Temperature PTC thermistor constant temperature control system

Most of the existing heaters are heated by traditional devices such as electric heating pipes and electric heating wires. The outer shell of the electric heating pipe is a steel pipe made of stainless steel. There is a heating element resistance wire inside. When heating, the heat is transmitted to the outside through the resistance wire and steel pipe. When flowing, the heat of the electric heating pipe cannot be dissipated, the temperature will become higher and higher, and the electric heating pipe will be burned in severe cases, or even a fire will occur. As a heating material, PTC thermistors have the characteristics of energy saving, constant temperature, no open flame, good safety, easy adjustment of heat generation, little influence by fluctuations in power supply voltage, and rapid temperature rise. Therefore, PTC thermistors are designed to be used as heating materials. The constant temperature heating system is very meaningful for applications with high safety requirements.

Overall system design

This system uses AT89C2051 as the control core, PTC thermistor to heat the heating area, digital thermometer DS18B20 collects the temperature in real time, the upper and lower limits of the heating temperature value to be set by the external keyboard, and the temperature value and setting collected in real time The temperature value is compared to determine whether the set temperature range is reached. The AT89C2051 controls the multiple relays to realize the on-off control of the working state of multiple PTC thermistors (one relay controls one PTC thermistor) to maintain the temperature in the heating area. Within the set temperature range. The system schematic is shown in Figure 1.

MCU system

AT89C2051 is a low-voltage, high-performance CMOS 8-bit microcontroller produced by the ATMEL company in the United States. It was produced using ATMEL's high-density, non-volatile memory technology during the period. It is compatible with the MCS-51 instruction system. The chip has a built-in general-purpose 8-bit CPU and 2K Byte Flash flash memory, 128 bytes internal RAM, 15 I / O port lines, 2 16-bit timer / counters, 1 5-vector two-level interrupt structure, 1 full-duplex serial communication port, built-in 1 Precision comparator, on-chip oscillator and clock circuit [1].

PTC thermistor

PTC is the abbreviation of Positive Temperature Coefficie-nt, which is a positive temperature coefficient, and its resistance value increases with increasing temperature. Before the PTC thermistor reaches a specific temperature, the resistance value changes very slowly with temperature. When the temperature exceeds this temperature, the resistance of the PTC thermistor increases sharply, and the temperature at which the resistance value changes sharply is called For Curie temperature, it is an important technical index of PTC thermistors [2].

Digital thermometer DS18B20

DS18B20 digital thermometer is a 1-Wire, a single-bus device produced by DALLAS, with simple wiring and small size. The measurement temperature range is -55 ° C to + 125 ° C, and within the range of -10 ° C to 85 ° C, the accuracy is ± 0.5 ° C. DS18B20 accuracy check is ± 2 ° C. The on-site temperature is directly transmitted in the digital mode of "first-line bus", which greatly improves the anti-interference performance of the system.

Because DS18B20 uses the 1-Wire bus protocol, that is, two-way data transmission on a data line, and for the AT89C2051 microcontroller, the hardware does not support the single-bus protocol, so software must be used to simulate the single-bus protocol. The protocol sequence of the bus completes the access to the DS18B20 chip. The protocol defines the timing of several signals: initialization timing, read timing, and write timing. All timings are based on the master as the master device and the single bus device as the slave device. The data and commands are transmitted in the low order first.