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

Tymm Twillman tymmothy at gmail.com
Mon Jan 17 11:19:21 PST 2011


On Jan 17, 2011, at 10:32 AM, Dr. Jesus wrote:

> 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.

makes sense in the switched capacitor filter role... but need some way to integrate the output.

>> 
>> 
>>>>> 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?

more input / output voltage dependent than battery chemistry from what i've seen.  

efficiencies vary a lot; reasonable switching supply will generally be between 75% and 95%, but even board layout can impact that a bit, and the higher efficiencies will generally go to synchronous switchers (which use switched mosfets instead of a diode, and are more expensive).    in some cases, like buck/boost regulators where the output voltage can cross over the input voltage as the battery voltage drops, you will have ranges where efficiency tanks (though these are also applications where a linear regulator by itself just can't do the job).  

Lower output voltages will have lower efficiencies in standard switchers; much of your loss will generally be the .5v from the schottky diode drop, and if you have a very low output voltage that can be a high percentage... 

In both switchers and linear regulators, efficiency will change as the input voltage changes.


probably good to note too that you get switching noise with switchers, which in some designs is a bigger problem.


i'd be happy to supply some ICs, though the packages won't make clear what kind of regulator a part is... lots of switchers & LDOs in similar packages.  it's the surrounding parts that give them away.

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