Troubleshooting Steps and Precautions for Multimeters
A multimeter consists of three main parts: the meter head, measuring circuit, and conversion switch.
(1) Header: It is a highly sensitive magneto electric DC current meter, and the main performance indicators of a multimeter basically depend on the performance of the header. The sensitivity of the meter head refers to the value of the direct current flowing through the meter head when the pointer deviates to full scale. The smaller this value, the higher the sensitivity of the meter head. The larger the internal resistance during voltage measurement, the better its performance. There are four scale lines on the meter head, and their functions are as follows:
lines (from top to bottom) are marked with R or Ω, indicating the resistance value. When the switch is in the ohm range, this scale line is read.
The second line is marked with ∽ and VA, indicating the values of AC and DC voltage and DC current. When the conversion switch is in the AC or DC voltage or DC current mode and the range is in other positions except for AC 10V, this scale line is read. The third line is marked with 10V, indicating the AC voltage value of 10V. When the conversion switch is in the AC/DC voltage range and the measuring range is at 10V AC, this scale line is read. The fourth item is marked with dB, indicating the audio level.
(2) Measurement line
Measurement circuit is a circuit used to convert various measured signals into small DC currents suitable for meter measurement. It consists of resistors, semiconductor components, and batteries
It can convert various types of measurements (such as current, voltage, resistance, etc.) and different ranges into a certain limit of small DC current through a series of processing (such as rectification, diversion, voltage division, etc.) and send it to the meter for measurement.
(3) Conversion switch
Its function is to select various measurement circuits to meet the measurement requirements of different types and ranges. There are usually two toggle switches, each labeled with different gears and ranges.
Steps for fault detection of multimeter
(1) Turn the range of the tested meter to the lowest level of DC voltage;
(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.
