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Common Practice for Transistors to be used as fuses?

Discussion in 'Beginner Electronics' started by AggieSawDust, Jul 14, 2006.

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  1. AggieSawDust

    AggieSawDust Guest

    My buddy dropped his ReplayTV off at my place the other day for me to
    take a look at. It turns out that one of the power supply output pins
    had a blown transistor attached to it. After further inspection, I saw
    the middle leg (Base or Gate, couldn't tell what kind of transistor it
    was) was cut and not connected to anything. This was apparantly done
    by design since none of the other two transistors attached to output
    pins had their middle legs connected to anything either.

    So anyway, best I can tell, these were designed to be used as fuses
    (that must make these normally closed transistors JFETs, right?). Not
    being too experienced, I've never run across this. Is this a common
    practice? Wouldn't it make sense to use actual fuses to make servicing

  2. Ernie Werbel

    Ernie Werbel Guest

    Sometimes transistors are used as diodes.

    The middle leg, I have found, does not necessarily have to be the base. The
    configuration can be EBC, but it can also be ECB or CEB, these I have seen
    often among others.

  3. Ernie Werbel

    Ernie Werbel Guest

    Oh, one other thing that I remembered. It's a bit OT but in the 60's many
    transistors were used as diodes (the emitter-base junction), or even fakes
    were soldered onto the board (not actually going anywhere) to increase
    transistor count. Sure an AM radio could be built with 5 or 6 transistors,
    but if you got the 12-transistor model, it just HAD to be better, right?

  4. default

    default Guest

    Somewhat "common practice"

    The reversed biased base-emitter junction on a silicon transistor
    behaves as a low power zener diode with a breakdown voltage in the 6
    to 8 volt range.

    2N3904 is a good candidate

    Not fuses . . .
  5. AggieSawDust

    AggieSawDust Guest


    I guess I'm not 100% sure it's a transistor, but it is in a TO-92
    enclosure. The only thing I'm certain of is that I should be seeing no
    voltage drop across this device and each leg should show 12V with
    respect to ground (the other two behave like this).

    So if this indeed a transistor, anyone have a favorite replacment?

  6. AggieSawDust

    AggieSawDust Guest

    By replacement, since I don't have a recognizable part number to share,
    does anyone know of a good transistor that can be used as a 1.5 A fuse
    (based on writing on the circuit board)?
  7. catguy

    catguy Guest

    It's an IC protector, Just a fuse in a TO-92 package. Read about them
  8. How can there be a fuse in a TO-92 package? You need a class tube otherwise
    there will be no air for the wire to be in. A fuse can't work if it's solid
  9. ePerson

    ePerson Guest

    Sure not emitter-base junction? That give a definite 0.7V drop.
  10. AggieSawDust

    AggieSawDust Guest

    Based on the info from this link, I've determined that my part in
    question is definitely an IC protector. I even looked at the board
    again and saw the part's notation: PR102. Thanks for the assistance.
    In all my education, I've never seen or even heard of these. I guess
    nothing can take the place of good old-fashioned experience!

    My next question is, who sells these? My local parts peddlers don't
    carry them.
  11. catguy

    catguy Guest

    Mouser has a resettable protector.....Paul
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