On Mon, 24 Aug 2009 02:28:04 +0100, Arfa Daily wrote:
"alchazz" wrote in message
...
On Sun, 23 Aug 2009 15:26:08 +0100, Arfa Daily wrote:
"karotto" wrote in message
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
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?