[Noisebridge-discuss] driving multiple LEDs with minimal batteries
Michael Shiloh
michaelshiloh1010 at gmail.com
Tue Jan 4 22:59:28 UTC 2011
This info needs to be wikified. led and battery questions are easily in
the top 5 FAQs.
i was about to do it but wonder about the structure.
do we have a technical information category? i couldn't figure out how
to get a list of categories.
should there be a link to technical information from the front page?
where would you expect to find this sort of information?
On 01/04/2011 02:54 PM, Corey McGuire wrote:
> Typically with NiCad and NiMH batteries, Sub-C cells are the best bang
> for buck and have the best energy density. This is because they are
> used in just about every industrial/hobbie rechargeable device and
> battery companies focus on this packaging. These are the cells roombas use.
> http://www.batteryspace.com/subcsizeseriesbatteries.aspx
>
>
> On Tue, Jan 4, 2011 at 2:49 PM, Corey McGuire <coreyfro at coreyfro.com
> <mailto:coreyfro at coreyfro.com>> wrote:
>
> The trouble with rechargeable batteries is how you handle over
> charging and over discharging. You want the simplest solution possible.
>
> A drawback of LiPO batteries is over discharging. Other battery
> technologies can also be over discharged, but the advantages of the
> others is, their voltages drop to the point where the LED's would be
> very dim before the cells reached a critical charge level. LiPO's
> typically hold their voltage to the bitter end...at least to the
> levels that we humans can detect with our senses. Motor's will
> happily whir, LED's will burn bright, and you won't know it's too late.
>
> To prevent over discharge with LiPO's, you need to have a voltage
> cut off circuit of some kind.
>
> Using Alkaline cells (AAA, AA, C, D, etc) means people can opt to
> use NiMH or NiCad batteries. Then battery charging is their
> problem, and not yours.
>
> If you want to solve the recharging problem, your self, you can
> include NiMH or NiCad batteries ( http://www.batteryspace.com/ ) and
> just provide a wall wart that gives 1.5v per cell wired in series (2
> cells, 3v, etc.) at 50mah-100mah of current, and you won't have to
> worry about over charging.
>
> The same can be done for the A123, LiFePo4 cells I linked, only they
> require 3.6v per cell at a low current.
>
>
> On Tue, Jan 4, 2011 at 2:26 PM, Dr. Jesus <j at hug.gs
> <mailto:j at hug.gs>> wrote:
>
> On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff
> <satiredun at gmail.com <mailto:satiredun at gmail.com>> wrote:
> > 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?
>
> You want to wire them up in parallel:
>
> (+) -|>|- (-)
> (+) -|>|- (-)
> (+) -|>|- (-)
>
> Not series:
>
> (+) -|>|- -|>|- -|>|- (-)
>
> If you have too many LEDs on the same battery it won't work because
> they will draw too much power. How many is too many depends on the
> LEDs. If you hook them up directly to the battery, they may
> draw more
> current than they're rated for, which is bad for the LEDs and may
> cause the lipo battery to catch fire.
>
> The cheap and easy way to make sure they don't draw too much
> power is
> to put a resistor in series with the LED to limit the current.
>
> (+) -/\/\/\-|>|- (-)
> (+) -/\/\/\-|>|- (-)
> (+) -/\/\/\-|>|- (-)
>
> The resistor value in ohms is (battery volts)-(LED voltage drop) /
> (the LED current you want in amps). If you want 20 milliamps
> through
> a single 2 volt LED and you're using a LiPo battery:
>
> (4 volts - 2 volts) / 0.02 amps = 100 ohms
>
> The LiPo battery voltage is only 4 volts when it's fully charged.
> When it begins discharging, it drops to about 3.7 for most of its
> discharge curve and then to 2.7 right at the very end. Even though
> the "right" number is 3.7 volts for most of the time the battery is
> discharging, use 4 volts in your calculations to avoid using too
> little resistance and putting too much current through the LED.
>
> If you have too many LEDs in the circuit, the battery will try to
> supply too much current. If the battery is unregulated it might get
> hot and catch fire.
>
> The resistor "throws away" the extra energy going to the LED in the
> form of heat, but a resistor is really cheap and you can put lots of
> them in your circuit easily. To make the battery last longer, you
> need to build or buy a constant-current regulator or a switching
> regulator, which is harder and a little more expensive.
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--
Michael Shiloh
KA6RCQ
www.teachmetomake.com
teachmetomake.wordpress.com
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