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How to Test a TO-3 Power Transistor

Updated February 20, 2018

Transistors are used in amplifier and switch circuits. Depending on how much power the transistor is designed to dissipate, the transformer may be fabricated in a plastic or metal package. One of the most common metal packages is the TO-3 package. A transistor that has been produced in a TO-3 package contains two wire leads rather than three. This is because the metal case serves both as a heat sink and as an electrical connection. Therefore, it is necessary to check the transistor specification sheet when testing a TO-3 packaged transistor.

Turn on the multimeter, and set the measurement scale to "Diode Test." Connect the red multimeter probe to the transistor base lead. The base lead on the 2N3055 transistor is the first wire lead.

Place the black multimeter probe on the transistor emitter lead and check the multimeter display. If the multimeter displays the message "Pass" or "OK," proceed to the next step. If the multimeter displays a "Fail" message, remove the multimeter leads and discard the transistor. Please note: the emitter lead on the 2N3055 is the second wire lead.

Remove the black multimeter probe from the emitter lead. Place the black multimeter probe on the metal transistor case and check the multimeter display. If the multimeter displays a "Fail" message, discard the transistor. If the multimeter displays the message "Pass" or "OK," the transistor is working properly.

Warning

Not all TO-3 transistor packages have the same electrical connection on the case. Always check the transistor specification sheet to determine which transistor terminal connects to the case.

Things You'll Need

  • 2N3055 bipolar junction transistor
  • Digital multimeter with diode-check function
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About the Author

David Sandoval has served as a trainer and technical writer since 2000. He has written several articles online in the fields of home improvement, finance, electronics and science. Sandoval has an Associate of Applied Science in microelectronics from Northern New Mexico College.