<mrdarrett@[EMAIL PROTECTED]
> wrote in message
news:a731e6f8-d541-4301-9959-88d1f7d9106b@[EMAIL PROTECTED]
> On Jul 3, 6:54 pm, "Bob Eld" <nsmontas...@[EMAIL PROTECTED]
> wrote:
>> <mrdarr...@[EMAIL PROTECTED]
> wrote in message
>>
>>
news:7edf1122-2064-4799-9661-4828c556a5ce@[EMAIL PROTECTED]
>>
>>
>>
>> > 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
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