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

Dr. Jesus j at hug.gs
Mon Jan 17 18:32:30 UTC 2011

On Mon, Jan 17, 2011 at 9:59 AM, T <t at of.net> wrote:
>> > 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...

Which solution did you end up going with, Meredith?

Power supplies being what they are, it occurs to me that it would be a
good idea to have a poster near the components showing the fundamental
linear power supply types and the simpler switching ones.  I'll draw
one up.  If anyone has spare switching ICs they'd like to donate, let
me know before Friday and I'll add them to the poster.

It would also be nice to put the typical conversion efficiencies next
to each power supply type to help the reader select an appropriate
circuit, but a quick look through my handbooks doesn't give me any
useful data.  Is there a good reference that takes into account modern
battery chemistries?

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