[Noisebridge-discuss] driving multiple LEDs with minimal batteries
Dr. Jesus
j at hug.gs
Tue Jan 4 23:27:03 UTC 2011
http://en.wikibooks.org/wiki/Practical_Electronics/Diodes
On Tue, Jan 4, 2011 at 2:59 PM, Michael Shiloh
<michaelshiloh1010 at gmail.com> wrote:
> 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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>>
>>
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> --
> Michael Shiloh
> KA6RCQ
> www.teachmetomake.com
> teachmetomake.wordpress.com
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