Chris wrote...

>On Sun, Mar 14, 2004 at 02:49:13PM +0000, Dave Dilatush wrote:
>
>> Also, put in that 10K base-to-ground resistor Spehro suggested so the
>> circuit doesn't turn on due to transistor leakage.
>
>Ah, so the transistors leak if the base is open-circuit?

Yep.  Like all other PN junctions, the collector-base junction of
a transistor has a certain leakage current (designated Icbo on
the transistor data sheet).  While it's usually pretty small at
room temperature, it varies exponentially with temperature,
approximately doubling with with every 10 degrees C increase.

Without any external resistance bridging the base and emitter,
this collector-base leakage just goes right on through the base
into the emitter, causing an amplified current in the collector
the same way a base signal current does; and at a high enough
temperature, this can be enough to turn a transistor switch
effectively ON.

Hence the practice of putting a resistor between base and
emitter; the exact resistance is not usually important, and
values from 1X to 10X the base feed resistor are common.

Dave D.

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