If a multimeter with a “diode check” function is used in this test, it will be found that the emitter-base junction possesses a slightly greater forward voltage drop than the collector-base junction. Low resistance readings with the red (+) lead on the base are the “opposite” condition for the NPN transistor. Meter readings will be exactly the opposite, of course, for an NPN transistor, with both PN junctions facing the other way. If your meter has a designated “diode check” function, use that rather than the “resistance” range, and the meter will display the actual forward voltage of the PN junction and not just whether or not it conducts current. Some multimeters are equipped with two separate continuity check functions: resistance and “diode check,” each with its own purpose. Here I’m assuming the use of a multimeter with only a single continuity range ( resistance) function to check the PN junctions. PNP transistor meter check: (a) forward B-E, B-C, resistance is low (b) reverse B-E, B-C, resistance is ∞. The collector is very similar to the emitter and is also a P-type material of the PN junction. The P-type emitter corresponds to the other end of the arrow of the base-emitter junction, the emitter. On the symbol, the N-type material is “pointed” to by the arrow of the base-emitter junction, which is the base for this example. Low resistance readings on the base with the black negative (-) leads correspond to an N-type material in the base of a PNP transistor. As such, transistors register as two diodes connected back-to-back when tested with a multimeter’s “resistance” or “diode check” function as illustrated in the figure below. Bipolar transistors are constructed of a three-layer semiconductor “sandwich” either PNP or NPN.
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