[Noisebridge-discuss] driving multiple LEDs with minimal batteries

miloh froggytoad at gmail.com
Wed Jan 5 20:47:18 PST 2011


On Wed, Jan 5, 2011 at 12:18 PM, 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
>

forwarding an earlier personal reply to the thread:

to answer your last speculations:  above a certain voltage the device
will not be able to drive the current -- and it will fail.   Abstract
form is a transconductance or VCCS amp, but check out this current
source available from Linear:
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1778,C1764,P88400

We built Lumbrellas that had 7 white 5mm LEDS on them, so assuming a
Vd = ~3.5v the thing is able to drive about 24.5 volts.  That looks
like the LT3092, but I'm not sure and I don't have my Lumbrella board
anymore!


Now, what's all this SEPIC stuff, anyways?


-rma


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