Automatic Temperature Controller Using Lm35

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