![]() ![]() In this example, that wire is number 3, being common to both the 1-3 and the 2-3 test point combinations. It must be the base connection of the transistor, because the base is the only layer of the three-layer device common to both sets of PN junctions (emitter-base and collector-base). Now we look for the one wire common to both sets of conductive readings. ![]() These two readings must indicate forward biasing of the emitter-to-base junction (0.655 volts) and the collector-to-base junction (0.621 volts). The only combinations of test points giving conducting meter readings are wires 1 and 3 (red test lead on 1 and black test lead on 3), and wires 2 and 3 (red test lead on 2 and black test lead on 3). Which terminals are emitter, base, and collector? Ω-meter readings between terminals. Measuring between pairs of wires and recording the values displayed by the meter, the technician obtains the data in Figure below. Suppose a technician finds a bipolar transistor and proceeds to measure continuity with a multimeter set in the “diode check” mode. All bipolar transistors have three wires, of course, but the positions of the three wires on the actual physical package are not arranged in any universal, standardized order. This is important because transistor packaging, unfortunately, is not standardized. Knowing this, it becomes possible to determine which wire is which on an unmarked transistor. This forward voltage difference is due to the disparity in doping concentration between the emitter and collector regions of the transistor: the emitter is a much more heavily doped piece of semiconductor material than the collector, causing its junction with the base to produce a higher forward voltage drop. 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 is the “opposite” condition for the NPN transistor. Meter readings will be exactly 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 Figure below. \( \newcommand\)īipolar transistors are constructed of a three-layer semiconductor “sandwich” either PNP or NPN.
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