What Causes Corner of BiPolar Transistor to "Chip Off"?

Discussion in 'General Electronics Chat' started by technicom, Jun 24, 2015.

  1. technicom

    Thread Starter New Member

    Jun 29, 2010
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    Hi,
    We are using an MPSA42 NPN transistor (TO-92) package in a product to drive a string of 30 blue LEDs from 120 VDC through a dropping resistor. Occasionally, one of our products is returned to us by our customer with the failure mode "blue LEDs not illuminating." When we inspect the unit, we see that the corner of the TO-92 package that is closest to the emitter has broken off (see photo). Further, the transistor is "intermittent" in that we can press on it (i.e. physically move the transistor very slightly) and the circuit will begin to work again. I think that the customer is applying too much voltage to our circuit and that is causing the case to "pop". Does this make sense? It certainly appears to me to be caused by electrical stress (exceeding the Absolute Maximum Ratings of the MPSA42) and not physical stress. Any comments would be appreciated!

    Thanks,
    Mike
    Q1 Cracked By Emitter Lead.jpg
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    Uhm.... it's rather hard to guess how a circuit might be causing the transistors to fail without seeing the circuit.

    Two obvious candidates -- physical stress or thermal stress. I'm voting for the latter, but without seeing a schematic for that part of the circuit that's just a wild guess.
     
    ErnieM likes this.
  3. ScottWang

    Moderator

    Aug 23, 2012
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    Are you used the tranformerless power supply for leds?
    Please just using the normal font size, unless you want to emphasize something, the big font already modified to the normal.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    What is the purpose of the transistor in your circuit?
     
  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    No need to blame the customer;) -- it's almost certainly a thermal (and, hence, design) issue -- especially if, as you say, there have been multiple (similar) failures...

    As an aside; please be very, very careful with non-isolated line operated products...

    Best Regards
    HP
     
    Last edited: Jun 24, 2015
  6. technicom

    Thread Starter New Member

    Jun 29, 2010
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    Sorry about the font... I just signed on and that was the font that came up!
     
  7. technicom

    Thread Starter New Member

    Jun 29, 2010
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    The product is used by electric utilities and they have both 125 VAC and 277 VAC in the test lab.... We believe that they are accidentally connecting 277 VAC to the circuit instead of the 125 VAC for which it is designed. My purpose in posting the photo was to get some opinions about the failure being caused by an electrical stress (over-voltage, over-current, etc.) and not by an assembly process (mechanical damage during assembly and handling). If I know that the chip fractured off the case as a result of electrical stress, we can focus on that and not on assembly issues. It doesn't look like any type of mechanical failure to me.....

    Thanks all!
    Mike
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Aye! Now that's a scenario I can support!:D:D:D

    TTFN
    HP
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    Ta Ta for Now! HP you are such a rogue.

    The MPSA42 is a high voltage transistor. Apparently it is being subjected to excessive voltage or current. It is interesting that you can press on it and it begins working. Suggest you put a zener, or tran-sorb diode in parallel with the part.
     
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  10. WBahn

    Moderator

    Mar 31, 2012
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    I don't care who it's used by. How can we possibly do any kind of analysis on whether it is being stressed unless we know what it's operating conditions are. Just telling us that it's used to drive 30 blue LEDs through resistor from a 120 VDC supply doesn't tell us diddly about the operating conditions of the diode.

    But it appears that that is some deep secret that you aren't going to share.
     
    Hypatia's Protege likes this.
  11. ScottWang

    Moderator

    Aug 23, 2012
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    Give us a proof that this is a cuircuit through transformer and diodes rectifier, otherwise we may treat it as a topic about transformerless power supply and close the thread which is not allowed by the TOS.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    The obvious answer to that is a sidac and a fuse.
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    Pretty sure there'll be a transformer of some description if you follow the cables back to the substation.................
     
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  14. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Else 33kV directly off the 'Genny'? At an Ifault[max] of Ca. 45kA that could get nasty!:eek::eek::eek::D
     
  15. #12

    Expert

    Nov 30, 2010
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    It is common enough to have an air pocket in a transistor which allows "wiggling" to cause intermittent operation, but THAT air pocket is not a manufacturing defect. It is also common for Our Father who Art in Heaven to slap down lightning bolts on the electric utilities. I am intimately familiar with the crackle of exploding thermoplastic, and that's what your photo looks like. Either blame the designer for not protecting that transistor or blame Dog for striking it down.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAICR: 11kV is the highest I've actually seen cabled into factory premises, but I do know its substantially higher in the bigger smelting foundries.

    In the days before the old Rugby transmitter became famed for the MSF60 time signal, it was a major world wide wireless communications facility - I read somewhere there power was direct from overhead cables (no substation transformer) - not sure, but I think I vaguely remember reading 11kV for that.

    They had a power house full of rotary transformers, most of them were mounted on insulating pillars so the outputs could be any combination of series/parallel.

    The 400kV pylons are almost a stones throw from my flat, the main substation for the surrounding towns & villages is less than a mile from me.
     
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  17. ScottWang

    Moderator

    Aug 23, 2012
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    :D

    He didn't answer my first question, I'm not sure he is ignore it or avoid to answer it, I started to concerned this thread may closed soon.
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    The TS appears to be describing a commercial product that they have some responsibility for.

    From what I've read so far, the distinction between professional and amateur seems somewhat blurred in this instance.
     
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  19. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    My citation was in reference to a 'generating station' I am (was) familiar with (To wit: Prairie Island -- *NOT* to be confused with TMI:mad:;))

    All the better to 'tap' that stray EMR;););)

    TTFN
    HP
     
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  20. takao21203

    Distinguished Member

    Apr 28, 2012
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    always interesting to encounter commercial products failing, engineer with degree consulted datasheet but, not enough experience, taking it literally, fail.

    Makes me think of my 3v 20 LED circuit, safe to touch, and just the first throw, for sure, much more could be archieved.

    Makes me think of that General Electric mini fluoro nightlight- after a short while, arcing inside and holes + molten plastic visible on outside. Total fail.

    Capacitive mains supplies are serious issue/source of risks even when done professionally.

    For a couple of reasons...

    Accepting bids from investors for the blueprints for my LED driving circuit btw.
    Its not difficult but apparently, PNP based, works with plain low voltage transistor.
     
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