Questions about "Testing transistors in circuit."

Discussion in 'General Electronics Chat' started by UnnamedUser159, Aug 23, 2016.

  1. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    Hi there.
    i have asked in forum from my country about checking on the board. They said me "the device is multimeter"
    I have read before minutes that :

    1. More than 0.7V difference between base and emitter voltages indicates an open circuit b-e junction.

    2. The same voltage on two or more terminals MAY indicate one or more short circuit junctions.

    3. A LOWER than expected collector voltage generally means that the transistor is conducting heavily (turned on).

    4. A HIGHER than expected collector voltage generally means that the transistor is not conducting (turned off).

    Note: Whilst conditions 2,3 and 4 can indicate a faulty transistor, they can also be caused by other circuit conditions. For this reason further voltage tests on other transistors (mainly transistors or supply lines feeding the suspect transistor) should be carried out, before deciding the location of the fault.

    I want for begining to check transistors on one random my motherboard.

    The above things are written for bipolar transistors. I should make an analog with FETs(terminals), i guess ?

    Which is the motherboard ground - where is it ? I should make these measures "to" Zero.

    About the collector voltage - It`s written "than expected." I doubt that if we have 30 transistors we check in WWW this volgate. What is accepted as principal?

    Thanks in advance!

  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    Please rephrase the above quoted text for clarity...:confused:

    The ground plane is typically located on an internal 'ply' -- That said, all mainboard PSU connector leads clad in black insulation are connected to ground (i.e. 0V, if you will...)

    Assuming the (typical) common emitter arrangement - collector EMFs closely approaching the appropriate rail are generally indicative of a non-conducting (which is NOT to say defective) device -- that said, please be advised that a scope (i.e. TDO) is a far more useful instrument applied such analysis.

    Best regards
    Last edited: Aug 23, 2016
  3. hp1729

    Well-Known Member

    Nov 23, 2015
    What type of transistor?
    Can you compare readings on the questionable board with readings on a known good board?
    Do you have a schematic that might help you tell what "normal and expected" should be?
  4. ramancini8


    Jul 18, 2012
    There is really only one method to check a transistor in-circuit and here it is.Short the base-emitter junction and observe the collector voltage rises on a good transistor. Clip one half the base resistor value (or 1K) from the positive supply to the base and watch the collector voltage fall. If the transistor passes these tests it is alive; now, touch the base through a 1K resistor and notice collector noise rise on the scope. That was the AC test. I've worked on transistor circuits for over 50 years, and these tests have always worked for me because when they seem to fail beta is low. How does beta become low; breaking down the b-e junction, dirt accumulation, temperature, time, and life.
    cmartinez, Hypatia's Protege and absf like this.
  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    100% agreed but with two caveats:
    1) Please be advised that, in rare instances, direct shunting of of B to E may damage preceeding stage{s}.

    2) Inasmuch as device polarity (i.e. NPN vs PNP) may be unknown, the OP is advised to consider only the absolute value of the EMF apparent upon the collector -- In either case a 'cut-off' condition is marked by collector EMF 'moving toward' the collector supply rail...

    Best regards
  6. Veracohr

    Well-Known Member

    Jan 3, 2011
    The flying probe machines I use at work use these methods to test transistors in-circuit:

    - B-E diode test (constant current)
    - B-C diode test (constant current)
    - C-E resistance (open) test (constant voltage) - default open threshold is 100 ohms
    - C-E voltage test while applying a specified B-E voltage, base current, and collector current (DC)
    - some kind of DC current gain test which I don't remember the specifics of. I just found it today and didn't get an expected result

    - S-D diode test (constant current)
    - S-D resistance (open) test (constant voltage)- default open threshold is 100 ohms
    - D-S voltage test while applying a specified G-S voltage, gate current, and drain current

    Any of these test values may and will be affected by the rest of the circuit, so often you can't do these tests with any definite idea of what test value to expect.
  7. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    I begin... :)

    If we have BJT we have base emitter and collector right ? if we have FET we have Ground Drain and Source. I am talking about some comparison between these three terminals of the both types transistors.

    If we watch

    to follow where goes the black wire of the PSU and make the measurement to this metal pin?
  8. UnnamedUser159

    Thread Starter Member

    May 3, 2016
    What is a "scope" or TDO? If it`s what i think can i use some device which is cheaper and to connect it to my notebook?