How to Determine Which Pin Is the Base with a Multimeter
Suppose we do not know whether the transistor under test is NPN or PNP, nor can we identify which pin is which electrode.The first step in testing is to determine which pin is the base.First, select any two electrodes (e.g., pins 1 and 2). Use the two multimeter probes to measure forward and reverse resistance repeatedly, and observe the deflection angle of the pointer.Next, repeat the same forward and reverse resistance measurement for pins 1 and 3, then pins 2 and 3, each time noting the pointer deflection.
Among these three pairs of reversed measurements, two will show similar results: one large deflection and one small deflection in each pair.The remaining pair will show only very small pointer deflection in both directions.The untested pin in this pair is the base (b) we are looking for.
II. Determining the Transistor Type
Once the base is identified, we can determine the conductivity type based on the direction of the PN junctions between the base and the other two electrodes.Touch the base with the black multimeter probe, and touch either of the other two electrodes with the red probe.
If the pointer shows a large deflection, the transistor is NPN.
If the pointer shows only small deflection, the transistor is PNP.
III. Identifying the Collector and Emitter
(1) For an NPN transistor:Use the red and black probes to measure forward and reverse resistance between the two remaining pins. Although both deflections are small, one will be slightly larger.In this case, the pin connected to the black probe is the collector (c), and the pin connected to the red probe is the emitter (e).
(2) For a PNP transistor:Again measure forward and reverse resistance between the two non-base pins.The reading with slightly larger pointer deflection indicates that the pin connected to the black probe is the emitter (e), and the pin connected to the red probe is the collector (c).
