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

Jonathan Foote jtfoote at ieee.org
Sun Jan 16 19:27:09 UTC 2011

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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