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

T t at of.net
Wed Jan 19 09:21:48 PST 2011


IYHO, why focus on linear regulators?  As Jonathan Foote pointed out,
they aren't really more efficient than a simple resistor.  Are they
simpler in any sense?

If you want efficiency (which IMHO is a must when powering with
batteries) don't you need a boost or buck or other switching supply?

Best Regards.



On Tue, Jan 18, 2011 at 23:53, Dr. Jesus <j at hug.gs> wrote:
> On Mon, Jan 17, 2011 at 11:19 AM, Tymm Twillman <tymmothy at gmail.com> wrote:
>>
>> 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.
>
> I threw up a poster with descriptions of the two linear regulators
> that are easy to make with the parts at Noisebridge.  There isn't a
> lot of space left on an 11x17 so I think only one switching regulator
> type will fit in the remaining area on the poster.
>
> If someone writes up some text and example circuits for a simple
> switching regulator I'll add it to the poster.
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