BJT are made of three layer which are semiconductor "sandwich" which are either PNP or NPN. In BJT two diodes are connected back to back which can be tested with a multimeter's "Diode Check" function shown in the figure below.
I have a multimeter which has Diode test function for checking the PN junctions which will display the actual forward voltage of PN junction and not just whether or not it conducts current.
If both PN junctions facing other way in NPN transistors then the meter reading would be exactly opposite.
Low voltage reading with the red (+) on the base is the opposite condition for the NPN transistor. If the multimeter with diode function is used in this test it will be found that the emiter-base junction possesses a slightly Higher forward voltage drop than the collector base junction.
Transistor symbol and semi conductor construction
Identify the legs of your transistors with a multimeter. For identifying and testing purposes refer to the representation shown above.
Diode test (V) meter readings | ||||||
Transistor Number | VBE | VEB | VBC | VCB | VCE | VEC |
NPN C557 | .894 | OL | .885 | .885 | OL | OL |
PNP C547 | .886 | .885 | .883 | .894 | OL | OL |
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