> 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?
>
> Thanks,
> Alice
>
> snipped segment follows:
>
> >From this explanation it seems like the circuit should work with a
> >higher
> voltage LED. Has anyone tried to power a blue LED from a single cell?
>
> snip%<-----------------------------------------------------
> >>> On 2/2/98 Pasi T Mustalahti wrote:
> >>>
> >>> -------8<--------
> >>> >
> >>> > ------------------------------------- +U (0.6..1.55 V)
> >>> > | |
> >>> > R20K || <
> >>> > | || <
> >>> > ---------------> || <
> >>> > | > || <
> >>> > = 10nF /------> |--|
> >>> > | ----| BC337 |
> >>> > | \, |
> >>> > | | |
> >>> > --------------------------------------
> >>> >
> >
> snip %< ----------------------------------------------------
> >The circuit is best described as a self-oscillating flyback
> >converter. Each time the transistor turns on, it charges the
> >transformer with current until it saturates. When it saturates, the
> >voltage induced in the base winding decreases, causing the transistor
> >to turn off. The energy stored in the transformer is then dumped to
> >the load (LED in this case).
> >
> >For a given frequency of operation and transformer core, the circuit
> >will deliver approximately constant *power* to the load, regardless
> >of the load voltage. The power is distributed in pulses having the
> >energy that the transformer core can hold before it saturates. Since
> >it always charges until it saturates, the output power doesn't depend
> >on the input voltage, if (and this is a big "if"), the frequency
> >doesn't vary.
> >
> >If the DC supplied to the transistor base circuit is adequate, the
> >circuit will oscillate continuously at the highes practical frequency
> >(depends on the input voltage and the inductance of the winding).
> >More likely, Pasi's circuit is running in "relaxation mode". The base
> >current required by each cycle discharges the capacitor somewhat, to
> >less than Vbe so the transistor doesn't turn on right away after the
> >transformer voltage reaches zero (all energy having been delivered to
> >the LED). There is a delay during which the resistor has to charge up
> >the capacitor to start the transistor conducting again. So the
> >frequency probably decreases significantly with input voltage as the
> >current available thru the resistor decreases. Varying the resistor
> >should vary the frequency, and thus the brightness of the LED. A
> >diode in parallel with the transistor base (to keep it from going too
> >far negative) would supply current to the capacitor while the
> >transistor is off, and probably get the circuit to oscillate
> >continuously rather than in relaxation mode. However, the resulting
> >output power would likely be too high, and difficult to control.
> >
> >However, the experiment shows that it had satisfactory performance
> >over the life of the battery. The self-regulating properties of the
> >circuit are apparently working well enough.
> >
> snip%<----------------------------------------------------------------
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