT transistor
that is "pointing in".
The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the
previous tutorial. Basically, in this type of transistor construction the two diodes are reversed
with respect to the NPN type giving a Positive-Negative-Positive configuration, with the arrow
which also defines the Emitter terminal this time pointing inwards in the transistor symbol.
Also, all the polarities for a PNP transistor are reversed which means that it "sinks" current into
its Base as opposed to the NPN transistor which "sources" current through its Base. The main
difference between the two types of transistors is that holes are the more important carriers for
PNP transistors, whereas electrons are the important carriers for NPN transistors. Then, PNP
transistors use a small base current and a negative base voltage to control a much larger emitter-
collector current. In other words for a PNP transistor, the Emitter is more positive with respect to
the Base and also with respect to the Collector.
The construction of a "PNP transistor" consists of two P-type semiconductor materials either side
of an N-type material as shown below:
Fig 6: The construction and symbol of a PNP Transistor
The construction and terminal voltages for an NPN transistor are shown above. The PNP
Transistor has very similar characteristics to their NPN bipolar cousins, except that the
polarities (or biasing) of the current and voltage directions are reversed for any one of the
possible three configurations looked at in the Common Base, Common Emitter and Common
Collector.
Fig7: PNP A1015 Transistor
Absolute Maximum Ratings of A1015:
• Maximum Temperatures
Storage Temperature............................................................................................ -55 ~ +150 °C
Junction Temperature................................................................................... +150 °C Maximum
• Maximum Power Dissipation
Total Power Dissipation (Ta=25°C) ............................................................................... 400 mW
• Maximum Voltages and Currents (Ta=25°C)
VCBO Collector to Base Voltage ....................................................................................... -50 V
VCEO Collector to Emitter Voltage .................................................................................... -50 V
VEBO Emitter to Base Voltage............................................................................................ -5 V
IC Collector Current..................................................................................................... -150 mA
SPDT RELAY
SPDT Relay (Single Pole Double Throw Relay) an electromagnetic switch, consist of a coil
(terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one
normally open terminal (87) (Figure 1).
When the coil of an SPDT relay (Figure 1) is at rest (not energized), the common terminal (30)
and the normally closed terminal (87a) have continuity. When the coil is energized, the common
terminal (30) and the normally open terminal (87) have continuity.
The diagram below center (Figure 2) shows an SPDT relay at rest, with the coil not energized.
The diagram below right (Figure 3) shows the relay with the coil energized. As you can see, the
coil is an electromagnet that causes the arm that is always connected to the common (30) to pivot
when energized whereby contact is broken from the normally closed terminal (87a) and made
with the normally open terminal (87).
When energizing the coil of a relay, polarity of the coil does not matter unless there is
a diode across the coil. If a diode is not present, you may attach positive voltage to either
terminal of the coil and negative voltage to the other, otherwise you must connect positive to the
side of the coil that the cathode side (side with stripe) of the diode is connected and negative to
side of the coil that the anode side of the diode is connected.
Working of ATC
The circuit switches a miniature relay ON or OFF according to the temperature detected
by the single chip temperature sensor LM35DZ. When the LM35DZ detects a temperature higher
than the preset level (set by VR1), the relay is actuated. When the temperature falls below the
preset temperature, relay is de-energized. The circuit can be powered by any AC or DC 12V
supply or battery (100mAmin.)
Remark: There are several versions of LM35 temperature sensors :
LM35CZ & LM35CAZ (inTO-92case)−40°Cto+110°C
LM35DZ (inTO-92case)0~100oC
LM35H & LM35AH (in TO-46 case) −55°C to +150°C
The one we supply with this project is the -DZ version with temperature detection range from 0
~ 100oC.
The heart of the circuit is the LM35DZ temperature sensor which is factory-calibrated in the
Celsius (or Centigrade) scale with a linear Degree->Volt conversion function. The output voltage
(at pin 2) changes linearly with temperature from 0V (0oC) to 1000mV (100oC). This greatly
simplifies the circuit design as we only need to provide a precision voltage reference (TL431)
and an accurate comparator (A1 of LM358) in order to construct a complete thermal-controlled
switch. The preset (VR1) & resistor (R3) from a variable voltage divider which sets a reference
voltage (Vref) form 0V ~ 1.62V. The op-amp (A2) buffers the reference voltage so as to avoid
loading the divider network (VR1 & R3). The comparator (A1) compares the reference
voltage Vref (set by VR1) with the output voltage of LM35DZ and decides whether to energize or
de-energize the relay (LED1 ON or OFF respectively). The purpose of R2 is to provide a bit of
hysteresis which helps to prevent relay chattering. Hysteresis is inversely proportional to the
value of R2. Lower value of R2 gives higher hysteresis.
Calibration : No special instrument is required. The relay can be set to "trip" (change state) at
any temperature form 0 ~ 100oC. For example: To set a 70oC trip point ( switch over
temperature) :
1. Connect a precision digital volt meter or multimeter across the test points "TP1" &
"GND".
2. Slowly Adjust VR1 until you get a exact reading of 700 mV ( or 0.7V) on your voltmeter
or multimeter.
Bibliography
www.google.co.in
http://www.eeweb.com/blog/circuit_projects/temperature-controlled-electronic-relay-by-
lm35
http://www.escol.com.my/Projects/Project-03(Thermostat-1)/Proj-03.html
http://electroschematics.com/628/lm358-datasheet/
http://www.datasheetdir.com/TL431+Voltage-References
http://en.wikipedia.org/wiki/Bipolar_junction_transistor#PNP
http://www.datasheetcatalog.org/datasheet/hsmc/A1015.pdf