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

T t at of.net
Sun Jan 16 16:59:02 PST 2011


Wrong.  You are limiting the power rather than burning a portion of it off
as heat. Yes, there *is* an inegrated resistor, but it's very low
resistance- only enough so the JFET can sense the current (V=IR) and turn
off when there's too much current and turn back on when there's not enough.


It's a switching power supply with just two leads.

It's simpler and easier to use than a resistor (you don't even have to
calculate a value- you just get one that's got a lower millamp rating than
the target LED and make sure the battery voltage exceeds the sum of the
voltage in the LED string)

And it's way more efficient anytime the supply voltage is higher than the
the LED voltage.  Like if you want to drive LEDs off 110V AC and a bridge
rectifier.

Sure, if you happen to match the supply voltage closely to the LEDs, then a
resistor is fine.  Like in the LED throwies- a resistor can even be zero.

But if you want a circuit that works with white and red LEDs, that works
when you add more in series, etc., you want current limiting not voltage
limiting.

T

On Sun, Jan 16, 2011 at 11:27, 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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