Introduction to digital multimeter techniques
For a faulty instrument, you should first check and determine whether the fault phenomenon is common (all functions cannot be measured) or individual (individual functions or individual ranges), and then distinguish the situation and solve it accordingly.
1. If all gears fail to work, focus on checking the power circuit and A/D converter circuit. When checking the power supply part, you can remove the laminated battery, press the power switch, connect the positive test lead to the negative power supply of the meter under test, and connect the negative test lead to the positive power supply (for a digital multimeter). Turn the switch to the diode measurement position. If it displays is the forward voltage of the diode, it means that the power supply part is good. If the deviation is large, it means there is a problem with the power supply part. If there is an open circuit, focus on checking the power switch and battery leads. If a short circuit occurs, you need to use the circuit breaking method to gradually disconnect the components using power, focusing on checking the operational amplifier, timer, A/D converter, etc. If a short circuit occurs, generally more than one integrated component will be damaged. The A/D converter can be checked at the same time as the basic meter, which is equivalent to the DC meter of an analog multimeter. The specific inspection method is:
(1) The measuring range of the meter under test is turned to the lowest DC voltage range;
(2) Measure whether the working voltage of the A/D converter is normal. According to the A/D converter model used in the table, corresponding to the V+ pin and COM pin, whether the measured value is consistent with its typical value.
(3) Measure the reference voltage of the A/D converter. The reference voltage of commonly used digital multimeters is generally 100mV or 1V, that is, measure the DC voltage between VREF+ and COM. If it deviates from 100mV or 1V, use an external potentiometer. Make adjustments.
(4) Check the display number whose input is zero, short-circuit the positive terminal IN+ and the negative terminal IN- of the A/D converter, so that the input voltage Vin=0, the meter displays "00.0" or "00.00".
(5) Check the monitor for full bright strokes. Short-circuit the test terminal TEST pin and the positive power supply terminal V+, so that the logic ground becomes high potential and all digital circuits stop working. Since DC voltage is applied to each stroke, all strokes light up and the alignment meter displays "1888", and the alignment meter displays "18888". If there are missing strokes, check whether there is poor contact or disconnection between the corresponding output pin of the A/D converter, the conductive adhesive (or connection), and the display.
