Re: Can't get enough current for 1W Luexon LED even without resistor

On Tue, 25 Aug 2009 09:42:15 +0100, Arfa Daily wrote:

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On Mon, 24 Aug 2009 02:28:04 +0100, Arfa Daily wrote:

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On Sun, 23 Aug 2009 15:26:08 +0100, Arfa Daily wrote:

"karotto" <biz@xxxxxxxxxxxxxxxxxxx> wrote in message
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Trying to power a 1W Luxeon star LED. It says: Forward Voltage:
3.5V, Forward Current 650ma. So I tried a 3.6V battery with a
minimal resistor. Very low current. Then I tried without the
resistor but still only about 100ma current. In order to get 650ma
to flow through the LED I have to raise the voltage to about 11V
and of course I already blew the LED. So... how can I get 650ma
while only using 3.5V. Seems like my battery has too much internal
resistance. Thanks much for your help.

If you want to get any decent length of life out of it, you don't
want to be thinking in terms of voltage. It needs a constant current
source. Try Googling "LM317 constant current source". It's a cheap
and simple way of doing it with just a couple of resistors to set
the current. The voltage that's used is then arbitrary, as long as
it's enough. The voltage across the LED will settle to whatever is
its natural value for the colour in question - maybe 3.5v, as you're
expecting. Alternatively, use one of the electronic drive modules
made by Luxeon, especially for the job.

Arfa

What is the response time of an LM317 current source? Will there be a
current spike larger than the amount set by the resistors before it
settles out?

I've used LM317's as a constant current source for driving all sorts
of LEDs for a long time, and never suffered a problem with a switch-on
failure, so I guess the answer to "how fast is an LM317"?" is "fast
enough ..." It's not a bad idea to have a decoupling cap across the
output anyway, and the initial charging current that this will
'steal', should be plenty enough to ensure that the '317's output
current has settled to the desired value for the LED, by the time that
the cap's effective resistance has come up towards that of the LED.

Driving with any kind of constant current source, is superior to
current limiting with a resistor from a constant voltage source for
any kind of demanding use, although just using a resistor is fine for
simple indicator type uses. The way to get the best performance and
life from any high power LED, is to pulse drive it. As someone else
commented, the max forward current quoted in specs, is for a short
duration pulse. The level of these that some high power LEDs can
withstand, is staggering, compared to the maximum continuous forward
current. It is not impossible to put together a little circuit to
pulse drive a LED satisfactorily, but it is easier to just use one of
the ready made modules.

Arfa

I have a IR LED rated at 10A pulse current. I would like to do some
experiments with it. I have hesitated as I do not want to damage it
with overcurrent. The LM317 is rated at most at 1.5A depending upon the
package type. I would probably start out with a current of 1A and
increase it. How could I beef up the LM317 current carrying capability?
There must be a circuit available that uses the LM317 and a power
boosting transistor. Yes, I know, the radiant power is deadly to the
eyes and all experiments would be in a light tight box.

I asked about the speed of the LM317 response as I would like to pulse
the diode with pulse widths ranging from, say 1 uS to several mS. I
would use a fast FET on the anode side for the switch.

Am I barking up the wrong tree?

I would not try to use an LM317 to pulse drive a LED. It is better
suited as a linear constant current source. Fundamentally, it is
actually designed as a variable linear voltage regulator, but its
internal topology happens to make it particularly suited to use as a
simple (and cheap) constant current source. If you are looking for a
'quick and dirty' experimental set up for pulse driving this LED, you
could probably just cobble up a 555 timer IC as an adjustable astable,
and use the output to drive a power MoSFET. Stick a suitable resistor in
series with the LED to limit the current to say 8A peak, and away you
go. Alter the 555's mark space ratio to effectively PWM the current to
the LED, and you then are able to control its (invisible) intensity.

You are probably better to put the switching FET in the cathode side of
the LED, as you can then return its source straight to ground, which
makes the drive arrangement to the gate a lot simpler. Doesn't really
matter where you put the current limiting resistor, but I would probably
put it in the anode side 'out of the way'.

If your drive circuitry is going to be powered from the same supply as
the LED, then you will need to get seriously good with the decoupling on
the supply rail to that circuitry. Pulses of 8A or so are going to cause
some big spikes to fly about. You would probably be as well to supply
the drive electronics via its own regulator - LM7808 or something - with
a small choke filter on its input, and all the recommended Cs,
especially if you are going to be driving with some very narow pulses. I
can't remember what the minimum pulse width from a standard bipolar 555
is, but if not that short, then one of the other versions possibly is.
Bear in mind also, that if yiu are going to start switching currents of
several amps at those sorts of pulse width, you are quite likely to
start generating quite broadband RF noise. You might want to have a
little transistor radio nearby, just to make sure that you are not
interfering with the local airport ... :-)

Arfa

It makes sense to not use a LM317 as a current source to limit current in
a pulse situation.

I have a couple of problems with putting series resistors into the
circuit. First of all, I don't know what the forward voltage drop of the
LED will be. It will probable change as it self heats during the
excitation. And it will vary with the current. Perhaps a very low duty
cycle will mitigate the self heating.

Secondly, most of the power resistors in my junk box are wirewound. I'm
not sure how the diode would react to the inductive reactance. Yes, I
know, put in a clamp diode. But those have a switching time involved.
Will a clamp diode that can handle many amperes have a fast transient
recovery time?

The IR LED I have acquired is quite old, > 15 yrs. It does not have an
integrated switch circuit in it like some of the recent ones I have seen.

Yes, I am doing this as a hobby. I manage to get everything I need from
either recycling stations or electronic flea markets. Most of my
experiments work. Some just let the smoke out ;-)
.

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