On Sun, 20 May 2007 15:38:06 -0500, Vampyre wrote:

I'm completely new to electronics so forgive my ignorance. I've read that
capacitors can be used to drop dc voltage, can they be used in the opposite
way? can i build a dc to dc transformer that increases voltage while
decreasing amperage with just capacitors and maybe transistors?

I'd like to use some small solar cells to slowly charge a 5 volt super
capacitor.. but ofcourse my cells only put out 4 volts in bright sunlight


any help would be greatly appreciated.

You can use capacitors and transistors to boost voltage efficiently.
Basically what you are doing is charging caps in parallel and
reconnecting them in series before discharging them into the load. It
is a very efficient way to step up voltage - but takes a lot of
transistors and caps or an integrated circuit designed for the
purpose. It is called "switched capacitor" voltage booster or
converter. If you want to search it.

All white leds require ~3.5 volts to light and single cells only put
out 1.2 or 1.5 volts (sans lithium which puts out 3+) so there's lots
of circuits for driving leds off low voltage. They would also be
ideal to charge your super cap. Put in 3-4 volts and you will get out
more than when you used just 1.2 volts.

http://cappels.org/dproj/PulseBoostL...White_LED.html
Shows a switched capacitor voltage booster using 6 transistors and no
inductors.

But inductors do the same job with fewer parts and still do it with
good efficiency.

It is far easier to use just a single transistor to charge a
rechargeable battery from a solar panel using something called the
"joule thief." The circuit is a blocking oscillator and uses
inductive kick to raise the voltage a bit. I use one for some "garden
lights" I built with 2 volt 100 ma solar panels. Open circuit the
output of my 15 turn transformer is about 50 volts DC which is plenty
to charge the 1.2 volt NMH battery. A second transistor uses the same
panel to tell the oscillator to shut down when it is light out. Neat
and as simple as it gets.

You may need to protect the super cap from over charging with
something like a zener diode to shunt XS voltage once the cap is
charged.

http://www.emanator.demon.co.uk/bigclive/joule.htm The original
circuit

http://www.joulethief.com/kit.php sells a kit using two transistors
and a classic oscillator boost converter - schematic is on the site
and can be built for $2-3 Their kit is only $10

http://www.cappels.org/dproj/ledpage/leddrv.htm Check out this site.
Circuit is identical to the original joule thief, but he winds the
transformer differently. Lots of tinker potential on this stuff.
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