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Check of transistor

Discussion in 'Electronic Repair' started by Don, Oct 22, 2003.

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  1. Don

    Don Guest

    How do you measure whether a transistor are damaged/dead? What is it that
    typically happens to a transistor, why does this happen and how do you
    measure it?
    I have read somewhere that shortcuts between some of the three pins could
    occur, is this correct?

    Best Regards
  2. Jeff

    Jeff Guest

    There are three basic types of failures in electrioncs.
    Open, shorted or leaky.
    These are the most common problems in transistors
    BUT are not the only ones, you can add the word "intermittantly" to the
    front of each of those three
    and still have others to learn.
    The first thing to know when measuring is what to
    expect as a measurement, that means knowing your
    equipment and its use and knowing what is a good/bad measurement on what you
    are measuring.
    Not all transistors will check the same, not all
    transistors that check good will be good, not all meters
    read the same, in circuit tests are not perfect, out of
    circuit tests are not perfect, sometimes leaky is OK.
    The short answer to your first question is experience.
  3. Spudley

    Spudley Guest

    Generally speaking though, a short circuit "Zero Ohms Resistance"
    between any legs of a transistor
    is a good indication that it has failed.

    One has to know how to use the meter to do the test though, as
    the results and method of doing so can vary for different meters and
    give you a false impression of a good or bad transistor. Which is what
    Jeff said.

    Use a Multimeter with a diode checking function 'So that it can bias
    the transistor". This will allow for reading the device in an active
    state and should give rise to more accurate assessment of whether the
    device is operational at all.
    If it measures zero ohms in this test then it's a good chance that the
    transistor is no good.
    If it measures with some resistance "There may be a scale indicating
    good or bad on the meter" showing a voltage potential of say .7volt
    for a Silicon or Germanium type Transistor or .2 Volt for a FET,
    shottky. These values can vary dramatically depending upon transistor
    type. But it can give you a general idea of the transisitors
    Swapping the + lead and - leads of the multimeter over will forward or
    reverse bias the transistor under test "That's if the potential across
    it is sufficient" and that's why I said to use a meter that has a
    diode test function. Else it may indicate open circuit "High
    resistance" in both directions, which can also give you an impression
    that the transistor is faulty. Or it may infact be faulty.
    Do these tests on a known good transistor first so that you know what
    results to expect before checking other transistors that are unknown.

    I hope this helps.
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