Transformer temperature intelligent monitor circuit module design

 An intelligent temperature monitor with ATMET's 89C51 microcontroller as the core is given. The monitor has the function of automatically recording the three-phase temperature, phase, historical maximum temperature and other data before power outage, as well as the black box function. 89C51 microcontroller is an 8-bit Flash microcontroller from ATMET Company. Its biggest feature is that it contains 4KB of reprogrammable Flash memory, 128×8B internal RAM and two 16-bit timers/counters. It is easy to modify the program during the development process. And compatible with MCS-51. For the LED control driver, PS7219 is selected. Its interface adopts synchronous serial peripheral interface (SPI), which can drive 8-bit digital tubes at the same time (using 5 bits of them). Because PS7219 has 15×8 RAM function control registers inside, it can be easily found. address, each digit can be refreshed independently, the display brightness can be numerically controlled, and each digit has flicker enable control and full-bright function testing, which can simplify the design of the hardware circuit, save CPU interface lines and reduce the time occupied by the CPU. Because the working status setting, parameter setting and other keys use a composite keyboard, there are fewer keys. The keyboard is designed as an independent key interface. It can easily determine which key is pressed by detecting the level of the input line. The communication with the upper level meter uses the MAX232 ($1.5790) chip. This chip has a dual set of driver/receiver. It contains a capacitive voltage generator that can provide EIA/TIA-232-E level when powered by a single power supply. A receiver converts the EIA/TIA-232-E level input to 5V TTL/COMS level, and the instrument needs to save some special data. The circuit is designed and equipped with an EEPROM and power monitoring circuit to achieve the above functions. The output driver circuit uses the ULN2003 ($0.1437) driver. This chip has 7 Darlington tubes. Its collector can collect a maximum current of 500mA. Because it is internally connected with a freewheeling diode, it can accurately drive relays with inductive loads.

Pre-signal processing circuit : CD4052 ($0.1163) is a dual 4-select-1 multi-channel analog switch. Its 4 sets of inputs are 3-channel 3-phase winding temperature signals and ambient temperature signals (optional). These signals are switched by the program through the microcontroller. The mouth line is controlled. Because the detected temperature signal is a slowly changing signal. In order to effectively improve the anti-interference ability of the instrument, RC low-pass filtering is performed. In order to improve the input impedance and high common-mode rejection ratio, the first stage of the preamplifier adopts differential connection. The two pairs of resistors R4, R5, R6, R7, R8, and R9 must be selected carefully, and the amplifier should be selected with micro-drift and low offset. , high gain and high common mode rejection chopper type automatic zeroing operational amplifier TIL7650. The amplification factor of the second-stage inverting amplifier is adjusted through the RW2 multi-turn potentiometer.