Transistor lead identification plays an important part in transistor maintenance; because, before a transistor can be tested or replaced, its leads or terminals must be identified.
Since there is no standard method of transistor lead identification , it is quite possible to mistake one lead for another. Therefore, when you are replacing a transistor, you should pay close attention to how the transistor is mounted, particularly to those transistors that are soldered in, so that you do not make a mistake when you are installing the new transistor.
When you are testing or replacing a transistor, if you have any doubts about which lead is which, consult the equipment manual or a transistor manual that shows the specifications for the transistor being used.
There are, however, some typical lead identification schemes that will be very helpful in transistor troubleshooting. These schemes are shown in the figure below. In the case of the oval-shaped transistor shown in view A, the collector lead is identified by a wide space between it and the base lead. The lead farthest from the collector, in line, is the emitter lead. When the leads are evenly spaced and in line, as shown in view B, a colored dot, usually red, indicates the collector. If the transistor is round, as in view C, a red line indicates the collector, and the emitter lead is the shortest lead. In view D the leads are in a triangular arrangement that is offset from the center of the transistor.
The lead opposite the blank quadrant in this scheme is the base lead. When viewed from the bottom, the collector is the first lead clockwise from the base. The leads in view E are arranged in the same manner as those is view D except that a tap is used to identify the leads. When viewed from the bottom in a clockwise direction, the first lead following the tab is the emitter, followed by the base and collector.
Transistor lead identification.
In a conventional power transistor as shown in views F and G, the
collector lead is usually connected to the mounting base. For further
identification, the base lead in view F is covered with green sleeving.
While the leads in view G are identified by viewing the transistor from
the bottom in a clockwise direction (with mounting holes occupying 3
o'clock and 9 o'clock positions), the emitter lead will be either at the
5 o'clock or 11 o'clock position. The other lead is the base lead.