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TRANSISTORS TESTING GUIDELINES

Transistors are semiconductors that are used as amplifiers or electronic switches. Transistors are made in either one

of two standard types, NPN or PNP, which refer to the configuration of the layers of the semiconductor materials

that are used to manufacture the transistor. Transistors have three connections -- the base, the emitter and the

collector. For an NPN transistor, the transistor is turned on when the base is at high relative to the emitter. The PNP

transistor is turned on when the base is low relative to the emitter. Transistors can be tested with a multimeter, which

can verify a transistor is working correctly and also help to identify its connection

Use the ohms function. First check a diode with known orientation so you know how the ohmmeter responds to a

diode. Then check the transistor. It should look like two diodes. For the NPN, it's a diode C B with anode at B,

and a second diode EB with anode also at B. In other words CBE. For PNP, the diodes are reversed, CB

and EB, with cathodes at the base. In other words CBE

Label the pins of the transistor to be tested as 1, 2 and 3, working from left to right. This is used as a

guide for test purposes only. These are not the actual connections.

Using the diode test setting on a digital multimeter, 100connect the positive redand negative black

probes to the pins of the transistor in the following order, and record the output from the

multimeter:

1 POSITIVE 2 NEGATIVE OL OPEN

2 POSITIVE 1 NEGATIVE OL OPEN

1 POSITIVE 3 NEGATIVE 0.675 volts THE EMITTER-BASE JUNCTION ALWAYS HAS THE

HIGHERREADING -- 0.665 VOLTS

3 POSITIVE 1 NEGATIVE OL OPEN

2 POSITIVE 3 NEGATIVE 0.635 volts THEBASE-COLLECTORJUNCTIONHASTHELOWER

READING, HERE 0.635 VOLTS

3 POSITIVE 2 NEGATIVE OL OPEN1. Identify the pins. Find the pin that is common to both readings. In Step 3 that is pin 3. This is the

transistor base pin, which means that pin 1 is the emitter and pin 2 is the collector. As the voltage

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readings were obtained when pin 3, the base, was connected to the negative probe, the transistor

type is PNP. If the base was connected to the positive probe when voltage readings were obtained,

the transistor type is NPN.

Tips & Warnings

If results similar to those above are not obtained, such as two voltage readings and four open

circuit readings, it indicates the transistor is faulty.

If the transistor is part of an electrical circuit, it should be desoldered and removed before being

tested to get valid results.

Tested with a multimeter in the resistance or diode check modes, a transistor behaves like

two back-to-back PN (diode) junctions.

The emitter-base PN junction has a slightly greater forward voltage drop than the collector-base

PN junction, because of heavier doping of the emitter semiconductor layer.

The reverse-biased base-collector junction normally blocks any current from going through the

transistor between emitter and collector. However, that junction begins to conduct if current is

drawn through the base wire. Base current may be thought of as opening a gate for a certain,

limited amount of current through the collector.

If connected backwards in a circuit, a base-collector current will fail to control current between

collector and emitter. Despite the fact that both the emitter and collector layers of a bipolar transistor

are of the same doping type (either N or P), collector and emitter are definitely not identical

Current through the emitter-base junction allows current through the reverse-biased base-collector

junction. The action of base current can be thought of as opening a gate for current through the

collector. More specifically, any given amount of emitter-to-base current permits a limited amountof

base-to-collector current. For every electron that passes through the emitter-base junction and on

through the base wire, a certain, number of electrons pass through the base-collector junction and no

more.