<mrdarrett@[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.
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