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.

hi, you must have something wrong , i looked on the luxeon website , and
all their star 1 watt leds draw no more than 350 ma

mark




"karotto" wrote in message
...
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.

On Sat, 22 Aug 2009 21:35:57 -0700 (PDT), karotto
wrote:

Trying to power a 1W Luxeon star LED. It says: Forward Voltage: 3.5V,
Forward Current 650ma. So I tried a 3.6V battery


Well, 3.65 volts at 650 MA is only 2.34 watts, so that ain't right.
The 'correct' current rating is *probably* about 275 MA or a bit less.
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.

"karotto" wrote in message
...
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

Thanks everyone for your suggestions: Here are the exact specs of the
item:

Brand New PKG 1, Star Bright 1W Full Color 100 Lumen LM White Led

Lens Color : Water Clear

Emitted Color : White

Intensity Typ Iv (mcd) : 90~100mcd

Viewing Angle : 160°

Forward Voltage (V) : 3.0v-3.2v

Forward Current (IFM): 650mA

I guess it's not a Luxeon. Sorry. Are those specs wrong? Thanks again.

On Sun, 23 Aug 2009 09:12:41 -0700 (PDT), karotto
wrote:

Thanks everyone for your suggestions: Here are the exact specs of the
item:

Brand New PKG 1, Star Bright 1W Full Color 100 Lumen LM White Led

Lens Color : Water Clear

Emitted Color : White

Intensity Typ Iv (mcd) : 90~100mcd

Viewing Angle : 160°

Forward Voltage (V) : 3.0v-3.2v

Forward Current (IFM): 650mA

I guess it's not a Luxeon. Sorry. Are those specs wrong? Thanks again.

Post an actual specifications sheet (or link to it), not an abstract.
There's more informaton that is missing from the above list of specs.

The above specs are wrong in that they are missing critical
information about conditions for forward current for example.

"PeterD" wrote in :
: Post an actual specifications sheet (or link to it), not an abstract.
: There's more informaton that is missing from the above list of specs.
:
: The above specs are wrong in that they are missing critical
: information about conditions for forward current for example.

IFM is Max Forward Current and is usually for a very short pulse
condition, like 10mS on with a duty cycle of x%, and not steady
state.
Art

karotto wrote:

Thanks everyone for your suggestions: Here are the exact specs of the
item:

Brand New PKG 1, Star Bright 1W Full Color 100 Lumen LM White Led


Forward Voltage (V) : 3.0v-3.2v

Forward Current (IFM): 650mA

I guess it's not a Luxeon. Sorry. Are those specs wrong? Thanks again.

IFM is current(I) Forward Maximum - i.e., the absolute max you could
feed the thing before it burns out. Try to stay well south of that.

At a forward voltage of 3.2V, your current should be about 1W/3.2V =
313 mA.

So, your battery may well have too much internal resistance to drive
it to the 1W level. But don't try to put 650ma into the thing.

karotto wrote:
Thanks everyone for your suggestions: Here are the exact specs of the
item:

Brand New PKG 1, Star Bright 1W Full Color 100 Lumen LM White Led

Lens Color : Water Clear

Emitted Color : White

Intensity Typ Iv (mcd) : 90~100mcd

Viewing Angle : 160°

Forward Voltage (V) : 3.0v-3.2v

Forward Current (IFM): 650mA

I guess it's not a Luxeon. Sorry. Are those specs wrong? Thanks again.

An LM317 with 1.92R (1.8R + 0.12R) between the common & output pins will
give you a constant 650mA.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------

On 8/23/2009 7:26 AM Arfa Daily spake thus:

"karotto" wrote in message
...

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.

What he said. I've used the LM317 to drive a LED headlight in a model
railroad locomotive. Simple and works great; nice constant brightness
regardless of track voltage.


--
Found--the gene that causes belief in genetic determinism

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?

"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

"Arfa Daily" wrote in
:


"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




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

"Jim Yanik" wrote in message
...
"Arfa Daily" wrote in
:


"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




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

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?

In article , alchazz wrote:
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?

Simple. Make a Mosfet driver and use a resistor to adjust current.

greg

"alchazz" wrote in message
...
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?

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

In article
,
karotto wrote:

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.

Karotto-

Your use of 11 Volts suggests you may have connected the LED backwards!

There is something wrong with your interpretation of the specifications.
It is a one Watt device, but you seem to be trying to make it burn 2.275
Watts (3.5 Volts times 0.65 Ampere). I think the 3.5 Volts and 0.65
Amps are peak or maximum values, not for continuous use.

