[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Dr. Jesus j at hug.gs
Tue Jan 4 15:27:03 PST 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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