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PICList Thread
'Ringing Chokes and batteries'
1999\11\08@161449 by Alice Campbell

While rummaging the Archives i found the following discussion of
powering a circuit from a single battery.  However, the ascii diagram
was munched.  I tried to unscramble it, but i think i have it wrong.
It is alleged to be a self-oscillating flyback converter.   Does
anyone recognize this and can you tell me where the load should be
taken off, and how the transforemer goes?


snipped segment follows:

>From this explanation it seems like the circuit should work with a
voltage LED. Has anyone tried to power a blue LED from a single cell?

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1999\11\08@164650 by Chris Eddy


You were right, your ascii is pretty hard to read.  Just my two cents,
with a saturating core converter, the current spike at the very end of the
core charge is quite high.  When the core saturates, inductance drops and
the primary acts like a near short.  I suggest making or having made a
custom transformer, as opposed to using a junk drawer one.  Production
repeatablility will depend very heavily on the reporducibility of the
transformer.  You can experiment with the transformer by adding a low
value sense resistor (and an IA to get clear voltages) in the primary
circuit, and then compare time and current.  As an inductor, the current
will be a straight linear ramp, until saturation begins.  The abruptness
of this roll off depends on core material.  There are materials with sharp
BH loop curves that maximize this sharpness and minimize the high current
pulse at the end.  Also, with a flyback, the goal is to store energy.
Many flybacks have a gap cut into the core.  That increases the core
inductance and thus the storage capacity (think I got that the right way
around).  Make sure you have a diode in the secondary or you don't have a
valid circuit.  If the secondary is in phase, you have a forward
converter.  If it is reversed, you have a flyback converter.  The easy way
to keep it straight, with a flyback current flows in the primary and
secondary at different times, never the same time.  A forward converter
results in current flow simultaneously.

Just an idea, you might try a PIC trick.  If you can control the pulse
width, you could let the PIC drive the transistor.  Essentially, drive the
transformer in an open loop mode.  You would have to take production
variation and temperature effects into account, as there is no primary
loop current feedback.  If you leave the pulse on for too long, POOF, it
is over.  There is a trick you can do to limit the current, by adding a
resistor in the ground connection, which drives an NPN base emitter.  Set
the resistor value to 0.65V/Imax.  Then connect the collecor to the base
of the drive transistor.  (assuming you switched to an NPN power trans for
this trick).  Make sure there is some resistance between the PIC and base,
and voila, you have built in current limit.  Then size the drive
transistor to survive an infinite current limit.

Chris Eddy
Pioneer Microsystems, Inc.

Alice Campbell wrote:

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1999\11\08@170718 by Dan Larson

part 0 1666 bytes content-type:application/octet-stream; name="EOLE6O0.MSG"Attached is one of the original messages containing the circuit to which you ref
with the correct ASCII art. (If you are using a fixed point font, of course...)


On Mon, 8 Nov 1999 00:08:00 PST, Alice Campbell wrote:

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Content-Type: application/octet-stream; name="EOLE6O0.MSG"

Attachment converted: wonderland:EOLE6O0.MSG (????/----) (0000F3CD)

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