Relay temperature controller based on LM35 and TL431

This is a simple yet high-precision thermal control circuit for applications requiring automatic temperature control. The circuit switches a miniature relay based on the temperature detected by the single-chip temperature sensor LM35DZ. The relay is activated when the LM35DZ detects a temperature higher than a preset level (set by VR1). The relay de-energizes when the temperature falls below the preset temperature. The circuit can be powered by any 12V AC or DC power supply or a battery (minimum 100mA).

The LM35 temperature sensor comes in several versions:

LM35CZ and LM35CAZ (TO-92 package) --- −40°C to +110°C

LM35DZ (TO-92 package) -------------- 0 ~ 100°C

LM35H and LM35AH (TO-46 package) ------ −55°C to +150°C

Working principle:

The core of the circuit is the LM35DZ temperature sensor, which is factory-calibrated in degrees Celsius (or percentages) and features linear-to-voltage conversion. The output voltage (at pin 2) varies linearly with temperature, from 0V (0°C) to 1000mV (100°C). This greatly simplifies the circuit design, as we only need to provide a precision voltage reference (TL431) and a precise comparator (A1 of the LM358) to build a complete thermal switch. A preset (VR1) and resistor (R3) form a variable voltage divider, setting the reference voltage (Vref) from 0V to 1.62V. An operational amplifier (A2) buffers the reference voltage to avoid loading the voltage divider network (VR1 and R3). The comparator (A1) compares the reference voltage Vref (set by VR1) with the output voltage of the LM35DZ and determines whether to activate or deactivate the relay (corresponding to LED1 being on or off, respectively). R2 provides a slight hysteresis to help prevent relay bounce. The hysteresis is inversely proportional to the value of R2. The lower the R2 value, the higher the hysteresis.

calibration:

No special instruments are required. The relay can be set to "trip" (change state) at any temperature between 0 and 100°C. For example, to set the trip point to 70°C (switch temperature): connect a precision digital voltmeter or multimeter between the test points "TP1" and "GND". Slowly adjust VR1 until you get an accurate reading of 700mV (or 0.7V) on the voltmeter or multimeter.

Application example:

This is a practical example demonstrating how to incorporate this circuit into an automatic temperature control system to maintain the water temperature in a container at 70°C.

Parts list

IC1: LM35DZ Precision Celsius Temperature Sensor

IC2: TL431 +2.5V Precision Voltage Reference

IC3: LM358 Dual Single-Supply Operational Amplifier.

LED1 -- 3mm or 5mm LED

Q1 -- General purpose PNP transistor (A1015, etc.), pin arrangement ECB

D1, D2 -- 1N4148 silicon diodes (or 1SS133)

D3, D4 -- 1N400x (x=2,...,7) rectifier diodes

ZD1 --- Zener diode, 13V, 400mW

Preset (fine-tuning potentiometer):

2.2K (temperature set point) (No. 222 or 2k2)

Resistance: (1/4W or 1/6W)

R1 -- 10K

R2 -- 4.7M

R3 -- 1.2K

R4 -- 1K

R5 -- 1K

R6 -- 33Ω

Capacitor:

C1 -- 0.1 µF ceramic or polyester film capacitor (part number 104 or 100n)

C2 -- 470 µF or 680 µF electrolytic capacitor (minimum 16V)

Miscellaneous:

8-pin socket -- x 1

Miniature relay -- DC12V DPDT, coil = 400Ω or higher