What's the matter with the digital multimeter? The digital display function on the screen doesn't work.
1. Quick Judgment of the Quality of the Analog-to-Digital Converter (ADC) of a Digital Multimeter
The questioner stated that the display screen of the digital multimeter shows something, but the functions don't work. If the data displayed on the screen is disorderly, and when using the multimeter to measure parameters such as voltage and resistance, the numbers displayed on the multimeter don't change either. In this case, we need to suspect whether the ADC inside is broken. Here, taking the commonly used ADC ICL7106 in an ordinary 3½-digit digital multimeter as an example, we will introduce how to quickly judge the quality of the ADC.
As shown in the figure above, it is a 200mV DC voltmeter composed of ICL7106. When working normally, there is a stable voltage of 2.8 to 3.0V between the V+ terminal (pin 1) and the COM terminal (pin 32) of ICL7106, and this voltage will not change with the change of the battery voltage of the multimeter. Therefore, we only need to use another multimeter to measure the voltage between pin 1 and pin 32 of ICL7106 to see if it is within the above range. If the measured voltage deviates greatly from the above voltage, it can be basically determined that there is a problem with the ADC or its peripheral components. At this time, we can first check those components connected externally to ICL7106, and then determine whether ICL7106 is damaged.
2. Judgment of the Faults of the Test Leads and Input Channels of the Multimeter
After confirming that the ADC of the digital multimeter is okay, we can first adjust the range switch to the DC 200mV range and check whether the number displayed on the screen is "000.0". Then, adjust it to the resistance range and check whether the displayed number is "1". If the displayed number can change when rotating the range switch, it means that there is a broken wire inside the test leads. At this time, we can use the resistance range or the buzzer range to determine which test lead has a problem. In addition, a fuse is connected in series at the input terminal of the current range of 200mA and below of the digital multimeter. If this fuse is blown, it will cause the current range to be unusable.
