A high-precision thermostatic controller circuit without temperature step zone

The be junction of Q1 and Q2 is connected in series as a temperature sensor. The precision voltage stabilizing integrated circuit U3 (TL431) provides a high stable reference voltage source of about 2.5V for the sensor, window voltage comparator (U2C, U2D) and differential amplifier (U1C). Suitable reference input. The connection of U1A and U1B is to facilitate debugging and make the input impedance of the differential amplifier as balanced as possible. The voltage follower U1D plays the role of buffer isolation, and sends the temperature difference voltage Vd amplified by the U1C differential amplifier to the non-inverting input terminal of the voltage comparator U2B. The inverting input terminal of U2C is connected to the sawtooth wave voltage composed of U2A, etc. On the output end of the generator, when the sawtooth wave voltage is lower than Vd, U2B outputs high level, Q5 is saturated and conducts, so that the interface circuit U5 gets the input current signal, and its output end triggers the triac VS when the mains power crosses zero. conduction. The load gets electricity to work and generates heat. When the temperature in the incubator rises, LED3 is a green high-brightness light-emitting diode, which has three functions: 1. As a heating working status indicator; 2. To prevent possible malfunction of the circuit; 3. To offset the saturation voltage that may occur due to Q6 The drop is too large and Q5 cannot reliably cut off when over-temperature occurs. Because the load is powered and heated, the temperature in the incubator increases, and the temperature difference with the set point decreases, causing the differential voltage of U1C input to become smaller, U1C output to become smaller, and U1D output (Vd) to also become smaller (in Within a sawtooth wave cycle) U2B output high level time decreases, so that the temperature in the incubator rises slower. When the temperature in the incubator reaches the set temperature (37°C), the temperature difference is minimal at this time, and Vd is also Minimum, the load has the shortest power-on time. As a result, there is basically no change in the temperature in the incubator, and the heat generated by the load is equivalent to the heat emitted by the heat exchange between the incubator and the environment, thereby maintaining the temperature in the incubator at the set temperature. Since the load working time is closely related to the temperature difference (when the temperature difference is large, the load with large Vd will work for a long time and produce more heat, on the contrary, the load will work for less time and produce less heat), and the output of the sensor is amplified by about 100 times. As a result, the sensitivity and control accuracy of this circuit are relatively high. Basically there is no obvious temperature step. U2C and U2D are window voltage comparators. When the temperature in the incubator is lower than the set alarm lower limit temperature, U2C outputs a high level and the yellow light-emitting diode LED4 is lit, indicating an under-temperature state. At the same time, the alarm sound source is integrated Circuit U4 provides working power so that the audio signal output by U4 is amplified by Q7 and drives the speaker to sound an alarm; and when for some reason the circuit is out of control and the temperature in the incubator rises above the upper limit of the set alarm temperature, U2D outputs a high level , all the way to make the red light-emitting diode LED5 light up to indicate the over-temperature state, and supply power to the alarm circuit to make the speaker sound an alarm; at the same time, Q6 is saturated and turned on through R17, forcing Q5 to cut off, cutting off the input current of U5, and turning off VS . When the load loses power, it stops working to prevent the temperature in the incubator from continuing to rise, thereby protecting the safety of the culture in the incubator.