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Current Flow for Active Low

Discussion in 'General Electronics Discussion' started by Raven Luni, Jan 27, 2012.

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  1. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    (hey that rhymes :D)

    ...Anyway - I was just tracking down faults in my circuit and noticed this one had the exact same problem as a previous build of the same thing: a fried transistor (BC108) which is being used as an inverter to feed active low inputs. Emitter is grounded and the collector and base resistors are 1K and 10K respectively, and it currently feeds the reset input of a 74HC164 (shift register).

    The collector is connected directly to the reset pin. I was having a think about it on the toilet (where all the best ideas come from) and it occured to me that this could be a problem with current flow. Would I be right in assuming that an active low input is actually sinking current from the input? If so then I can see how that would easily exceed the max. 100mA rating for this transistor. All I would need to do is add a resistor right?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
    2,809
    Jan 21, 2010
    The 74HC series is CMOS. The input current is very low.

    The 1K resistor in the collector if the transistor is easily small enough to pull the input high, and the BC108 should have no trouble pulling it low. The base resistor also seems to be a reasonable value.

    By far, the highest currents that will flow are through the 1K resistor and the transistor to ground. The loading on this from the CMOS input will be very small.

    Any current that flows through a CMOS input is likely to be dominated by the leakage current from the reverse biased protection diodes on the input. It's really tiny. Check the specs.

    I can't see why the transistor would fail in this position.

    True, at really high speeds, the input capacitance of the CMOS gates need to be charged and discharged and this can lead to a larger input current, but not anywhere near 100mA.

    Oh, and BC108 -- that brings back memories. You can still get those? In a metal can?

    The metal can is connected to the collector, so I'd beware of the can touching V+. That would kill the transistor pretty fast.
     
  3. Raven Luni

    Raven Luni

    798
    8
    Oct 15, 2011
    That wil most likely be the case then. There are so many ways for that to happen right now :p
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
    2,809
    Jan 21, 2010
    Perhaps you should use TO-92 case transistors (epoxy aka plastic).

    Incidentally, if the case is metal then the case or heatsink, if connected to any lead, is (almost?) invariably connected to the collector.

    If you're trying to figure out what lead is what, that can be useful.
     
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