Methods for Judging the Condition of Components with a Multimeter:
1. Detection of ordinary diodes
Measure with an MF47 multimeter, connect the red and black probes to both ends of the diode, read the reading, and then swap the probes for measurement. Based on the results of two measurements, the forward resistance value of low-power germanium diodes is usually 300-500 Ω, while that of silicon diodes is about 1k Ω or greater. The reverse resistance of germanium tube is several tens of kiloohms, and the reverse resistance of silicon tube is above 500k Ω (the value of high-power diode is much smaller). A good diode has a low forward resistance and a high reverse resistance, and the greater the difference in forward and reverse resistance, the better. If the measured forward and reverse resistance is very small and close to zero, it indicates that the diode is short circuited internally; If the forward and reverse resistance is very high or tends to infinity, it indicates that there is an open circuit inside the tube. In both cases, the diode needs to be scrapped.
On road testing: Testing the forward and reverse resistance of the diode pn junction makes it easier to determine whether the diode is experiencing a breakdown short circuit or an open circuit.
2. Pn junction detection
Set the digital multimeter to diode mode and measure the pn junction with the probe. If it is conducting in the forward direction, the displayed number is the forward voltage drop of the pn junction. First, determine the collector and emitter electrodes; Measure the forward voltage drop of two pn junctions with a probe. The emitter has the highest voltage drop, while the collector has the lowest voltage drop. When testing the two junctions, if the red probe is connected to the common electrode, the tested transistor is of npn type, and the red probe is connected to the base b. If the black probe is connected to the common electrode, the tested transistor is of pnp type, and this electrode is the base b. After the transistor is damaged, the pn junction can have two situations: breakdown short circuit and open circuit.
In circuit testing: In circuit testing of a transistor is actually achieved by testing the forward and reverse resistance of the pn junction to determine whether the transistor is damaged. The branch resistance is greater than the forward resistance of the pn junction. Normally, there should be a significant difference in the measured forward and reverse resistance, otherwise the pn junction will be damaged. When the branch resistance is less than the forward resistance of the pn junction, the branch should be disconnected, otherwise the quality of the transistor cannot be determined.
3. Three phase rectifier bridge module detection
Taking the Semikron rectifier bridge module as an example, as shown in the attached figure. Set the digital multimeter to the diode test mode, connect the black probe to com and the red probe to v ω, and use the red and black probes to measure the forward and reverse diode characteristics between phases 3, 4, and 5 and poles 2 and 1, respectively, to check and determine if the rectifier bridge is intact. The greater the difference in the measured positive and negative characteristics, the better; If the forward and reverse directions are zero, it indicates that the detected phase has been broken down and short circuited; If both forward and reverse directions are infinite, it indicates that the detected phase has been disconnected. If one phase of the rectifier bridge module is damaged, it should be replaced.
4. Inverter IGBT module detection
Set the digital multimeter to the diode test mode and test the forward and reverse diode characteristics between IGBT modules c1. e1 and c2. e2, as well as between gate g and e1, e2, to determine if the IGBT module is intact.
