with transistors used as switches do they ever fail closed?

Discussion in 'General Electronics Chat' started by sdowney717, Oct 23, 2012.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    curious about that.
    Also how about using transistors as relays?
    What would be a transistor number that could easily handle 40 amps of DC power used as a switch?

    Should transistors replace relays? Or are mechanical relays better for switching on powerful loads.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    PackratKing likes this.
  3. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Transistors, when in distress in a circuit, tend to melt, short out, or just plain detonate in some cases. I would have to say no, they cannot fail closed.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Compare them to mosftes for instance. These when abused - typically with overvoltage on the gate - tend to fail shorted. And if you´re in luck and the circuit is a single-ended switch it is not hard to end up with a permanently closed switch. In a push-pull configuration this usually either blows the fuse if present or the transistors melt enough after a while to become open circuit.
    It just depends on the basic topology of the circuit and on the actual design.
     
  5. Stuntman

    Active Member

    Mar 28, 2011
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    I can tell you first hand that FETS can and do readily fail in a "closed" or "conductive" state.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I have replaced many a shorted out BU508D transistor with shorted C/E in a TV line output , so yes they do blow
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Yes, transistors and mosfets can go defective.
    A relay can also defective, the coil can burn or the contacts can melt together.

    Bertus
     
  8. Sensacell

    Well-Known Member

    Jun 19, 2012
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    My experience- semiconductors fail shorted more than they fail open.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    I once say a TO-3 power transistor, which was part of a high power sonar transmitter, with a small hole burned in the top of the metal case directly over the emitter pin. The apparent cause of this was that the transistor originally shorted which, because of the high available current supply, caused the emitter pin wire bond to blow. This created an internal arc which then traveled to the top of the case (which was at the collector potential) and burned the hole until the arc extinguished. The transistor failure started as a short but definitely ended as an open.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Cool story! I've never seen a TO-3 with a hole, but I have seen a few TO-220 with the entire plastic part blown off and just the metal tag and legs remaining.
     
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