troubleshooting transistors

7/28/2019 Troubleshooting Transistors

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Instructions1.Testing a Diode

o 1Set the control dial on your multimeter to the "diode" setting. Most digital multimeters havea diode test setting indicated by the schematic symbol for a diode, which is an arrowpointing at a line.

o 2Connect the red test lead of the multimeter to the anode of the diode, and the black lead tothe cathode. The cathode end of a diode is indicated by a band on the case. If the diode is

working, the meter will display the forward voltage, often about 0.7 volts.

o

o 3Reverse the test leads. The meter should read "OL," indicating an open circuit. Any otherreading means the diode has failed.

2.Testing a Transistor

o 4Set your meter to "diode" mode.

o 5Connect the red lead to the base of the transistor and the black lead to the emitter. If it's anNPN transistor, the meter should show the forward voltage drop of the transistor, usually a

value between about 0.5 and 1 volt. With a PNP transistor, the meter should read "OL,"indicating an open circuit. An open circuit means that there's no electrical connection.

o 6Switch the black test lead to the collector and leave the red lead connected to the base. Thereadings should be identical to the previous test.

o 7Reverse the test leads so that the red lead is connected to the collector and the black lead isconnected to the base of the transistor. The readings should be the reverse of the previoustest.

o 8Connect the red test lead to the emitter of the transistor, with the black test lead stillconnected to the base. If it's an NPN transistor, the meter should read that it's an opencircuit. If you're testing a PNP transistor, the meter should show the forward voltage drop.

o 9Connect one of the test leads to the collector of the transistor and the other to the emitter. Itdoesn't matter which lead you connect to which. The meter should indicate an open circuit.


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How to Determine the Base of Transistor With

the Use of Multi TesterBy David Sandoval, eHow Contributor

Print this article

You can use a multimeter or multitester to check a

transistor.

A bipolar junction transistor is made up of three semiconductor regions. Each regioncontains a specific impurity that has been introduced into the semiconductor material.These regions are called "p-regions" or "n-regions" depending upon the dopant.

For a bipolar junction transistor to conduct electrical current, it must have its emitter andcollector regions contain the same type of dopant. Therefore, an "NPN" transistor has itscollector and emitter terminals doped as "n-regions" whereas the base terminal is dopedas a "p-region." You can find the base terminal on an unmarked bipolar junctiontransistor by using the diode-check function on a digital multimeter or multitester.

Bipolar junction transistor

Multitester or digital multimeter with diode-check functionInstructions

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o 1Turn on the multitester or multimeter, and set the scale for "Diode Test."

o 2
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Connect the red probe from the multitester or multimeter to the center lead on thetransistor. Connect the black probe to the lead on one of the edges of the transistor.

o 3Check the multimeter or multitester display. If the display shows that the circuit passed adiode test, this indicates that the red probe is connected to the base lead and no furthertesting is necessary. Should the diode test fail, remove the black probe from the lead on theedge of the transistor. Attach this probe to the transistor lead on the opposite edge of thetransistor.

o 4Check the multimeter or multitester display. If the display shows that the circuit passed adiode test, this indicates that the red probe is connected to the base lead and no furthertesting is necessary. If the diode test fails, detach the black probe from the lead on the edgeof the transistor and place this probe on the center lead. Place the red probe on one of theleads on one of the edges of the transistor.

o 5Check the multimeter or multitester display. If the display shows that the circuit passed adiode test, this indicates that the red probe is connected to the base lead and no furthertesting is necessary. If the diode test fails, detach the black probe from the center lead andplace this probe on the edge lead opposite the red probe.

o 6Check the multimeter or multitester display. If the display shows that the circuit passed thediode test, this indicates that the red probe is connected to the base lead. If the circuit doesnot pass the diode test, the transistor is faulty and should be disposed of promptly.

How to Test Transistors in a Circuit

How to Test Transistors in a Circuit

An electronic transistor is essentially two diodes. Diodes and transistors are either inservice or not since neither are known to wear out gradually. Any component that goesbad in a circuit can prevent it from functioning. Transistors perform an important duty incircuits, and if they are shorted out or open they will cause the circuit to fail. Test thetransistors in faulty electronics to find out if problems can be fixed by replacing them.


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Things You'll Need

Insulated screwdriver

Ohmmeter or digital multimeter

Solder iron

Instructions1.

o 1Turn off the power to the circuit before testing. Unplug the AC power cord and/or removethe battery supplying power. Exhaust power from all capacitors on the board. Touch boththe terminals on a capacitor simultaneously with a metal screwdriver that has an insulatedhandle to cancel out the stored power.

o 2Identify the base, collector, and emitter leads on a transistor. The leads may be labeled B, E,and C or the orientation within the circuit may reveal which lead is which. Consult anelectronics supplier catalog for exact orientation of the leads on particular transistors if nopositive identification is present.

o 3Set a digital multimeter to a diode setting if it has one; use an ohms setting otherwise. Use alow ohms scale setting on analog meters.

o 4Check the base to collector readings in both directions. Touch one lead from the meter tothe base lead and the other to the collector lead. Look at the reading, then reverse the leads.

A good reading will show infinity in one direction and give a reading of about 600 in theother.

o 5Check the base to emitter readings in both directions. Touch one lead to the base and theother to the emitter. Check the meter and reverse the leads to read the opposite direction.Good readings should show infinity in one direction and close to 600 in the other.

o 6Remove the base lead if the numeric readings are far from 600. Other electroniccomponents like resistors may affect the readings from a transistor within a circuit. Use asoldering iron to melt the solder holding the base lead of the transistor to the circuit boardand pull the base lead out of the board so it's not connected to the circuit. Then use themeter to test both directions from base to collector and base to emitter again. Use the solder

iron to melt the solder over the hole the base lead occupied and push the base lead back intothat hole in the board after taking meter readings.

o 7Replace a transistor if either the base to emitter or base to collector readings show zero in

both directions or infinity in both directions. Zeros reveal a short while infinities indicate anopen diode within the transistor.


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How to Test a Transistor With an Ohm Meter

Used to amplify a current, a transistor consist of two diodes wired together in parallel. A transistoris either an NPN transistor, which has one input and two outputs, or a PNP transistor that has twoinputs and one output. Both styles of transistor have three leads known as the collector (C), base(B), and emitter (E). A transistor can quickly and easily be checked with an ohmmeter to insureproper functioning. Most digital multimeters also have a setting to measure resistance that willperform the same function. Knowing the type of transistor and location of its leads is important

when performing this type of test.

Instructions1.

o 1Identify the type of transistor you are working with and the location of each lead. The styleand location of the leads should be identified on the transistor packaging.

o 2Set your ohmmeter to measure resistance in the low ohms range, around 10 ohms.

o

o 3Touch the red meter probe to B, and the black probe to C for an NPN transistor. Touch the

black probe to B and the red probe to C for a PNP transistor.

o 4Observe the display. Note the reading on the display, which will either be zero or some othernumber above zero (the exact number is not important).

o 5Repeat steps 3-4 keeping the probe connected to B in the same manner, but connecting thesecond probe to E. If either or both the connections from B to C and B to E read zero, thetransistor is bad.

troubleshooting transistors

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