Composition and working principle of digital multimeter
Digital multimeters are generally welcomed by technicians in the electrical and electronic industry because of their intuitive display values, light overall design and manufacture, and flexible and convenient use. Since most of the digital universals do not have technical instructions and specific circuit diagrams. To make it difficult to analyze, eliminate and repair the common faults of the digital multimeter, taking this type of digital multimeter as an example, briefly introduce its internal structure and circuit composition, as well as its working principle.
Open the shell of the multimeter, and it can be found that the meter is composed of three printed circuit boards (PCB) except that the printed circuit board (PCBI) composed of the display screen and the driving circuit is fixed with the front panel. The top piece (PCB1) is the power circuit of the whole machine. The power board converts the built-in battery or external power supply into various positive and negative DC power supplies needed in the machine. It should be noted that the board will simulate the ground (Analog Ground) It is separated from the digital ground (Digital Ground) to eliminate the interference between the analog signal and the digital signal when each circuit is applied. The second board (PCB2) is the input measurement circuit board, which includes resistance, AC and DC current and voltage measurement input conversion and measurement circuits, as well as the adjustment and calibration of the multimeter, etc., the (PCB2) In addition to the resistors, capacitors, integrated circuits, operational amplifiers and other electronic components required to form the above circuit, there is also a small relay component on the board. It is necessary to pay attention. The third board (PCB3) includes a circuit for data acquisition and data processing of the measured quantity. This board converts the resistance, current, and voltage analog quantities that need to be measured into digital quantities after being sampled and held by the A/D converter. Then carry out data processing on these digital quantities. Transformed into data streams of resistance, current, and voltage in various ranges, and then sent to the display drive circuit on the display panel. Finally, the measured digital value is displayed on the display.
