[Noisebridge-discuss] driving multiple LEDs with minimal batteries
froggytoad at gmail.com
Thu Jan 6 04:47:18 UTC 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:
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
Now, what's all this SEPIC stuff, anyways?
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