TIP125 - Newbee Question

Discussion in 'General Electronics Chat' started by treehouse3911, Feb 12, 2016.

  1. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    Hi all;

    Just venturing into some basic electronics, and not at all familiar with transistors like TIP125.

    In diagnosing a printer logic board, I was checking continuity on pins of a TIP125 transistor (see photo). With the TIP125 still soldered into the board, and with the board un-powered, should I be reading continuity (a short) between the TIP125 center screw and EACH of its three pins?

    Or, is this indicating a TIP125 that is shot?

    Tim
     
  2. #12

    Expert

    Nov 30, 2010
    16,345
    6,829
    Measuring all zeros in both polarities is a good reason to unsolder it and check again.
     
    Lestraveled likes this.
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    That is one of the fundamental trouble shooting methods. Don't unsolder it unless it looks at you the wrong way. Dead shorts are a dead giveaway.
     
  4. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    Sorry - just to be clear. A normal, functioning TIP125 would NOT be reading a short between its center screw and each of its 3 leads when still connected into the board?
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    First the center pin and the tab are common, they should read a dead short. If the outside pins read very low resistance to the tab, this would be cause to remove the part from the board and retest.
     
  6. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    Okay, great. That's certainly the case. I'm reading NO resistance between pins 1 and 3 and the tab. Thanks very much for the guidance! I really hope I have found the troubled component here. It's on the logic board for a DECWriter III terminal printer which won't line feed. If it's the TIP125 at fault, I'm going to be ecstatic.

    Best regards,
    Tim
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    @treehouse3911
    Do the following test. In the ohms scale of your meter, one setting should have a diode symbol next to it. Select that setting. Place the negative on the left pin and the positive on the right pin. You should read about 1.4. Next, move the positive to the center pin. You should read .65.

    Meters measure resistance with voltages that are too low to forward bias transistors or diodes. That setting with the diode symbol is for semiconductor devices. Use it when testing transistor or diodes.
     
  8. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    OK, (while still soldered into the board) between left and right pins, I'm measuring 0.56 on the diode setting on my DMM. Between left and center, I'm reading 0.544.
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,166
    1,797
    I think what is measured on the "diode" setting is the forward voltage drop of the junction and any other components in parallel with the two pins. Those values are lower than I would expect in view of the TIP125 being a Darlington connected transistor. The Vbe drop of the BE junction should be closer to 1.4V without anything else connected to it.
     
  10. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    Looks like I need to get it out of the board and do some new measurements. Will report back tomorrow.
    Thanks all, Tim
     
  11. ramancini8

    Member

    Jul 18, 2012
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    I would test it powered up. Short the B-E junction and the collector voltage should rise. Connect a 1K resistor from V+ to the base and the collector voltage should drop. If it fails either of these tests unsolder it and use the resistance test.
     
    atferrari likes this.
  12. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    Thanks for the diagnostic tip, ramancini8.... I'll give that a shot and report back!
    Tim
     
  13. treehouse3911

    Thread Starter New Member

    Feb 6, 2016
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    Hi ramancini8 - just thought I'd report back that your diagnostic tip has helped me to identify a failed TIP125. Thanks so much.

    Now, with the component desoldered from the board, further tests reveal a short between its base and collector:

    Between B (Pin 1) and C (Pin 2) : 1.6 Ω
    Between B (Pin 1) and E (Pin 3) : 3.78 KΩ
    Between C (Pin 2) and E (Pin 3) : 3.78 KΩ
    There is continuity between B (1) and C (2).

    I'm happy to have found the failure, but haven't yet uncovered why it failed, so we're going further back in the circuit now to see if there is problem with the 7406 that the TIP125 is connected to.

    Thanks to all who pitched in.
    Tim
     
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