[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Jonathan Foote jtfoote at ieee.org
Wed Jan 5 17:04:29 PST 2011


Yes exactly. It's a current source: it delivers as much voltage as
necessary to result in the desired current, (up to some reasonable
limit which depends on the circuit but sounded like 30V in this case).

At 20 mA, the power requirement for that is VI = 0.6 watts,  which is
taxing but do-able for an alkaline AA (I'd use a few in a pack).
Typically these switching circuits are ~90%  efficient so you can more
or less ignore their power draw (another reason they rock).

So the forward diode drop is the practical limit on this. For blue or
white LEDs that's more than 3V typically, so you won't get more than
10 or so in series before you hit 30V. For reds Vf is closer to 1.7,
so you could do a few more.


On Wed, Jan 5, 2011 at 2:27 PM, jim <jim at well.com> wrote:
>
>
>   restating: so this gizmo gets input from a battery
> (or other DC source) and sends pulses to a transformer
> or a coil (or voltage doubler or...) depending on
> some sensor circuitry that detects load requirements?
>   generally, the question of power limit seems still
> unanswered. it doesn't seem reasonable that one can
> power an unlimited number of LEDs; what's the upper
> limit, i.e. how to figure upper limit for a variety
> of LEDs or other devices?
>   if there's some sensor circuitry that "does the
> right thing" with respect to delivering voltage and
> current, how does that part work? user configurable?
>
>   is this an off-the-shelf part or a fairly simple
> mod of one or more off-the-shelf parts or the design
> of someone using discrete components or what?
>
>   many thanks!
>
>
>
> On Wed, 2011-01-05 at 14:10 -0800, T wrote:
>>
>>
>> Outstanding.  A boost with a current limiter in a small package!
>>
>>
>> It that really is what it is, then Jim, in answer to your question, it
>> would seem that the output would be *up to* 30VDC, not 30VDC.  I.e. it
>> would raise voltage (as boost circuits do by definitiion) by cycling
>> inductors or flying caps or whatever at the appropriate rate to get
>> the spec'd current limit or 30VDC, whichever is lower.
>>
>>
>> The beauty of this family of circuits (boost and buck and active
>> current limiter) is that there's logic that controls the frequency or
>> duty cycle of switching components to get the desired voltage or
>> current instead of a resistor that wastes some of the energy.
>>
>>
>> T
>>
>> On Wed, Jan 5, 2011 at 12:18, jim <jim at systemateka.com> wrote:
>>
>>
>>           i'm curious as to the power aspect:
>>
>>           assuming an AA battery and a black box gizmo that
>>         converts input voltage to 30 VDC out, connecting one
>>         LED requires a current limiting resistor. connecting
>>         two LEDS requires a smaller current limiting resistor.
>>         more LEDs, smaller current limiting resistor....
>>           having not done it, i don't know that some number
>>         of LEDs requires no current limiting resistor, is
>>         that the case? if so, then i'm guessing the internal
>>         series resistance of the power supply (the black box
>>         gizmo and/or the battery) provides the limit, yes?
>>           regardless, it seems that each LED represents
>>         power consumption of current times voltage drop
>>         across the device. a few LEDs presents no problem,
>>         but above a certain number, it seems there may be
>>         some kind of failure, possibly dimmer LEDs or hotter
>>         battery or blackbox gizmo weirdness or....
>>
>>           what's the truth about this?
>>
>>         with thanks
>>
>>
>>
>>
>>
>>         On Wed, 2011-01-05 at 12:02 -0800, Michael Shiloh wrote:
>>         > It's Rolf, copied on this.
>>         >
>>         > On 01/05/2011 11:49 AM, Jake wrote:
>>         > > There was a guy who came to Circuit Hacking Mondays who
>>         made these tiny
>>         > > circuitboards with a Linear Technologies chip that boosted
>>         power from a
>>         > > low voltage to up to 30 volts with a set current limit.
>>          He brought them
>>         > > to CHM as a kit i think, and was making umbrellas with
>>         hanging lights.
>>         > >
>>         > > Mitch or Miloh will know who i'm speaking of.  He was
>>         really tall and thin
>>         > > and older than Miloh...
>>         > >
>>         > > anyway the great awesomeness of his kit is like this:  you
>>         hook up a
>>         > > battery to it (a single AA, rechargable or not) or a
>>         lithium-whatever, it
>>         > > doesn't care - and it will work.
>>         > >
>>         > > On the output side, you simply wire all your LEDs in
>>         series - one after
>>         > > another, any colors, no resistors necessary - and the
>>         thing will power
>>         > > them all with the same current.  Regardless of the state
>>         of the battery
>>         > > (until it is empty)  Up to 30v or 10-15 LEDs in series per
>>         board.
>>         > >
>>         > > I think the appropriate way to power a scarf is with a
>>         single AA sized
>>         > > battery.  People can use a disposable battery but a
>>         rechargable AA is
>>         > > ubiquitous and appropriate, and an be charged separately
>>         from the scarf.
>>         > > And a scarf is something that will hopefully outlast any
>>         LiFePo battery or
>>         > > NiMH cell, so it has to be a standard, replaceable cell
>>         like a AA.
>>         > >
>>         > > -jake
>>         > >
>>         > > ---- original mess ----
>>         > >
>>         > > I'd like to do a soft circuit scarf or three, but I'm
>>         always running up
>>         > > against the problem of power. I usually use fairly low
>>         power LEDs
>>         > > (<2v) driven by a 9v battery or one of sparkfun's LiPos.
>>         > > I've heard tell of somehow being able to power more, but
>>         I'm still
>>         > > learning
>>         > > this EE stuff. Could some kind person point me in the
>>         right direction?
>>         > > Meredith
>>         > >
>>         > > _______________________________________________
>>         > > Noisebridge-discuss mailing list
>>         > > Noisebridge-discuss at lists.noisebridge.net
>>         > >
>>         https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
>>         > >
>>         >
>>
>>         _______________________________________________
>>         Noisebridge-discuss mailing list
>>         Noisebridge-discuss at lists.noisebridge.net
>>         https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
>>
>>
>> _______________________________________________
>> Noisebridge-discuss mailing list
>> Noisebridge-discuss at lists.noisebridge.net
>> https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
>
> _______________________________________________
> Noisebridge-discuss mailing list
> Noisebridge-discuss at lists.noisebridge.net
> https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss
>


More information about the Noisebridge-discuss mailing list