An LED is a diode. Voltage drop is just that, the Voltage measured
across the device when it is conducting in its forward direction. You
apply a current and measure the Voltage drop, not the other way around.
For a 1 Watt LED, I would expect around 3 Volts drop for 1/3 Ampere of
current (3 Volts times 1/3 Ampere equals 1 Watt).

To use a 3.6 Volt battery in theory, the series resistance would be 1.8
Ohms (0.6 Volts divided by 1/3 Ampere). With this approach, the light
output will dim fairly rapidly since the internal resistance of the
battery increases as it drains.

Use of a higher voltage with the LM317 constant current arrangement
suggested by Arfa, is probably the best approach. Just make sure you
don't set it for too high a current!

Fred

"Fred McKenzie" wrote in message
...
In article
,
karotto wrote:

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.

Karotto-

Your use of 11 Volts suggests you may have connected the LED backwards!

There is something wrong with your interpretation of the specifications.
It is a one Watt device, but you seem to be trying to make it burn 2.275
Watts (3.5 Volts times 0.65 Ampere). I think the 3.5 Volts and 0.65
Amps are peak or maximum values, not for continuous use.

An LED is a diode. Voltage drop is just that, the Voltage measured
across the device when it is conducting in its forward direction. You
apply a current and measure the Voltage drop, not the other way around.
For a 1 Watt LED, I would expect around 3 Volts drop for 1/3 Ampere of
current (3 Volts times 1/3 Ampere equals 1 Watt).

To use a 3.6 Volt battery in theory, the series resistance would be 1.8
Ohms (0.6 Volts divided by 1/3 Ampere). With this approach, the light
output will dim fairly rapidly since the internal resistance of the
battery increases as it drains.

Use of a higher voltage with the LM317 constant current arrangement
suggested by Arfa, is probably the best approach. Just make sure you
don't set it for too high a current!

Fred

In general, it's not so much about applying a current and measuring the
voltage with LEDs, although your calculations are of course all correct. The
forward voltage drop on a LED is what it is - i.e. the voltage will be
pretty much constant irrespective of the current which that voltage causes
to flow. A bit like a zener diode. The actual voltage varies a great deal
between colours of LED, and even within colours, depending on LED power and
other factors. The key is that as long as you apply enough voltage to exceed
the forward voltage requirement for the LED in question, with a sensible
margin available, then that *actual* applied voltage is arbitrary, the key
requirement then being that the current is restricted in some way, to give
the desired light output / life expectancy, without exceeding the
manufacturers maximum figure for continuous (DC) operation, or pulsed drive.

It also has to be remembered that high power LEDs generate quite a bit of
heat, and this needs to be removed fairly efficiently. If the LED is allowed
to heat up exessively, its life will be considerably shortened. Its forward
voltage drop will also vary with temperature, so if you are feeding it with
a simple resistive current limiter, and running it close to max spec, you
might run into additional problems with the current increasing further than
you intend. Which is why it's an all-round better solution to drive with a
simple constant current source, as Fred agrees.

Remember also, that if you are intending to drive multiple examples of the
same LED in some kind of array, the normal way to do this is to put them
into series 'strings'. So, as an example, the LEDs you are using here have a
quoted forward voltage drop of around 3.5v. Lets say that you wanted to run
five of them at perhaps 150mA. So, multiply 3.5 by 5 to get 17.5v. Add a bit
of overhead to allow the LM317 to work, and call it 21v or so. A 15v
transformer with a bridge and decent sized filter cap on the end would be
just about right. Set the resistor values to give the 150mA, hook the five
LEDs in series, and away you go. You might need a small heatsink on the
LM317, but provided you don't go wild with the voltage that you're starting
out with, or the drive current that you're asking it for, the device should
not dissipate a lot of power.

Each LED in the string, will develop its own forward voltage drop, and this
may well be slightly different for each example, and may vary slightly
differently for each example as the temperature rises. No matter. The LM317
will adjust its output as required, to maintain 150mA through the string.
You don't have to worry about voltages, or matching currents.

If you wanted more than five LEDs, you just put more in series, and raise
the driving voltage appropriately, taking care of course, not to exceed the
maximum ratings of the LM317, and not to go to a level that could be
dangerous if touched. It is possible to parallel up strings to drive bigger
numbers of LEDs at lower voltages, but this requires some slightly more
complicated balancing arrangements, and protection against the current
through the remaining strings increasing, if one string fails by an LED
going open.

Arfa

In article , karotto wrote:
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.

I do not recognize the LED. 100 Lumens is pretty high. One important
spec, color, is missing. Price is very low.

I have used the 350 ma. drivers from
http://www.dealextreme.com/products.dx/category.917

greg