Brief description of circuit design of program-controlled voltage divider for impact test

Author： GOZ Electric Time：2024-05-10 21:48:44 Read：19

1 Overall block diagram of hardware circuit

The programmable voltage divider is mainly composed of five unit circuits: single-chip microprocessing, reset, keyboard display, voltage dividing ratio control, and GPIB interface, which are introduced separately below.

2 Circuit diagram and working principle of each unit

2.1 Single-chip microprocessing unit circuit

     Because the MCS-51 single-chip microcomputer application system has the characteristics of small size, high reliability, strong functions, and low price, it is widely used in intelligent instrumentation. AT89C51 is selected as the microprocessor chip here, and it has 4K EEPROM inside as the program memory. Since AT89C5l has many peripheral expansion circuits, considering the driving capability of the P0 port, a bidirectional three-state bus driver is added. Pl. 6 and Pl. 7 are respectively the REN (remote control) and ADDR (address strobe) during GPIB interface communication. Identification line, used to detect the status of instrument interface communication,

2.2 Reset circuit

     Because the programmable voltage divider is used in high-voltage impact tests, there is large electromagnetic interference. The instrument is easily disturbed and enters an infinite loop or an abnormal state, causing the interface to run out of control; adding a watchdog circuit can restore the program operation to normal. . X25045 is used here, which has 5l2x8 Byte serial EEPROM and programmable watchdog timer. The EEPROM is used to store the instrument address. Pl.0 - P1.4 of AT89C51 are connected to /CS, SO, SI and SCK of X25045 respectively. The monitoring program intermittently sends out the chip select signal to make X25045 re-time and keep the RESET - OUT terminal as normal when the program is running. low level. Once the instrument is disturbed and an infinite loop occurs or the program runs out of control and the program does not send a clear signal, RESET - OUT will turn to high level to reset the CPU and system and restart the program.

2.3 Keyboard display unit circuit

     The keyboard display unit circuit is composed of 8279 and peripheral chips. 6 buttons are used to manually set the partial pressure ratio. Six LED digital tubes display the voltage dividing ratio of the three input channels. The chip select address of the 8279 is 1000H. The segment driver uses 74LS367 and the bit driver uses 74LS07.

2.4 Voltage dividing ratio control unit circuit

     According to the current standard requirements for transformer impact testing, the transformer impact test needs to record the waveforms of three channels, so the programmable voltage divider is designed with three independent input channels. In order to prevent the digital control circuit and the voltage divider analog circuit from interfering with each other, a photoelectric coupler is used for isolation to separate the digital and analog circuits. In addition, in order to reduce the mutual interference between the three channels, metal circuits are used between each channel. Thin sheets serve as isolation shields. The expanded 8255A controls the voltage dividing ratio of the three channels. The voltage dividing ratio values are 4, 6, 8, 1O, 12, 14, and 16, a total of seven levels. The voltage divider circuit is a key part to ensure the performance of the whole machine. According to the input impedance of the digital oscilloscope used for impact testing, the voltage divider circuit uses resistors and capacitors in parallel to divide the voltage. In the voltage dividing circuit, the resistor determines the DC voltage dividing ratio. After the DC voltage dividing ratio is determined, the dynamic voltage dividing ratio is equal to the DC voltage dividing ratio by adjusting the capacitor value to achieve an ideal square wave response and an accurate voltage dividing ratio. In addition, in order to ensure the square wave response requirement, the adjustable capacitance value can be adjusted, and an inductor can be connected in series with the low-voltage output end, and the high- and low-voltage input lines should use shielded coaxial cables. High-precision resistors are used in the voltage dividing circuit, the error is 0. 1%, and the maximum input voltage amplitude is 1600V. .

2.5 GPIB interface circuit

     In shock testing, to automate the test, the voltage divider must have programmable functionality. The international general instrument standard interface, GPIB interface, is used here as the communication interface. Since the dedicated GPIB interface chip is expensive, and its timing requires special processing to match the AT89C51, the circuit is more complex. Here, 8255A is used together with a small number of peripheral chips to form the GPIB interface. Since the voltage divider has fewer interface functions and only has a listening function, the listening and speaking functions of the interface and the three-wire communication are all completed by software. Four pieces of 74LS240 are used as the bus driver and receiver, the A port of the 8255A is used as the data output port, and the B port is used as the data input port. The output and input control lines of the two 74LS240 are respectively provided by the P3. 0 and RD lines of AT89C51. Completed; Port C serves as a bit control port, connecting three handshake lines and five management lines. The output and input control lines of the two 74LS240s are completed by P3. 1 and RD respectively. Since AT89C51 uses a search mode to manage the interface, the SRO line is changed to a ground line.