[Noisebridge-discuss] A simpler circuit for ... [driving multiple LEDs with minimal batteries]

[Taylor Alexander]

Hey Guys,
If you look back at the scarf thread, I volunteered to design some
"proper" LED drivers for whatever you want. I've been interested in
High Brightness LEDs lately and recently made a board that drives 4
CREE XP-G LEDs at 800ma, to produce as much light as a 60W light bulb.
I'm working on a new board based off the CREE XM-L that will be
cheaper. The circuits are actually really simple, but you do have to
use a PCB. Luckily I'm trying to design up some LED driver boards that
I can open source, so I would gladly share the designs with you, and
even do the soldering myself, if some one else pays for the parts
(~$10 a board).

I can also easily design a board and you guys can order it yourself
from BatchPCB. This stuff is easy for me and I'd like to help - I just
need someone to give me some requirements. Like input voltage, # of
LEDs in series, current required, etc.

-Taylor

On Sun, Jan 16, 2011 at 11:27 AM, Jonathan Foote <jtfoote at ieee.org> wrote:
> As in all engineering solutions, optimizing one variable (efficiency,
> say) comes at the cost of another (simplicity).
>
> A little teaching moment here: there will be a voltage drop V across,
> and a current I through, the CLD.
> Power = V x I.  How much power is this? Where does it go?
>
> And how much power would an equivalent resistor use?
>
> Seeing as how neither the battery voltage nor the load is changing
> appreciably, what's the advantage to using a CLD over a resistor?
>
>
>
> On Sun, Jan 16, 2011 at 10:08 AM, T <t at of.net> wrote:
>>
>>
>> Here's another idea in the thought of not getting overwhelmed by building
>> complex circuits.
>>
>> You can get a device called a "Constant current diode (also called CLD,
>> current limiting diode, constant-current diode, diode-connected transistor
>> or CRD,current-regulating diode)"
>>
>>  https://secure.wikimedia.org/wikipedia/en/wiki/Constant_current_diode
>>
>> As long as your battery has a higher voltage than your string of diodes (add
>> up the voltage drops if you run them in series as others have advised) and
>> is capable of producing the current (milliamps), picking a constant current
>> diode that has a current rating at or below the rating of your LEDs should
>> do the job in a very simple circuit:
>>
>>  --- - battery + ---- CLD |> ----- LED ---- LED ---- LED .... ---
>> |                                                                |
>>  ----------------------------------------------------------------
>> So something like a 12V camera battery should be able to drive up to 3 white
>> 3V LEDs or a few more of the lower-voltage colored variety, a 9V "transistor
>> battery" should be able to drive 2.
>> And it lends itself to experiment too... you can hood up the battery and the
>> CLD and one LED, and it should work fine (since it's current-limited it will
>> limit voltage too), and you can hook up two, and you can hook up three, and
>> if you hook up too many they just won't light up, no harm done.
>>
>> Best Regards.
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