<mrdarrett@[EMAIL PROTECTED]
> wrote in message
news:832a69e0-dafd-41e3-aae9-56eb27d658d2@[EMAIL PROTECTED]
> 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.
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
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