Research and Design of a Constant Temperature Control System Based on DC PTC Thermistor 2

3.2 PTC Thermistor

PTC stands for Positive Temperature Coefficient in English, which means positive temperature coefficient. Its resistance increases with temperature. Before reaching a specific temperature, the resistance value of PTC thermistor changes very slowly with temperature. When it exceeds this temperature, the resistance value of PTC thermistor increases sharply. The temperature at which the resistance value undergoes a drastic change is called Curie temperature, which is an important technical indicator of PTC thermistor.

Figure 2 shows the current time characteristics of PTC thermistor.

3.3 Digital thermometer DS18B20. html "target=" -blank "title=" DS18B20 ">DS18B20

DS18B20. html "target=" -blank "title=" DS18B20 ">DS18B20 digital thermometer is a 1-wire, single bus device produced by DALLAS company, which has the characteristics of simple circuit and small size. The measurement temperature range is -55 ℃ to+125 ℃, with an accuracy of ± 0.5 ℃ within the range of -10 ℃ to 85 ℃. DS18B20. html "target=" -blank "title=" DS18B20 ">The accuracy check for DS18B20 is ± 2 ℃. The on-site temperature is directly transmitted digitally through a "one line bus", greatly improving the system's anti-interference ability.  diode

Due to the fact that DS18B20 uses the 1-Wire bus protocol, which enables bidirectional data transmission over a single data line, hardware does not support the single bus protocol for AT89C2051 microcontrollers. Therefore, software methods must be used to simulate the protocol timing of a single bus in order to access the DS18B20 chip. This protocol defines the timing of several signals: initialization timing, read timing, and write timing. All timing sequences use the host as the master device and the single bus device as the slave device, and the transmission of data and commands is in the low order first [3].

3.4 Single chip microcontroller controlled multi-channel relay

The microcontroller controls the relay as shown in Figure 1, which only shows the control circuit connection of one relay. It occupies one bit of the P3 port of the microcontroller, and multiple relays can be implemented by occupying different pins. This system uses the 3-bit of the P3 port to control three relays. PTC thermistor is selected with a DC 12V voltage provided by a switching power supply. Due to the fact that the driving current of the P3 port of the microcontroller is 20mA, and the driving of the relay is equivalent to the driving of a high-power device for the microcontroller, the driving current of the P3 port must be amplified. The PTC thermistor is powered by a DC 12V voltage, which may affect the stability of the microcontroller's operation. Therefore, the photoelectric coupler 4N33 is selected to improve the driving capability of the P3 port. At the same time, because the photoelectric coupler 4N33 uses an electric optical electric conversion form to amplify the driving current, it isolates the microcontroller and the relay, eliminating potential unstable factors in the system. When the pin of the relay connected to the P3 port of the microcontroller is set to a low level, the light-emitting diode at the 4N33 input terminal will conduct, and the output terminal will output an amplified current to drive the relay to close, causing the PTC thermistor loop to conduct and start heating [4-5].diode