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


<mrdarrett@xxxxxxxxx> wrote in message
news:832a69e0-dafd-41e3-aae9-56eb27d658d2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jul 3, 11:45 pm, "Paul E. Schoen" <pst...@xxxxxxxxx> wrote:
<mrdarr...@xxxxxxxxx> wrote in message

news:a731e6f8-d541-4301-9959-88d1f7d9106b@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



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.

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.

But, also remember that you must have very little DC current in any
transformer winding, or you will get saturation. Pulsed DC can be used if
you are building a circuit where you are alternately storing and releasing
energy in the magnetic field, in which case you really have an inductor,
which may have additional windings for various reasons (especially
isolation, multiple outputs, and large differences in input to output
voltage/current).

You might have a look at the Microchip PIC16F616, which has multiple
on-board PWM outputs that can be programmed to drive various power supply
circuits ranging from simple single inductor buck or boost converters, to
center tapped transformers, and full bridge circuits (using MOSFETs, of
course). I'm using that PIC (and its similar cousin, PIC16F684) to make a
single inductor 100 kHz boost converter from 12 VDC to anything from 20 to
60 VDC at close to 1 amp, and it would only take a few changes in the power
components (MOSFET, inductor, Schottky diode, and capacitors) to scale that
up to several hundred watts. And some simple additional diodes and
capacitors can also provide a negative output that tracks the positive
output for balanced loads. I have posted the LTSpice file before. If you
want to learn PIC programming this might be a fun project.

Paul


.

Quantcast