On Jul 3, 11:45 pm, "Paul E. Schoen" <pst...@[EMAIL PROTECTED]
> wrote:
> <mrdarr...@[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
Yep, that's why I thought pulsed DC would be easier.
Michael
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