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

<mrdarrett@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 of the nice things about 555s and 556s is that they always start if you keep within the specifications in the data ***. 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

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