Re: astable multivibrator issues - 2N3904s work, but not TIP31As

On Jul 3, 6:54 pm, "Bob Eld" <nsmontas...@xxxxxxxxx> wrote:
<mrdarr...@xxxxxxxxx> wrote in message

news:7edf1122-2064-4799-9661-4828c556a5ce@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



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 *** 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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