This circuit introduces a simple AI-powered temperature control circuit. To control the temperature using this circuit, simply turn the switch to position 2, set the desired temperature, and view the temperature value displayed on the 3.5-digit digital display. This allows for precise temperature control, making operation very convenient.

I. Circuit Working Principle
The circuit uses an LM35 voltage-type integrated temperature sensor, simplifying the circuit.
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The LM35 is an internally calibrated integrated temperature sensor. Its output voltage is proportional to the Celsius temperature, exhibiting good linearity, high sensitivity, and moderate accuracy. Its output sensitivity is 10.0 mV/℃, with an accuracy of 0.5℃. Its measurement range is -55 to 150℃. It has low self-heating effect (0.08℃) at rest. It has a wide operating voltage range, functioning normally within a 4-20V supply voltage range, and consumes very little power, with an operating current typically less than 60uA. Its output impedance is low, at 0.1Ω with a 1mA load.

Based on the output characteristics of the LM35, when the temperature varies between 0 and 150℃, the corresponding voltage at its output terminal is 0-150V. This voltage is divided by potentiometer W3 and sent to the detection signal input terminal of the 3.5-digit digital display (composed of ICL7107 and related circuitry). When the input voltage is 150V, adjusting the potentiometer to display a value of 150.0 indicates the actual measured temperature.

Temperature control selection can be achieved using potentiometer W2. Adjusting W2 allows the voltage at its intermediate terminal to vary between 0 and 1.65V, corresponding to a control temperature range of 0-165℃, which is sufficient for general heating needs. With switch K in position 2, the voltage at the intermediate terminal of potentiometer W2 is sent to the digital display input terminal via voltage follower A to display the controlled temperature value.

Adjusting potentiometer W2 changes the displayed temperature value, which is the controlled temperature value. Potentiometer W1 is used for preset temperature adjustment, with a voltage adjustment range of 0-0.27V, corresponding to an adjustable temperature range of 0-27℃. After adjustment, the voltage at its intermediate tap and the voltage at the intermediate tap of potentiometer W2 are respectively fed to the inverting and non-inverting inputs of comparator amplifier B (amplification factor of 1). The output voltage of B is the difference between the two input voltages. This voltage corresponds to the difference between the two set temperature values. For example, adjusting W1 to 0.10V corresponds to a temperature of 10℃; adjusting W1 to 0.80V corresponds to a temperature of 80℃. The output voltage of B is 0.70V, indicating a temperature of 70℃. This voltage is compared with the voltage output from the integrated temperature sensor to voltage comparator C.

When the voltage output by LM35 is less than the output voltage of B, C outputs a high voltage. The thyristor T1 receives bias current and remains conducting, directly applying 220V AC to the heating element for high-power rapid heating.

When the output voltage of LM35 is greater than the output voltage of B but less than the output voltage of A, it indicates that the actual temperature is close to the control temperature. C outputs a low voltage, SCR T1 is cut off due to no bias current, voltage comparator D outputs a high level, and SCR T2 remains on. AC 220V needs to be applied across the heating element through diode D2 for low-power, slow heating (the heating power is only 25% of the original).

When the actual temperature rises above 80℃, the output voltage of LM35 is greater than 0.80V, voltage comparator D outputs a low level, SCR T2 is also cut off, and the heating element is de-energized.

Because the heating power is low at this time, coupled with heat dissipation, the temperature will not rise significantly. The actual temperature fluctuates within a small range around the control temperature, thus achieving high-precision automatic temperature control.

II. Precautions for Use
1. Switch K is in position 2 when setting the control temperature and in position 1 during normal heating control. The digital display shows the actual temperature value.

2. Potentiometers W1 and W2 can be ordinary organic solid-core potentiometers. Potentiometer W2 can be a multi-turn precision potentiometer with an indicator, and both should be mounted on the panel for individual adjustment.

3. Thyristors T1 and T2 should be 220V bidirectional thyristors with a current rating greater than the actual operating current. Heat sinks should be added during use to prevent overheating and damage.

4. D2's current rating should be greater than half of the actual operating current, and an additional heat sink should be added.

5. One end of the thyristor is connected to the ground wire of the control circuit. Therefore, the entire circuit is powered by AC mains. Insulation measures should be taken during installation and use to prevent electric shock.

6. The adjustment of W1 should be appropriately selected according to the actual heating conditions.