Re: Can't get enough current for 1W Luexon LED even without resistor
- From: "Arfa Daily" <arfa.daily@xxxxxxxxxxxx>
- Date: Tue, 25 Aug 2009 09:19:50 +0100
"Jim Yanik" <jyanik@xxxxxxxxx> wrote in message
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"Arfa Daily" <arfa.daily@xxxxxxxxxxxx> wrote in
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On Sun, 23 Aug 2009 15:26:08 +0100, Arfa Daily wrote:
"karotto" <biz@xxxxxxxxxxxxxxxxxxx> wrote in messageb0aa-9a304bcdff1b@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
news:5d43962f-06f8-4ce1-
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
only problem with the LM317 current source is the large V-drop across it.
(for a battery supply)
and a ONE watt Luxeon LED is only going to draw about 300ma,with a 3.5Vf.
--
Jim Yanik
jyanik
at
kua.net
Yes indeed, Jim. There are better solutions than a '317 if you are battery
powering, but if you are going to start driving 300mA through the things,
you're going to be needing some pretty good batteries anyway. I got the
impression that the OP was just trying to test his purchase with a battery.
As to the current that is going to pass though the LED, it is not determined
by the forward voltage. Rather, it is determined by how much you *allow* it
to draw. Once you have enough voltage available across the device, it will
settle at whatever value that particular colour and design of LED, normally
has. It's then up to you how much current you let it draw. Less than 300mA,
and it's not quite so bright, with a longer life expectancy. More than
300mA, and it will burn like a miniature sun. Briefly ... :-)
Arfa
.
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