How to Diagnose a Circuit Board With a Bad Transistor

Written by david sandoval
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How to Diagnose a Circuit Board With a Bad Transistor
While JFETs and BJTs look alike, they operate differently. It is important to know how to test each type. (Hemera Technologies/PhotoObjects.net/Getty Images)

Electronic circuits require that all of the components contained within that circuit operate properly. If any of the components fail, it can have catastrophic consequences for any devices connected to that circuit. Failed active components --- such as transistors, diodes and microchips --- are often more difficult to diagnose than failed passive components --- such as resistors; active components behave differently than passive components when subjected to a range of voltages. If you suspect that a transistor has failed, the transistor must be tested before you power the circuit up again.

Skill level:
Moderate

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Things you need

  • 2 1000-Ohm resistors
  • Needle-nose pliers
  • Digital multimeter

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Instructions

    Junction Field Effect Transistor (JFET) Test Procedure

  1. 1

    Twist one lead from the first resistor to the drain terminal on the transistor. Twist one lead from the second resistor to the source terminal on the transistor. Twist the free leads from both resistors together with the gate terminal on the transistor. Wait 30 seconds, and then remove the resistors from the transistor terminals.

  2. 2

    Turn the multimeter on; set the measurement scale to "Diode Test." For an n-channel JFET, place the red multimeter probe on the transistor gate terminal, and place the black multimeter probe on the drain terminal. For a p-channel JFET, place the red multimeter probe on the drain terminal and place the black probe on the gate terminal.

    How to Diagnose a Circuit Board With a Bad Transistor
    A multimeter can test diodes by applying voltage across the probe leads. (Polka Dot Images/Polka Dot/Getty Images)
  3. 3

    Check the multimeter display. If the multimeter displays a "Pass" rating, the JFET is working properly. If the multimeter displays a "Fail" rating, replace the JFET.

    Bipolar Junction Transistor (BJT) Test Procedure

  1. 1

    Turn on the multimeter and sent the measurement scale to "Diode Test." For an NPN transistor, place the red multimeter probe on the transistor base terminal, and place the black probe on the collector terminal. For a PNP transistor, place the black multimeter probe on the base terminal, and place the red probe on the collector terminal.

  2. 2

    Check the multimeter display. If the multimeter displays a "Pass" rating, remove the multimeter probe from the collector, place it on the emitter terminal, and proceed to the next step. If the multimeter displays a "Fail" rating, remove the multimeter probes from both terminals and replace the transistor.

  3. 3

    Check the multimeter display. If the multimeter displays a "Pass" rating, the transistor is functioning properly. If the multimeter displays a "Fail" rating, the transistor must be replaced.

Tips and warnings

  • You may need to use the needle-nose pliers to twist the resistor leads to the transistor terminals if the transistor is installed in a small space.

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