Research and Design of a Constant Temperature Control System Based on DC PTC Thermistor 3

At the beginning of operation, PTC thermistor has a surge current, which can be seen from its current time characteristics (as shown in Figure 2). After a period of time, the current drops to a stable working state, and the current is very small, about 0.8-1A. Therefore, when starting to heat, it is not possible to turn on all three relays at the same time, because the power supply of PTC thermistor is selected as DC 12V, 12A. If all three relays are turned on at the same time, when the surge current of PTC thermistor reaches its maximum value (5.7-7A), the current borne by the switching power supply will exceed its rated value, causing the power supply to stop working or even be destroyed, which affects the continuity and reliability of system operation. Given this factor, the time S1 for the current of a single PTC thermistor to start working and reach its maximum value, as well as the time S2 for the current to decrease from its maximum value to below 4A, were measured. Based on these two times, the opening of the relay can be determined. Now set the three relays as 1, 2, and 3. After the microcontroller is powered on, the first relay is turned on. After the current rises from the minimum value to the maximum value for time S1, the second relay is turned on (if the temperature provided by the first PTC thermistor does not reach the set temperature range). The opening of the third relay also takes time S1. The on/off status of each relay is determined by comparing the real-time temperature measured by the digital thermometer DS18B20. html "target=" -blank "title=" DS18B20 ">DS18B20 with the upper and lower limits of the set temperature. The order of relay opening is set to 1, 2, and 3, and the order of closing is set to 3. 2. 1. These settings are implemented by software.  diode

3.5 Keyboard and Display

The peripheral keyboard of this system adopts the HD7279A. html "target=" -blank "title=" HD7279A ">HD7279A keyboard display chip. Due to the need to display the upper and lower limits of the system set temperature and the real-time heating temperature, only the keyboard function of HD7279A. html" target="-blank" title="HD7279A">HD7279A is used. The display part of the system adopts the LT12864I LCD display module. When the system starts working, the heating temperature is set through the keyboard, and the data is transmitted to the microcontroller, which then sends the data to the display module for display; During the operation of the system, the real-time temperature values measured by the digital thermometer DS18B20 are also transmitted to the microcontroller, which converts the data and sends it to the display module for display [6-7].diode

4. System software design

The overall flowchart of the system software is shown in Figure 3. After the system is powered on, the first step is to initialize and set the registers and I/O ports. When a valid key signal is detected, the key data is read, transmitted to the microcontroller and sent to the display module for display. After the keyboard setting is completed and the flag is set, the microcontroller detects this signal and turns on the first relay for heating. At this point, initialize the digital thermometer DS18B20. html "target=" -blank "title=" DS18B20 ">DS18B20 for temperature measurement, and send the real-time temperature data to the microcontroller for processing, which is then displayed by the display module. DS18B20. html "target=" -blank "title=" DS18B20 ">After each temperature measurement, DS18B20 compares the measured temperature value with the upper and lower limits of the set temperature value. If the measured temperature is higher than the upper limit of the set temperature, it goes to the relay shutdown processing program to check the on/off status of each relay, and shuts off the relays in the order of the third, second, and first relays (one relay is closed every time it is executed); If the measured temperature is lower than the lower limit of the set temperature, switch to the relay activation processing program, check the on/off status of each relay, and activate the relays in the order of the first, second, and third relays (activate one relay every time). In the subsequent system operation, the process of temperature measurement, temperature value comparison, and turning on or off the relay is always cycled to achieve constant temperature control of the heating area.diode