In the maintenance process, it is essential to use a multimeter to assess the condition of electronic components based on the fault symptoms. Incorrect measurement techniques can lead to misdiagnosis, complicating the repair process and potentially causing unnecessary financial losses. There are two main measurement approaches: component testing and circuit board testing. During a road test, the inverter power supply should be disconnected, and components on the board should be tested without removing them. This method allows for quick identification of faulty components such as those with short circuits or open circuits. However, it's important to consider the impact of other components connected in parallel during the measurement to avoid errors.
The following outlines the methods for assessing component quality:
**First, Diode Testing**
Using an MF47 multimeter, connect the red and black leads to the diode’s terminals and record the readings. Then reverse the leads. A good germanium diode typically has a forward resistance of 300–500 Ω and a silicon diode about 1 kΩ or higher. The reverse resistance should be in the tens of kilo-ohms for germanium and over 500 kΩ for silicon. If both forward and reverse resistances are close to zero, the diode is shorted; if they are very high, it is open. In either case, the diode must be replaced.
During a road test, checking the forward and reverse resistance of the PN junction helps determine if the diode is shorted or open.
**Second, Transistor Testing**
Switch the digital multimeter to the diode mode and measure the PN junctions. If conducting, the display shows the forward voltage drop. To identify the collector and emitter, measure the voltage drops across the two junctions. The emitter will show a higher drop than the collector. For NPN transistors, the red lead connects to the base, while for PNP, the black lead does. When a transistor is damaged, its PN junction may short or open.
During a road test, measuring the forward and reverse resistance of the PN junctions helps determine if the transistor is damaged. The reverse resistance should be significantly higher than the forward resistance.
**Third, Three-Phase Rectifier Bridge Module Testing**
Take a SEMIKRON module as an example. Switch the multimeter to the diode test mode. Connect the black lead to COM and the red to VΩ. Measure the forward and reverse characteristics between the 3, 4, 5 and 2 poles. A healthy module will show a large difference between forward and reverse readings. If both directions read zero, the phase is shorted; if both read infinity, it is open. Replace the module if any phase is damaged.
**Fourth, MOSFET Testing**
Connect the black lead to the D terminal and the red to the S terminal. The resistance should be around 500–600 Ω. Touch the G terminal with the red lead, then measure the S terminal again—conduction should occur. Reversing the leads and repeating the test confirms the MOSFET is functioning properly. If the resistance between S and G is less than 30 Ω, the MOSFET is likely damaged.
**Fifth, IGBT Module Testing**
Set the multimeter to the diode test mode and check the forward and reverse characteristics between C1/E1, C2/E2, and G/E1/E2. For a German EUPEC 25A/1200V module, the readings should be consistent across all phases. If there is a significant difference, the module may need replacement. A reading of zero indicates a short, while infinity suggests an open circuit.
**Sixth, Electrolytic Capacitor Testing**
Use the MF47 multimeter, selecting the appropriate range based on capacitance. Capacitors below 47μF are tested at R×1K, and above 47μF at R×100. Connect the red lead to the negative terminal and the black to the positive. The needle should swing right and then slowly return to infinity, indicating normal leakage. Reverse the leads and observe the same behavior, but with slightly lower resistance. If the needle doesn’t move, the capacitor is faulty.
**Seventh, Inductor and Transformer Testing**
Test the inductor’s resistance using the multimeter’s ohm setting. Zero resistance indicates a short, while infinite resistance suggests an open. For transformers, check insulation by measuring resistance between core and windings (should be infinite). Also, test winding continuity with R×1, ensuring the primary and secondary resistances match expected values.
**Eighth, Resistor Testing**
Ensure the circuit is powered off before measuring. Discharge any capacitors and avoid interference from other components. Use the multimeter’s center scale for accurate readings.
**Ninth, Surface Mount Components**
Surface mount devices (SMD) are widely used in modern electronics. They come in various shapes and types, including resistors, capacitors, and ICs. Removing and soldering SMD components requires care. Use a low-wattage iron and special tools for fine-pitch ICs. Clean the board before re-soldering and apply a thin layer of solder to the pins for better adhesion.
This detailed guide provides practical insights into diagnosing and repairing common electronic components using a multimeter, ensuring accuracy and efficiency in the maintenance process.
Rack Mounted Line Interactive UPS
Rack Line Interactive UPS
Shenzhen Unitronic Power System Co., Ltd , https://www.unitronicpower.com