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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