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

On Jul 3, 11:45 pm, "Paul E. Schoen" <pst...@xxxxxxxxx> wrote:
<mrdarr...@xxxxxxxxx> wrote in message

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

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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.

I tried simulating various forms of this circuit with both 2N3904 and
2N3055, and it always seemed to work, at least down to 2.5 volts or so. It
seemed to woek better if I connected C1 and C2 directly to the collectors,
which have a bit more voltage swing. I would suggest connecting a logic
level MOSFET to drive a transformer, so you will have minimal loading.
Without the LEDs, you will have plenty of voltage swing for the gate. And
you can use an N-channel to sink a higher voltage on a transformer CT, or
P-channel to source the voltage. You might even be able to make a full
bridge, but you need to make sure there is dead time where both the
high-side and low side are off. This is why they have dedicated circuits
for that.

Paul


Yep, that's why I thought pulsed DC would be easier.

Michael
.

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