To use a multimeter to identify base of transistor
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In a bipolar transistor, each "PN" junction is essentially a diode. Consequently, you can use a multimeter to test each pair of pins of the device (with the third pin left floating in each test) to determine the location and orientation of each diode. Not surprisingly, this task is simplified if you have a "diode test" function on your multimeter. Regardless, it is relatively easy to find the "base" of the transistor, but disambiguating which of the two other pins is the collector or the emitter is more difficult. Still, it's possible. I outline methods for finding the base and then methods for finding the other two pins below.
Finding the Base
For example, in an NPN bipolar transistor, there is a PN junction formed from base to collector, and another PN junction formed from base to emitter. So by testing each "diode candidate", you can determine which pin is the base. Some digital multimeters have a "diode" test function that can determine whether a diode is forward biased. In general, any multimeter should have an ohmmeter function to measure resistance (set to a low setting to detect short circuits (low resistance) or open circuits (high resistance)). Test each of the 3 pairs (pins 1+2, 1+3, and 2+3) in both orientations (forward and reversed) to find the two PN diodes (which are forward biased (i.e., low resistance) with + connected to P and - connected to N). The common "P" will be the base. The other two pins will be the collector and emitter. For a little graphical help in the NPN case, see:
Testing a transistor with a multimeter from ElectronicsClub.info
Determining Collector and Emitter Pins
Unfortunately, there is no easy way to reliably tell the difference between the two using a conventional multimeter. Because both PN junctions are diodes with steep non-linear current--voltage relationships, you cannot reliably test the "resistance" (which is not well defined in this case) of each PN junction to determine which one is which. So you have two options to disambiguate the collector and emitter:
If your multimeter does have a "diode test" function that gives you the voltage drop across the forward-biased diode being tested, you can compare the voltage drop across the two diode candidates. The one with the higher voltage drop will correspond with the emitter of the transistor.
You can get more details about this diode-test procedure at:
Meter check of a transistor AllAboutCircuits.com
If you do not have a sophisticated multimeter, you will need a little extra setup to determine which pin is the emitter and which pin is the collector. You need to build a simple circuit to effectively test the "gain" of the BJT for the two possibilities.
For example (in the NPN case), to determine which one is the collector and which is the emitter, you can setup a simple degenerated common-emitter circuit. You can follow these steps:
Connect one of the collector/emitter candidates to a 2kOhm resistor that is connected to 5V on its other end. This pin is your "guess" at which is the collector.Connect the other collector/emitter candidates to a 2kOhm resistor that is connected to 0V on its other end. This pin is your "guess" at which is the emitter.Connect the discovered base to a DC voltage source of 1V.Measure the voltage across EACH of the two resistors. If they are both approximately 0.3V (i.e., 1V minus a 0.7V diode drop), then you guessed correctly. The emitter follows behind the base by 0.7V, and the current through the emitter matches the current through the collector, and so both resistors carry the same voltage drop. However, if the resistor connected to 0V has a much larger drop across it, then you guessed incorrectly and your transistor is "upside down."
Finding the Base
For example, in an NPN bipolar transistor, there is a PN junction formed from base to collector, and another PN junction formed from base to emitter. So by testing each "diode candidate", you can determine which pin is the base. Some digital multimeters have a "diode" test function that can determine whether a diode is forward biased. In general, any multimeter should have an ohmmeter function to measure resistance (set to a low setting to detect short circuits (low resistance) or open circuits (high resistance)). Test each of the 3 pairs (pins 1+2, 1+3, and 2+3) in both orientations (forward and reversed) to find the two PN diodes (which are forward biased (i.e., low resistance) with + connected to P and - connected to N). The common "P" will be the base. The other two pins will be the collector and emitter. For a little graphical help in the NPN case, see:
Testing a transistor with a multimeter from ElectronicsClub.info
Determining Collector and Emitter Pins
Unfortunately, there is no easy way to reliably tell the difference between the two using a conventional multimeter. Because both PN junctions are diodes with steep non-linear current--voltage relationships, you cannot reliably test the "resistance" (which is not well defined in this case) of each PN junction to determine which one is which. So you have two options to disambiguate the collector and emitter:
If your multimeter does have a "diode test" function that gives you the voltage drop across the forward-biased diode being tested, you can compare the voltage drop across the two diode candidates. The one with the higher voltage drop will correspond with the emitter of the transistor.
You can get more details about this diode-test procedure at:
Meter check of a transistor AllAboutCircuits.com
If you do not have a sophisticated multimeter, you will need a little extra setup to determine which pin is the emitter and which pin is the collector. You need to build a simple circuit to effectively test the "gain" of the BJT for the two possibilities.
For example (in the NPN case), to determine which one is the collector and which is the emitter, you can setup a simple degenerated common-emitter circuit. You can follow these steps:
Connect one of the collector/emitter candidates to a 2kOhm resistor that is connected to 5V on its other end. This pin is your "guess" at which is the collector.Connect the other collector/emitter candidates to a 2kOhm resistor that is connected to 0V on its other end. This pin is your "guess" at which is the emitter.Connect the discovered base to a DC voltage source of 1V.Measure the voltage across EACH of the two resistors. If they are both approximately 0.3V (i.e., 1V minus a 0.7V diode drop), then you guessed correctly. The emitter follows behind the base by 0.7V, and the current through the emitter matches the current through the collector, and so both resistors carry the same voltage drop. However, if the resistor connected to 0V has a much larger drop across it, then you guessed incorrectly and your transistor is "upside down."
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