[Noisebridge-discuss] A simpler circuit for ... [driving multiple LEDs with minimal batteries]

T t at of.net
Mon Jan 17 17:59:21 UTC 2011

> > Ooh! Another teaching moment. How Switching Power Supplies Work.
> >
> > Switching supplies are efficient because the transistor "switch" is
> > either fully OFF, (in which case negligible current) or in saturation
> > (fully ON) in which case there's negligible voltage across it. In
> > either state the power consumption is tine (recall P=V*I), compared to
> > linear mode which has both significant current and voltage drop.  The
> > JFET in that current limiting circuit is precisely in linear mode, and
> > it will dissipate precisely the same power as a resistor (minus a
> > little for the feedback resistor).  In fact a popular use of JFETs are
> > as voltage-variable resistors.

Hmmm... I'm not sure that's not a switched mode JFET.

But without more details I will defer to the guy with at @ieee.org
e-mail address and his teaching moments.

> > > It's simpler and easier to use than a resistor (you don't even have to
> > > calculate a value- you just get one that's got a lower millamp rating than
> > > the target LED and make sure the battery voltage exceeds the sum of the
> > > voltage in the LED string)
> >
> > It's not that much simpler seeing as how it's not a discrete component
> > that I'm aware of. It's a handy circuit if your supply voltage or load
> > is variable (though I would use a LM317 for any appreciable current,
> > as Igs for JFETs is rarely better than 50mA). Otherwise, simpler and
> > easier to pick a resistor using Ohm's Law.
> For all switching regulators, you need some element that can store
> energy (a capacitor or, in these cases, an inductor), not just dissipate
> it (like a resistor or a switch).

All transistors have some inherent capacitance.  That's why the
original zero-capacitor zero-inductor joule thief circuit works.

> So Ohm's Law becomes a little more complex.
> If you don't want to use a boost converter, ...

The OP was looking for a simple way to add lights to a scarf, IIRC,
and didn't have much electronics experience and some trepidation at
building anything complex.  I fear we have driven her away.  Or
perhaps I misjudge...


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