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Testing Bipolar Transistors

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Bipolar transistors are solid state devices that are capable of operating for extremely long periods of time without failure.

If they are exposed to excessively high temperatures, currents, voltages or mechanical stress the transistor may short or develop an open.

Theses devices can be tested for shorts or opens using an ohm-meter.

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Test equipment is available for testing transistors either in or out of an electronic circuits.

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Sophisticated transistor testers can indicate if there are shorts, opens or if the transistor is passing an excessive amount of undesirable leakage current.

The most common troubles, shorts or opens can be tested with an ohm-meter or a digital multi-meter (DMM).

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The DMM is used to determine if a short or open exists between the transistor’s:

Emitter and baseBase and collectorEmitter and collector

You are checking for either very low (near zero) or very high (nearly infinite) resistances.

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Since a BJT has two PN junctions, it resembles two diodes that are connected back to back.

Each PN junction will exhibit a low resistance when forward biased and a high resistance when reverse biased.

The meter’s battery is the source of the forward and reverse bias voltage.

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To check the forward resistance (bias) of each junction in an NPN transistor the meter’s leads are connected as shown.

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NPN: the positive (RED) lead is connected to the base and the negative (BLACK) lead is connected FIRST to the emitter.

For the next test, the positive (RED) lead is connected to the base and the negative (BLACK) lead is connected to the collector.

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NPN: the DMM should indicate a relatively low resistance (several hundred ohms or less) for the base emitter and base collector test.

The next test will require the leads to be reversed and the same test will be performed.

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NPN: the negative (BLACK) lead is connected FIRST to the base and the positive (RED) lead is connected to the emitter.

For the next test, the negative (BLACK) lead is connected to the base and the positive (RED) lead is connected to the collector.

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NPN: with the reverse bias test the DMM readings should indicate that both the base to emitter and base to collector junctions have a relatively high resistance (thousands of ohms).

Remember that the first test is forward biased and the second test is reverse biased because of the polarity of the test leads, the ohm-meter or DMM provides the voltage source.

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To check the forward resistance (bias) of each junction in an PNP transistor the meter’s leads are connected as shown.

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The testing procedures are for the PNP transistor are very similar to the tests done for the NPN, however the lead polarity is different.

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When checking the forward resistance of the junctions, you should use a low resistance range that will allow the meter to present a mid-range indication that is easy to read.

The specific meter readings that you obtain have no real meaning, you are verifying either a low or high resistance reading.

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Transistor made from silicon usually have higher forward and reverse resistances than transistors made from germanium.

A good rule of thumb for these test is a 1 to 100 ratio of low to high resistance readings.

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When a transistor has a very low forward and reverse resistance (almost equal) the junction is shorted.

When a transistor has a very high forward and reverse resistance then the junction is open.

In both of these scenarios the transistor is defective.

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In some cases it may be necessary to determine if a transistor is an NPN or PNP component.

This can be done using an ohm-meter as well, special attention has to be paid to the polarity of the test leads Red is positive and black is negative.

The results will have to plotted (written down) because you will be performing at least six tests.

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With the plotted information below the #1 lead (+) and the #3 lead (-) show a low reading.

Also lead #2 (+) and the # 3 lead (-) has a low reading, this indicates that the transistor is forward biased.

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Since lead #3 is low in both readings it is the base lead.

Since forward bias was achieved when a negative test lead was applied to the base, the base must be N type material.

Therefore this is a properly operating PNP transistor.