Dave Dilatush wrote:
>
> Roman Black wrote...
>
> >Ha ha! :o) Thanks guys, well I did say it was
> >done in a hurry, and no obviously not tested.
> >But i am curious as to what you see is the
> >problem with it? That it won't oscillate or that
> >Q1 (top) won't turn off? Or am I really thick
> >today and missing something? :o)
>
> What's missing, Roman, is that you skipped a crucial step in designing
> things like this: a walk-through of its operation, looking at the
> voltages and currents that occur in each state to verify (1) that they
> are what you need them to be and (2) that they are physically possible.
>
> Had you done this, you would have discovered (referring to the redrawn
> diagram below) that when Q2 turns on, the circuit node connecting the
> emitter of Q1, the left end of L1, and the cathode of D1 cannot possibly
> go more negative than [Vbe(Q1) - Vcesat(Q2)].  In order to function, and
> deliver current to the -15V output, this node would have to go down to
> -15.7 volts to forward-bias D1, and that it clearly cannot do because it
> would require the collector of Q2 to go negative down to -15 volts which
> would destroy Q2.
>
> In other words, this circuit "fails by inspection" and a very brief
> inspection, indeed, is all it should take to confirm that it doesn't do
> the job.
>
> Dave D.
>
>   +9v------+----------------+
>   In       |                |
>            |                R2
>        Q1  C                |
>        NPN  B---------------+
>            E                |
>            |                |
>            |                C  Q2
>            +---L1--+---+---B   NPN
>            |       |   |    E
>            |       |   |    |
>            - D1    C1  R1   |
>            ^ 0.1uF |   |    |
>            |       |   |    |
>   Gnd------|-------+---+----+
>            |       |        |
>            |  10uF C2       ~ 15v
>            |       |        ^ Zener
>            |       |        |
>  -15v------+-------+--------+
>   Out
>


Thanks Dave! Very nice looking circuit there too. :o)
R1 adjust (even a trimpot) should still give a
decent constant current control and the thing should
oscillate well and be pretty efficient even with a
flat 9v battery. :o)
-Roman

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