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More Basic Questions From Electronman

Discussion in 'Electronic Basics' started by electronman, Feb 13, 2007.

  1. electronman

    electronman Guest

    Basics group,
    Thanks again for all your helpful responses last week. Also, I
    purchased an old copy of Forest Mims' "Getting Started in Electronics"
    for $5.00 and am finding it clear, concise, and fairly easy to
    understand. Here are my latest newbie questions.

    TRANSISTORS

    In experimenting with some of the simple circuits in Mims' book, I
    burned out several 5mm round LEDs ... they're cheap ... and I learned
    a lot from my mistakes. But I haven't quite figured out how to
    determine when I've blown out a transistor. Here are some things I've
    learned about transistors by using my digital multimeter.

    Transistor HFE Resistance between Base/Emitter
    2N2907 312 80,400 ohms
    2N2222 170 81,000 ohms
    2N3906 156 80,000 ohms
    2N3904 170 80,200 ohms

    So I'm deducing that all (?) small transistors have a base/emitter or
    base/collector resistance of about 80,000 ohms. I've learned that HFE
    is a measure of forward gain (what unit of measurement is attached to
    HFE?) But this experimentation hasn't helped me to determine if I've
    blown out a transistor. What method can I use to determine if a
    transistor is bad?
     
  2. Eeyore

    Eeyore Guest

    No. There's a diode junction there. That's mainly what you're measuring.

    Graham
     
  3. It depends how you blew it.
    If by destroing the junction between two legs and having open cirquit
    you have infinite resistance in both directions.
    If by heating the junction and welding it you have "zero" resistance in
    both directions.
    In a _normal_(whatever it means?!) transistor you should measure
    different resistances on the same junction depending on direction of the
    diode and your leads.

    HTH

    Stanislaw.
     
  4. Phil Allison

    Phil Allison Guest

    "electronman"

    ** Since there are many ways for a transistor to go bad - more than one
    test is required.

    Fitting it into a circuit where it ought to work and having it do so is the
    good proof it is not faulty.

    Otherwise, a DMM that has a diode check function that lets you read the
    actual voltage across the BE and CE junctions when forward biased is a good
    start. If they are OK ( compared to a new example) then try the Hfe
    est - if it is within the normal range of values for the type you have a
    goer at low voltages.

    Whether it is still OK at higher voltages, is noisy or behaves
    intermittently when it heats up is another matter.

    Fitting it into a circuit where you need it to work and having it do so is
    very the best proof it is not faulty.



    ........ Phil
     
  5. g.knott

    g.knott Guest

    Have a look at http://homepage.ntlworld.com/g.knott/elect56.htm
     
  6. electronman wrote:
    (snip)
    Any diode junction does not have a constant resistance
    (volts per ampere), but one dependent on the current passing
    through it. Each resistance scale on your meter uses a
    different test current, so, if you can switch resistance
    scales, you should be able to see this effect. Each scale
    should show a different resistance, with the lower ohm
    ranges (that use a higher test current) showing a lower
    junction resistance.
    It is a ratio of two currents,
    (collector current)/(base current),
    so the units cancel. It is just a number.
    The diode test scale on your meter applies a constant
    current across the leads, and displays the voltage drop
    across them, usually with the decimal point showing
    millivolts. A good silicon diode junction (base to emitter
    or base to collector) should show something like 500 to 600
    mV. If you see both those junctions, and the voltage goes
    off scale indicating an open circuit emitter to collector in
    both directions, proceed to the Hfe measurement. It should
    show a much higher gain with the emitter and collector
    correctly oriented, and a much lower gain with them
    interchanged. If you get all this, the transistor is
    probably fine. Failures usually short one of the junctions,
    and kill the gain.
     
  7. Jamie

    Jamie Guest

    Learn the basic's about diodes. learn how to test them using your
    meter.
    After that, picture a transistor as 2 diodes connected to each other
    on one end with the polarity back to back.
    Now at this point, the leads that are twisted together would
    be the "BASE". the other leads could be the Collector/Emitter.
    study that with your meter and you then will understand how to do a
    simple quick test of a transistor.
     
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