Temperature control systems are widely used in industrial, agricultural, medical and other industries, and the most widely used temperature control systems are designed for single-chip or microcomputer systems. The hardware part of the system consists of input modules such as input output interface, central processing unit, A/D conversion, timing counting, etc. The system software part needs to be programmed with a large amount of PID algorithm. The design and implementation of the whole control system is complicated and cumbersome. The analog circuit signal input, amplification, operation and control output composed of discrete components are all completed by hardware circuits and do not require software design. Compared with the digital circuit, the design and implementation process is more simple, so the simple and practical analog circuit is used to realize the design of the temperature control circuit.
1 temperature control total circuit composition
The temperature control circuit is mainly composed of power supply part, temperature detecting component, signal amplification, proportional integral, voltage comparison, phase shift trigger control relay, over temperature protection, heating furnace and LED display. The circuit structure is shown in Figure 1.
Figure 1 Temperature control system circuit composition diagram
The temperature detecting component can detect the temperature value signal, and the signal is amplified and sent to the proportional integration circuit and compared with the temperature setting voltage, and the comparison result is sent to the phase trigger circuit to generate a variable period pulse to trigger the thyristor in the solid state relay. The conduction angle can control the heating power of the heating device to achieve the purpose of controlling the temperature. The temperature compensation circuit reduces the influence of room temperature on the accuracy of temperature measurement; the over-temperature protection circuit can ensure that when the heating temperature exceeds the set value, the device stops heating and functions as a protection device.
2 each circuit design
2.1 power circuit
The devices that require DC voltage in the temperature control circuit are operational amplifiers and electronic information display modules. The voltage is rectified and filtered by a 220V AC voltage. The output to the three-terminal regulator is obtained. Its circuit is shown in Figure 2.
Figure 2 power circuit diagram
2.2 Input temperature signal amplification and temperature compensation circuit
The temperature sensor is collected by a temperature sensor with a nickel-nickel-nickel-chromium K-type thermocouple as a temperature sensor. The temperature signal is mV level, and the actual measurement needs to be amplified. The thermocouple measurement temperature signal is affected by the working temperature and the free end ambient temperature, so the compensation signal is needed in the measurement to eliminate the influence of the ambient temperature change on the temperature measurement. The specific circuit is shown in Figure 3.
Figure 3 signal amplification and temperature compensation circuit
2.3 over temperature protection circuit
For example, the heating device is heated to 750 ° C with a power of 60 w. The temperature signal in FIG. 3 is amplified by 100 times and then added to the proportional integration circuit and compared with the temperature setting voltage. The comparison result is that the phase trigger circuit generates a variable period pulse. To trigger a solid state relay. In order to avoid setting the over-temperature protection circuit too high, the heating voltage is cut off when the temperature is too high. The specific circuit is shown in Figure 4.
Figure 4 Proportional integral, voltage comparison, phase shift trigger and over temperature protection circuit
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