Digital Multimeter Maintenance Techniques and Purchase Guide
Digital instruments have high sensitivity and accuracy, and their applications are almost ubiquitous in all enterprises. However, due to the multifactorial nature of its malfunctions and the high randomness of encountering problems, there are not many patterns to follow, making it difficult to repair. Therefore, this article will summarize some repair experience accumulated in practice for everyone's reference.
For a faulty instrument, the first step is to check and distinguish whether the fault phenomenon is common (all functions cannot be measured) or individual (individual functions or individual ranges), and then differentiate the situation and solve it accordingly.
1. If all files are not working, the power circuit and A/D converter circuit should be checked carefully. When checking the power supply, remove the stacked battery, press the power switch, connect the positive probe to the negative power supply of the tested meter, and connect the negative probe to the positive power supply (for a digital multimeter). Turn the switch to the diode measurement mode. If the display shows the positive voltage of the diode, it indicates that the power supply is good. If the deviation is large, it indicates that there is a problem with the power supply. If there is an open circuit, focus on checking the power switch and battery leads. If a short circuit occurs, it is necessary to use the circuit breaking method to gradually disconnect the components that use the power supply, with a focus on checking the operational amplifier, timer, and A/D converter. If a short circuit occurs, it usually damages more than one integrated component. Checking the A/D converter can be performed simultaneously with the basic meter, equivalent to the DC meter head of an analog multimeter. Specific inspection methods:
(1) Turn the range of the tested meter to the lowest DC voltage level;
(2) Measure whether the operating voltage of the A/D converter is normal. According to the model of the A/D converter used in the table, corresponding to the V+and COM pins, compare the measured values with their typical values to see if they match.
(3) Measure the reference voltage of the A/D converter. The commonly used digital multimeter reference voltage is generally 100mV or 1V, which measures the DC voltage between VREF+and COM. If it deviates from 100mV or 1V, it can be adjusted by an external potentiometer.
(4) Check the display number with zero input, short circuit the positive terminal IN+and negative terminal IN - of the A/D converter to make the input voltage Vin=0, and the instrument will display "00, 0" or "00, 00".
(5) Check the fully lit strokes on the monitor. Short circuit the test terminal TEST pin to the positive power terminal V+, causing the logic ground to become high and all digital circuits to stop working. Due to the direct current voltage applied to each stroke, all strokes are lit up and the alignment table displays "1888" and "18888". If there is a missing stroke, check whether there is poor contact or wire breakage between the corresponding output pin of the A/D converter and the conductive adhesive (or wiring), and between the A/D converter and the display.
