How can we say that a transistor which we are using is good or faulty ?
we can test it by simply using multi-meter. We all know that three junction are in transistor, which are Emitter, Base ,Collector.
Base-Emitter junction is always forward bias.
Base-Collector junction is always reverse bias.
Therefore Base-Emitter junction should have low resistance,and Base-Collector junction should have high resistance.
we simply check it by using multi meter. we set multi-meter on Ohms value.
(i) We connect one Emitter to negative terminal and Base to positive terminal of the transistor.
Typically ohmmeter displays value 100 to 1 K . So forward side resistance is low, now we have to check on output side which is Collector Base side.
(ii) When we connect positive terminal to Base and negative terminal to Collector if it shows 100K or higher ohms, So it is good transistor.
If multi-meter displays any other value like forward side high and reverse side low than transistor is faulty.
above example is for NPN transistor, but you can apply for PNP only polarity is reversed.
