A simple and practical temperature control circuit

Temperature control is a widely used automatic control, which can be found in industrial and agricultural production and daily life. However, many temperature control circuits are relatively complex and have a high failure rate.

The simplest temperature controller introduced below is very practical. The attached figure is the circuit diagram of the temperature controller. Its core circuit uses the TL431 variable voltage-dividing type voltage-stabilizing integrated circuit. TB1 in the circuit uses a small 220V/12V transformer (capacitor voltage reduction power supply can also be used, but it is not isolated from the power grid). The output is rectified by D1~D4 and filtered by CI to output a 14V DC voltage to supply the temperature control circuit.

According to the equivalent circuit of TL431, it is an operational amplifier circuit with a reference power supply. When Uref>2.5V (reference voltage 2.5V), the output transistor is turned on and the output is low level. When Uref<2.5V, the operational amplifier outputs a low level and the output transistor is cut off. Changing the voltage value of Uref can change the output voltage, so it can be found in the voltage reference of the voltage regulator (ordinary linear voltage regulator and isolated switching power supply). Its output voltage Uo=Uref(1+R11/R2). Since Uref=2.5V, the output voltage can be determined by the ratio of R11/R2.

In this example, R11 is composed of a thermistor Rt, an upper limit temperature adjustment resistor Rp1, and a fixed resistor Rl in series, and R2 has a value of 5.1 kΩ, and Rp2+R3 is the lower limit temperature threshold control resistor controlled by the normally open contact of the relay.

1. Working Principle After the power is turned on, the heating resistor is connected to the 220V AC circuit through the normally closed contact of the relay, and heating begins. At this time, the temperature is room temperature, and the negative temperature coefficient of the thermistor is 10kΩ. As the heating progresses, the resistance of Rt continues to decrease, and Uref begins to rise. At this time, adjusting Rpl can also change the upper limit temperature control point T1 that determines the temperature.
When the temperature reaches the control point. Rt=Rtl, Uref=UCC*R2/(R2+R11>2.5V, the output of the operational amplifier is high, the internal transistor is turned on, the relay is energized, the normally closed contact is disconnected, and the heating stops. At the same time, the other set of normally open contacts of the relay is closed, so that Rp2+R3 is connected in parallel with R11, so that Uref rises further. This circuit is a simple hysteresis circuit.
　　
By adjusting Rp2, the lower limit temperature control point T2 of the thermostat can be adjusted. As the heating stops, the temperature begins to slowly drop. Rt gradually increases, that is, when Rt=Rtl, due to the connection of the Rp2+R3 parallel circuit, Uref is still greater than 2.5V, the output transistor continues to conduct, maintaining the relay in the energized state, and the heating resistor is still in the power-off state. Only When the temperature drops to the lower threshold value T2, Rt=Rt2, Uref=Uc-cxR2/(R2+RI1)<2.5V, the operational amplifier outputs a low level, the internal triode is cut off, and the relay is released. The normally open contact is disconnected and exits the connected circuit, while the normally closed contact is reset and the heating starts again. Repeatedly, the temperature is stabilized within the range of T1~T2 by controlling the heating resistor. It was found in the experiment that even if the resistance Rp2+R3 is not needed, the circuit will not have thermal oscillation (that is, the relay keeps switching at the point Tl), because of the inertia of heat. However, it will be more reliable after adding Rp+R3, and there is a temperature threshold range T1~T2. This value can be achieved by adjusting Rpl and Rp2.
　　
2. Circuit debugging
　　
This circuit is very simple. Because TIA31 has a driving capacity of 100mA, it can directly drive a small relay. Therefore, the circuit board can be made of a perforated board. The more difficult part is circuit debugging. Here, a 10kΩ negative temperature coefficient thermistor is used for temperature measurement, which has a relatively high accuracy. After the circuit is connected, heating begins. The 10kΩ temperature measuring resistor is placed in the temperature control room, and a thermometer is placed at the same time. When the temperature rises to the set upper limit temperature value T1, Rp1 is adjusted to turn on TL431 and the relay is energized. Continue to observe. When the temperature drops to the lower limit temperature control value T2, Rp2 is adjusted to turn off TL431 and the relay is released. Due to the nonlinearity of the temperature measuring resistor, the markings of the potentiometers Rp1 and Rp2 may also appear nonlinear. Just mark a few key points.
　　
This temperature control circuit uses only one TL431 to complete the setting and control of the temperature within a range. It is simple, practical, and cost-effective. It is very suitable for students and electronics enthusiasts to make.