On Jul 3, 6:54 pm, "Bob Eld" <nsmontas...@[EMAIL PROTECTED]
> wrote:
> <mrdarr...@[EMAIL PROTECTED]
> wrote in message
>
>
news:7edf1122-2064-4799-9661-4828c556a5ce@[EMAIL PROTECTED]
>
>
>
> > I built an astable multivibrator with blinking lights (much simpler
> > than using a 556!), as a test for a circuit that will use a power
> > transistor (or MOSFET) to pulse a transformer primary for future
> > experiments.
>
> >http://mrdarrett.googlepages.com/blinkenlights002.pdf
>
> > Strangely, when I replaced the two 2N3904 transistors with TIP31As,
> > the circuit would not oscillate unless I briefly disconnected then re-
> > connected R2 or R3. (I bumped the voltage up from 3V to 6V during
> > these tests.)
>
> > As a work-around, I'm considering just putting the 2N3904s back in,
> > and connecting the positive end of C2 to the base of a TIP31A.
> > Inelegant, but I think it will work.
>
> > I'm trying to figure out why TIP31As won't work, but it also doesn't
> > help me any that the TIP31A data sheet does not specify a minimum
> > V_BE_on.
>
> >http://www.st.com/stonline/products/literature/ds/12292/tip31a.pdf
>
> > Input?
>
> > Michael
>
> One possible reason it doesn't work with certain transistors is that the
two
> transistors come on together and lock up the operation. It fails to
> flip-flop. Also, there might not be enough base current for TIP31's
with R2
> and R3 at 10K. Lower these resistors. The minimum beta is 25 at 1 amp.
so
> you're likely not getting enough current to drive these transistors.
>
> To insure that an astable won't lock up, disconnect R2 and R3 from the
> positive rail and connect the junction of the two resistors to the
cathodes
> of two diodes. Connect the anodes of the diodes to the anodes of the
LED's,
> one on each.
>
> With this arrangement, if the transistors both come on together, the
base
> drive is reduced and the circuit will always start.
Thanks!
Michael
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