Can't get enough current for 1W Luexon LED even without resi

[GregS]

In article <5d43962f-06f8-4ce1-b0aa-9a304bcdff1b@f18g2000prf.googlegroups.com>, karotto <biz@theplayspace.com.au> 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

 

[Arfa Daily]

"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns9C715420653AFjyanikkuanet@74.209.136.83...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:H0mkm.232339$m%4.124985@newsfe25.ams2:


"alchazz" <no.spam@wanted.com> wrote in message
news:hGhkm.531$FV4.509@nwrddc01.gnilink.net...
On Sun, 23 Aug 2009 15:26:08 +0100, Arfa Daily wrote:

"karotto" <biz@theplayspace.com.au> wrote in message
news:5d43962f-06f8-4ce1-
b0aa-9a304bcdff1b@f18g2000prf.googlegroups.com...
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

 

[Arfa Daily]

"alchazz" <no.spam@wanted.com> wrote in message
news:UAwkm.626$FV4.562@nwrddc01.gnilink.net...

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

"alchazz" <no.spam@wanted.com> wrote in message
news:hGhkm.531$FV4.509@nwrddc01.gnilink.net...
On Sun, 23 Aug 2009 15:26:08 +0100, Arfa Daily wrote:

"karotto" <biz@theplayspace.com.au> wrote in message
news:5d43962f-06f8-4ce1-
b0aa-9a304bcdff1b@f18g2000prf.googlegroups.com...
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

 

[alchazz]

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

"alchazz" <no.spam@wanted.com> wrote in message
news:UAwkm.626$FV4.562@nwrddc01.gnilink.net...
On Mon, 24 Aug 2009 02:28:04 +0100, Arfa Daily wrote:

"alchazz" <no.spam@wanted.com> wrote in message
news:hGhkm.531$FV4.509@nwrddc01.gnilink.net...
On Sun, 23 Aug 2009 15:26:08 +0100, Arfa Daily wrote:

"karotto" <biz@theplayspace.com.au> wrote in message
news:5d43962f-06f8-4ce1-
b0aa-9a304bcdff1b@f18g2000prf.googlegroups.com...
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 ;-)