> >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).

> 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.

All good stuff. Just note that if you use a small value (eg 1X base feed
resistor), you will halve the driving voltage available to turn the
transistor on. This is usually unimportant but can make the difference
between operation and no-operation when small drive voltages are used.
(Proper design of the input circuit would make this obvious soon enough, but
it's something that can be overlooked when just throwing things together).
So eg for a 100k base drive resistor use of something like a 1M from base to
ground would be advisable.

Note that this tends to be relevant only when the driving circuit goes high
impedance when off. If the drive circuit instead pulls down to ground when
off then the base drive resistor provides a path for leakage current.

        RM

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