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INTRODUCTION
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 inorder 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 is 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 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>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.
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 ±1⁄4˚Cat room temperature and ±3⁄4˚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.
Features
 Calibrated directly in ˚ Celsius (Centigrade)
 Linear + 10.0 mV/˚C scale factor
 0.5˚C accuracy guaranteable (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 ±1⁄4˚C typical
 Low impedance output, 0.1 Ω for 1 mA load
TL431
The TL431/TL431Aare three-terminal adjustable regulator series with a guaranteed thermal stability over applicable temperature ranges. The output voltage may be set to any value between VREF (approximately 2.5 volts) and 36 volts with two external resistors These devices have a typical dynamic output impedance of 0.2W Active output circuitry provides a very sharp turn-on characteristic, making these devices excellent replacement for zener diodes in many applications.
Features
 Programmable Output Voltage to 36 Volts
 Low Dynamic Output Impedance 0.20 Typical
 Sink Current Capability of 1.0 to 100mA
 Equivalent Full-Range Temperature Coefficient of
 50ppm/°C Typical
 Temperature Compensated For Operation Over Full Rated
 Operating Temperature Range
 Low Output Noise Voltage
 Fast Turn-on Response