Digital multimeters are widely used in people’s daily life and study due to their simplicity, digital display, ease of operation, and strong anti-interference capabilities. This article briefly describes the common faults encountered during the use of digital multimeters and provides some daily handling methods.
1. Several Common Fault Analysis Methods for Digital Multimeters
1.1 Sensory Judgment Analysis Method
This method involves judging whether there are faults based on sensory organs without needing any tools, mainly including the eyes, hands, nose, and ears. For example, by visually inspecting whether the multimeter’s wires are intact and whether there are any internal damages, using the nose to detect any strange smells that indicate burning, and using touch to judge based on temperature. The sounds from the multimeter can also be heard to assess its operational status.
1.2 Voltage Judgment Analysis Method
During normal operation, the digital multimeter has specific voltages at its pins, contacts, or measurement points, and these voltages always have a standard value. If the value displayed on the multimeter deviates significantly from the normal standard value during operation, it indicates that the multimeter has a problem. It is best to use a multimeter of the same model and size for comparison to ensure accuracy.
1.3 Component Judgment Analysis Method
The digital multimeter is composed of multiple components. When a fault occurs, the internal components can be disassembled for testing to find solutions. During the disconnection testing of components from the circuit, comparisons must be made with the standard values of good components. If a component’s values cannot be measured directly or indirectly when disconnected, and the standard value is unknown, a good component can be installed in the multimeter to determine its operational status. During testing of some components, the impact on other components should also be considered.
1.4 Interference Signal Analysis Method
This analysis method is also commonly used in current circuit repairs. When inspecting faults in circuits or devices, interference signals can be applied, and the changes in the multimeter’s display can be observed to determine where potential issues may lie.
2. Fault Handling of Digital Multimeters
2.1 Display Issues
2.1.1 No Response Upon Powering On
Generally, electrical appliances or instruments display some response after power is connected, indicating they are in working condition, and the displayed content varies based on different situations. However, if there is no response on the display after powering on, the following analyses should be conducted:
2.1.2 Battery Issues — No Display Upon Powering On
First, it may be due to the battery being nearly depleted from previous use, resulting in insufficient voltage for the display to show content. The battery should be replaced. Second, batteries can corrode over time; if the battery cover is corroded, it affects the battery’s tightness, and the corrosion should be cleaned off with a hard tool. Third, if the battery is unsoldered, the leads can be replaced or resoldered as needed. Fourth, if the battery leads are leaking, they should be discarded to avoid safety accidents.
2.1.3 Incomplete Display
(1) If the display is missing a number or message, the a/d converter should be checked or replaced. If the related components in the display section are damaged, an oscilloscope should be used to check the signal waveform output at the corresponding lead terminals for repair.
(2) Check whether the display is damaged or worn due to friction or if the chips are intact, and replace or repair as necessary.
2.1.4 Incorrect Low Voltage Indicator Display
If a new battery is installed but the display still shows low voltage, this is typically due to a fault in the gate circuit, or possibly due to a faulty transistor or a deformed resistor connection that has become unsoldered.
2.2 Functional Issues
Generally, multimeters have common and individual faults in functionality. For common faults, the internal structural components of the multimeter should be checked, while for individual faults, different analyses should be conducted based on the specific situation.
2.2.1 Issues in DC Voltage Range
(1) Check whether the corresponding switch is functioning properly. Poor contact in the switch can cause sudden connections or disconnections of voltage, creating significant trouble for users. If the switch is open-circuit, it can cause problems in measuring DC voltage, leading to inaccurate voltage results.
(2) Check whether the resistors in series with the DC voltage are properly connected. If the series resistors are open-circuited, accurate voltage measurement will be impossible.
2.2.2 Measurement Results of DC Voltage Deviate from Standard Values
(1) Check for any series connection issues with the switch. Due to prolonged use, the positioning device may fail to operate properly, causing the multimeter to be stuck between two ranges, leading to measurement inaccuracy.
(2) Check the differences between the measured values of the voltage divider resistors and the standard values. The multimeter may become damp over time and weather conditions; therefore, the voltage divider resistors should be cleaned.
2.2.3 Issues in DC Current Range
(1) Check whether the switch is functioning properly.
(2) Check the integrity of the fuse.
(3) Check the completeness and smoothness of the current-limiting diode. If the diode is short-circuited, it can cause severe short circuits between the tested object and the COM terminal.
(4) For AC voltage range issues, check the contact points of the switch, as well as the integrated operational amplifier, filter capacitors, and whether the resistors in series at the rectification output are short-circuited or open-circuited.
2.2.4 Issues in Resistance Range
First, check the contact of the switch, then check the thermistor, observing its open-circuit status, standard resistance, and whether the capacitor (0.1μf) between the c-e terminals of the protection transistor is intact and free from leakage. Ensure that the resistors connected to the reference voltage are in normal condition.
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