Current regulation confusion

Hi, Michael. Luxeon is staying away from all the customer's stuff by specifying a current source. But how the customer develops a current source is his problem.

Your feeling that a good voltage source with a series LED to limit current would work just as well is correct. There are some variations in LED voltage between LEDs, and also the forward voltage will tend to decrease as the LED warms up, but if your voltage source is at least twice the LED forward voltage (6VDC or so for the Vf=3V of the white LED), you shouldn't have any problem in just using a current-limiting resistor and being done with it.

Good luck Chris

You would however be well advised to provide each LED with its own series resistor for this approach, since variations in forward voltage in the LEDS could otherwise result in one LED hogging most of the current.

--
T

If it\'s not broken, don\'t fix it.

Read the last line of what Chris said.

You would however be well advised to provide each LED with its own series

Exactly.

That's why the "yuck" when I mentioned series.

Why do they push the series arrangement so much? You should see there data sheets for this part....unreal matrixes of led's in series/parallel.

Heck, I just want to light 2 of them for a sign!

In fact, the 317 I have now as a current regulator and 2 in series as per their instructions is showing one of the leds dimmer.....(maybe this bin stuff they tlak about cures that, really confusing, the bin numbers).

Maybe I should adjust my resistors in my parallel arrangement for the variation in forward voltage and resultant brightness......we'll see. Maybe the Luxeons are more critical than the average LED.

Thanks you guys.

"Good Luck"?

What you say is completely true for a fixed voltage source, as you get with a regulator.

However.

For circuits that use a battery, the current can change considerably as the battery voltage drops. Or, if you were to change to another type of LED, you'd have to recalculate and change the resistor value.

There are some simple constant-current circuits that are easy to build:

A two-transistor ciruit that I use, except instead of the 2n4403's shown here I use two 2n3906's (Vbe = 0.66V):

(The circuit can be modified to use npn transistors instead of pnp's)

Using a transistor and two diodes. While I don't recommend recharging standard batteries as shown at this site, it does look like an easy-to-build current source that should work fine for LED's. Just put your LED (or LED's) in place of the battery shown here, and then connect a battery between ground and where it says "12ish V":

Regards,

Mark

Multiple LED's would be connected in series where the battery is drawn. Oh, and also change the value of R2 to run a current suitable for an LED.

Mark

He already has the simplest:

-----

• --+--Vin|LM317|Vout---+ | ----- | [.1uF] Adj [R] | | | Gnd +----------+-----> Constant I output

Only 3 parts. I = 1.2/R

Ed

So he does.

What about a cap on Vout?

Mark

They generally use series LEDs because it is simpler to have one current source feed a string of them.

CC has the advantage of keeping the current constant with changes in the voltage drop due to temperature variations. The data sheet on the Luxeon, that I'm looking at, doesn't give a chart with temperature and voltage, but it would be unusual if it wasn't a factor.

And there's nothing holy about constant current - a voltage source with dropping resistors will be fine. Run them conservatively and there's no need for a regulated supply - just take into account the voltage fluctuation of the mains and the ambient temperature you will be working at and de rate them for temperature and anticipated voltage fluctuations if need be.

If you are building one of a kind - measure the current and set it with a resistor. If you are manufacturing many - it is easier to use CC and not have to worry about the individual led variations in voltage drop. 3.2 to 4.0 for white.

The White Luxeon Stars are specified with bins- and both the intensity and color temperature vary with the bin letter. A is highest with maximum light and coldest color temperature and E is weak and warm.

De rate the LEDs and keep the power supply simple or run them at their maximum output and regulate the current - and use a good heatsink.

I'm running four Cree red leds in series mounted to a 24 square inch piece of aluminum with a resistor. At 15 volts in they are close to their maximum power limit, and my voltage runs between 12 and 15 volts ambient temperature to about 40 C.

A place calling themselves the ledshoppe in Hong Kong is selling 1.5 watt Luxeon star flashlights for $5 post paid - about the best price that I've seen. I've bought LEDs from them without problems but not a Star flashlight.

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I would think a cap on Vout would cause instability when running it as a CC source and possibly make it oscillate or slow the tracking to the point large current excursions could take place, possibly frying the load.

Jim

Not needed. Ed

Because in a series circuit, every element passes the same current. They recommend that LEDs be driven with a constant-current source, because the voltage/current curve is very exponential, and biased by temperature.

If you want to use parallel strings of LEDs, each with its own resistor, then you can use any power supply you want, and you could even trim the resistor values to equalize the brightness. In series, if a given LED has a different brightness from another at the same current, that's what shows up.

Have Fun! Rich

When I ordered my Luxeon 1 stars from future electronics, I ordered 5 of them.

When I placed the order, there is nothing on the order form that says anything about being able to designate a bin number.

On the parts I have, there are some numbers printed on the back, one looks like a common part number (all 5 have this) and the other number is.... RX0JW 0122186 and RX0JW 0122205

When you order, how do the bin numbers come into play and how can you choose?

Like I say, I have 2 of the stars in series and one is obviously a mellower white and dimmer YET they both have the 0122205 number.

(always something).

THANKS.

redbelly wrote:

ehsjr wrote:

James Beck wrote:

Ed, Jim, Thanks for answering. I still have some learning to do when it comes to linear regulators and when to use capacitors. ON Semiconductor's datasheet for the LM317 does say:

"Although the LM317L is stable with no output capacitance, like any feedback circuit, certain values of external capacitance can cause excessive ringing. An output capacitance (CO) in the form of a 1.0 uF tantalum or 25 uF aluminum electrolytic capacitor on the output swamps this effect and insures stability."

I'll have to do a fair amount of self-study before I understand this better.

Mark

That only applies to using it as a voltage regulator, not using it in current source configuration. For using it as voltage regulator, generally a 10 uf cap is good enough.

You might want to read up on Luxeon codes here:

The first part is the bin code. All LEDs from a particular bin are similar, but there is still some visible variation within bins as you've discovered - up to 20% for your LEDs, which are from intensity bin 'R'. If you want better intensity matching than Lumileds do at the factory you've got to do it yourself - either by testing a whole bunch and selecting similar ones or by adjusting the current to the LEDs individually.

You could order several thousand from Lumileds and pick exactly the bin you want :) Assuming you don't want that many you'll have to shop around for a vendor who carries the bin you want in small quantities.

Tim