How to Check the Legs of a Transistor

Written by jonathan cadieux
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How to Check the Legs of a Transistor
Transistors are a key component in today's electronic circuits (Thinkstock Images/Comstock/Getty Images)

Transistors are a common component in many electronic circuits used to switch voltages, or amplify current. Transistors are manufactured as an NPN type or a PNP type, and both types will have three legs that are the emitter, collector and the base. Although PNP and NPN transistors are wired differently internally, they can both be tested the same way using the diode setting on a digital ohmmeter.

Skill level:
Moderately Easy

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Things you need

  • Transistor data heet
  • Ohmmeter

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Instructions

  1. 1

    Identify the base, collector and emitter legs on the transistor by viewing the transistor manufacturer's data. The data sheet will clearly define each leg.

  2. 2

    Set the ohmmeter to the diode check setting. Connect the positive lead of the meter to the base of the transistor and the negative lead to the collector of the transistor. The meter should indicate an open circuit or a short depending on the type of transistor (NPN or PNP). Connect the negative lead of the meter to the base of the transistor and the positive lead to the collector of the transistor. If you read a short before, you should now read an open or vice versa.

  3. 3

    Connect the positive lead of the meter to the base of the transistor and the negative lead to the emitter of the transistor. The meter should indicate an open circuit or a short depending on the type of transistor. Connect the negative lead of the meter to the base of the transistor and the positive lead to the emitter of the transistor. If you read a short before, you should now read an open or vice versa.

Tips and warnings

  • A low ohm setting can be used on your meter if it does not have a diode check.

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