Testing Diodes and Transistors with a Multimeter
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 using 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 pole, 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 pole, the tested transistor is of PNP type, and this pole 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.
Mechanical zeroing of pointer multimeter
Before measuring with a pointer type multimeter, it is necessary to observe whether the pointer of the multimeter is aligned and coincides with the zero scale line. If it does not coincide, zero adjustment operation must be carried out, otherwise it will affect the reading and make the measurement result inaccurate. The method of mechanical zeroing is to separate the two probes and use a properly sized straight screwdriver to adjust the mechanical zeroing screw located in the middle of the dial until the pointer coincides with the zero scale line, as shown in the figure. Mechanical zeroing should pay attention to the following two points: (1) Do not adjust the mechanical zeroing screw randomly. First, observe carefully. If it is necessary to adjust, adjust it half a circle left and right to make the pointer return to zero. (2) Mechanical zero adjustment is not required every time it is used. It is two different things from Ohm zeroing.
