astable multivibrator issues - 2N3904s work, but not TIP31As

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

news:7edf1122-2064-4799-9661-4828c556a5ce@s21g2000prm.googlegroups.com...



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.
Click to expand...

Thanks!

Michael
 
<mrdarrett@gmail.com> wrote in message
news:7edf1122-2064-4799-9661-4828c556a5ce@s21g2000prm.googlegroups.com...
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
Click to expand...

One of the nice things about 555s and 556s is that they always start if you
keep within the specifications in the data sheet. The main issue folks have
is that they forget to bypass the power supply, which often causes
retriggering problems.

If you have a cmos 555 laying about, you can do this in a way that is pretty
much guaranteed to start.

As a side note, the easy way to build a square wave oscillator from a cmos
555 is to connect the output pin to a resistor, then connect the other side
of the resistor to ground through a capacitor. Now, connect both the trigger
and threshold inputs to the junction of the resistor and capacitor.

Using this configuration, you can also use the discharge pin as an 'open
collector' output, and use it to drive arbitrary bits of circuitry. For your
application, you might use the discharge to directly drive one LED, and have
the other LED driven by a power PMOS or PNP transistor.

If the resistor you use is a pot, you can then adjust the frequency without
affecting the duty cycle by varying the resistance. You can work out the
limits of the frequencies you can obtain, given your parts.

Note that the TTL outputs of NE555 variants don't easily lend themselves to
this approach, due to the asymmetric current draw of the output for high and
low output signals. CMOS versions offer nice rail-to-rail square wave
output.

This is a nice way to do it, which I learned here from a post by John Fields
a couple of years ago. (Yet another example of what a nice resource this
group is). I'm not sure if it was his idea, or if he learned it from
somebody else. I haven't seen this idea in any of the data sheets I've
looked at.

Regards,
Bob Monsen
 
On Jul 3, 11:45 pm, "Paul E. Schoen" <pst...@smart.net> wrote:
mrdarr...@gmail.com> wrote in message

news:a731e6f8-d541-4301-9959-88d1f7d9106b@q27g2000prf.googlegroups.com...



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

news:7edf1122-2064-4799-9661-4828c556a5ce@s21g2000prm.googlegroups.com...

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.

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
Click to expand...

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

Michael
 
On Fri, 4 Jul 2008, Bob Monsen wrote:


This is a nice way to do it, which I learned here from a post by John Fields
a couple of years ago. (Yet another example of what a nice resource this
group is). I'm not sure if it was his idea, or if he learned it from somebody
else. I haven't seen this idea in any of the data sheets I've looked at.
Click to expand...
If you're talking about the resistor from the output to pins 6 & 2, that
was in "Electronics" over thirty years ago, albeit with a pullup resistor
on the output since it was the original 555 at the time. It was either
in "Designer's Notebook" or "Engineer's Casebook", I guess the former
is most likely.

It's included in Walter Jung's "IC Timer Cookbook" (which is a lot
easier to dig out than the original article) and credits it to the
June 21, 1973 issue of "Electronics". It does not bother with
the pull-up resistor, though it then suggests it as an improved
version of the simple circuit.

When I saw it, I thought it was great, no fussing with two timing
resistors, and no worry that the pulse width would get too narrow
over a wide range. So any time I need a non-specific clock, I use
the circuit, without the pullup reisstor. It works fine, it gives
far less variation in output pulse width than the standard circuit
with the two timing resistors, that require you to fuss with the
resistors to keep the pulse width reasonably constant.

For a lot of breadboarding, and even finished circuits, "close
enough" is good enough, so you don't need the pullup resistor
on the output, or a CMOS 555.

Michael
 
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