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hFE on DMM

Discussion in 'Electronic Basics' started by [email protected], Mar 17, 2007.

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  1. Guest

    I found out that my DMM can measure hFE for transistors, but when a
    correctly place a transistor in the slots the numbers wobble a little
    then settle to a 1 as the only digit shown on screen. What am I
    supposed to be looking for? Does this mean the transistor is not
  2. Phil Allison

    Phil Allison Guest

    ** Maybe - do any transistors test OK ?

    Sure you know which legs are what ?

    ........ Phil
  3. Chris

    Chris Guest

    I'd agree that you've probably got the E, B and C leads turned
    around. It does make a difference. You should see a number greater
    than one -- possibly 20 to 200.

    Go look at the data sheet. The quickest way to do that would be to go
    to the NTE site and punch in the manufacturer part number:$$Search?OpenForm

    It will then show the NTE equivalent repair part. Click on the data
    sheets link to see the part pinout, and insert the leads into the DMM

    Good luck
  4. Eeyore

    Eeyore Guest

    What do you mean by "correctly" ?

  5. Jamie

    Jamie Guest

    ha, i have a cheap meter that can turn the leads in the circuit to find
    the highest Hfe. i don't have to make sure the leads are in the correct
    hole! :)
  6. Guest

    I mean that I have correctly identified the pins. It is a 2N2222A
    transistor. I looked up the datasheet and found the correct
    information, but I still get the solitary "1" on the screen.
  7. John Fields

    John Fields Guest

    If you've connected the transistor to your DMM correctly and you
    can't tell whether the transistor is good or bad, then you either
    have a bad meter or a bad transistor.

    In order to find out where the fault lies, (assuming your DMM has a
    diode test function), do this:

    Set your meter for diode test and connect it across the base to
    emitter junction of the transistor. If it reads "OL", reverse the
    connection of the leads. If, after that, it still reads "OL", then
    the base to emitter junction has burned open and you have a bad
    transistor. If it doesn't, and it reads something like "0.7", then
    the base to emitter junction is OK.

    However, if it reads close to zero, then the base to emitter
    junction has been burned shut and you have a bad transistor.

    If those tests are successful, then without disturbing the lead
    connected to the base of the transistor, disconnect the lead
    connected to the emitter and connect it to the collector. The
    reading should still be around 0.7V.

    If it isn't, and it's close to zero, then the collector to base
    junction has been burned shut.

    If it isn't and your meter reads "OL", then the collector to base
    junction has been burned open.

    If both of those preliminary tests were successful then, for a
    final test, set your DMM to an "OHMS" range where 10k ohms can be
    easily detected and connect the lead with a positive output to the
    collector of the transistor and the other lead to the emitter of the
    transistor. Your meter should read "OL".

    If it doesn't, you've got a bad transistor.

    If it does, then lick your fingers and pinch the collector lead
    between the thumb and forefinger of one hand and pinch the base lead
    between the thumb and forefinger of the other hand. If you do that
    and you can change the resistance indicated by the meter by
    squeezing and relaxing the pressure exerted on the base lead, you've
    got a good transistor.
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