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

Jonathan Foote jtfoote at ieee.org
Sun Jan 16 18:05:42 PST 2011

On Sun, Jan 16, 2011 at 4:59 PM, T <t at of.net> wrote:
> 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.

Ooh! Another teaching moment. How Switching Power Supplies Work.

Switching supplies are efficient because the transistor "switch" is
either fully OFF, (in which case negligible current) or in saturation
(fully ON) in which case there's negligible voltage across it. In
either state the power consumption is tine (recall P=V*I), compared to
linear mode which has both significant current and voltage drop.  The
JFET in that current limiting circuit is precisely in linear mode, and
it will dissipate precisely the same power as a resistor (minus a
little for the feedback resistor).  In fact a popular use of JFETs are
as voltage-variable resistors.

> 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)

It's not that much simpler seeing as how it's not a discrete component
that I'm aware of. It's a handy circuit if your supply voltage or load
is variable (though I would use a LM317 for any appreciable current,
as Igs for JFETs is rarely better than 50mA). Otherwise, simpler and
easier to pick a resistor using Ohm's Law.

> 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.

I wouldn't recommend that unless you want to smoke some JFETs when you
exceed rated Vds.

> 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.

 For small currents you don't have to match the voltage: resistors
work just fine.

> 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.

Exactly: see slide7 and 8.

> 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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