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Wednesday, October 14, 2009

Thermal Control Circuit Using LM35

This is a design circuit for accurate thermal control circuit that can be used in applications where automatic temperature control is required. Active circuits in the miniature relay ON or OFF according to temperature detected by single-chip temperature sensor LM35DZ. LM35DZ detect when the temperature is higher than the preset level (set by VR1), the relay is actuated. When the temperature has fallen below the temperature, relay is de-energized. The circuit of thermal control can be supported by AC or 12V DC supply or batteries (100mA min). This is the figure of the circuit.

How is the circuit works? First, the voltage output (at pin 2) changes linearly with temperature from 0V to 1000mV. This highly simplifies the circuit design because we only need to provide precision voltage reference (TL431) and an accurate comparator (A1 of LM358) to create a complete thermal-controlled switch. The preset (VR1) & resistor (R3) of a variable voltage divider to set the reference voltage (Vref) form 0V ~ 1.62V. The op-amp (A2) is the voltage reference buffer to avoid loading the divider network (VR1 & R3). The comparator (A1) with a reference voltage Vref (set by VR1) with a voltage output from LM35DZ and to decide whether or workers who relay (LED1 ON or OFF respectively). R2 goal is to provide a bit of hysteresis to help prevent relay chattering. Hysteresis is inversely proportional to the value R2. Low R2 value is higher hysteresis.


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